Initiation, Structure and Sprouting of Dormant Basal Buds in Betula pubescens

Flora (1987) 179,55-83 VEB Gustav Fischer Verlag Jena

Initiation, Structure and Sprouting of Dormant Basal Buds in Betula pubescens ANN ELI KAUPPI, PiIYI RINNE

and

ARI FERM

Department of Botany, University of Oulu and Forest Research Station, Kannus, Finland

Contents Introduction

56

Material and Methods

56

A. Seedlings « 1 year) B. Seedlings (2 years) C. Stumps . . . . . D. Macroscopic Examinations E. Light Microscopy . . . . F. Scanning Electron Microscopy

56 56

57 57 57 57

Bud Structure and Inferred Development Patterns A. Terminology . . . . . . . . . . . B. First Buds on Seedlings . . . . . . C. Dormant Basal Buds on Adult Trees D. Clustering of Buds . . . . . . . . . 1. Initiation of Bud Clusters in Seedlings 2. Initiation of Bud Clusters in Adult Trees 3. Experimental Induction of Bud Clusters . E. Bursting of Dormant Basal Buds and Development of Young Sprouts

57 57 57 60 63 63 65 69 72

Discussion

76

Conclusions

81

Acknowledgements

81

References

81

. . . .

Summary The morphological basis for sprouting in Betula pubescens EHRH., is studied with special reo ference to the initiation, structure and number of buds at the base of the tree and their development into sprouts. The material consists of seedlings of varying ages and stumps of mature trees. The dormant basal buds begin life as axillary buds at the seedling stage, positioned in accordance with the leaf arrangement. These primary basal buds are initiated in the axils of retarded leaves, and it is perhaps partly for this reason that they remain in a protracted state of dormancy. This dormancy is no more than a superficial feature, however, as, unlike the axillary buds higher up the stem, they are engaged in constant growth. They also differ structurally from ordinary axillary buds, possessing a growth point and a few scales after the first growing season, but no foliage leaf primordia, and gaining more scales as they age, normally one whorl per year. Similarly the vascular connection of a dormant basal bud grows year by year in accordance with the radial growth of the tree, thus ensuring that the bud does not become buried within the wood. The basal buds normally increase in number as the seedling grows, the primary buds branching

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to form clusters of secondary buds located in the axils of their scales. This branching requires a some degree of bursting of the buds. The resulting clusters are also found to vary in structure. Felling causes the majority of the dormant basal buds to burst, but only a certain number develop into sprouts. These sprouts differ morphologically from seedlings, especially in their pattern of ramification.

Introduction It is sprouting that holds the key to almost all shortrotation cultivation of trees. Although sprouting from the stump is very common in deciduous trees, the mechanism by with it takes place remains largely unknown. Thus there has been general agreement recently on the necessity for research into the biological foundation for this phenomenon (BLAKE 1981; FERM et al. 1983a und b). Attention has previously been concentrated on axillary buds located high on the stem or branches (GARRISON 1949; TUCKER 1963), while those situated at the base of the tree and responsible for generating the sprouts, although noted earlier (see TABATA 1971), have scarcely been studied morphologically at all (STONE & CORNWELL 1968). It has been normal either to describe their structural features at the macroscopic level or merely to make undocumented references to the existence of these buds. The Pubescent Birch, Betula pubescens, is one of the deciduous trees under investigation in Finland with a view to cultivation for fuel purposes (FERM et al. 1982), and with this in mind it is important to determine the structure, manner of initiation, development and growth of the buds which serve as a source of sprouts in order to be able to promote coppicing. There are many coppicing investigations which make no mention at all of what kind of buds the sprouts develop from (cf. BLAKE & RAITANEN 1981). The purpose of the present paper is to examine sprouting in Betula pubescens by morphological methods, to describe the types of bud produced by this species, and to monitor the bursting of the buds and the early growth of the sprouts. There is also a need to clarify some of the relevant terminology, since the variety of disciplines engaged in the study of sprouting and the inadequacy of the existing botanical terminology would appear to have led to discrepancies in the use of certain terms related to sprouts and buds.

Material and Methods The buds of Betula pubescens were studied mainly on the stumps of mature trees. The results and eventual hypotheses were then tested experimentally on seedlings. A. Seedlings «

1 year)

Seeds of Betula pubescens were germinated on 2.3.1984 under controlled conditions in a growth chamber (after VAL ANNE 1973). Once their first foliage leaves had emerged, 100 of the plants were transplanted into a 3 : 1 mixture of peat and sand containing fertilizer and lime (pH 5-0). They were then grown in a greenhouse with natural illumination, a fine mist spray and day temperatures of approx. 19°e. Their progress was monitored for about 6 months, including examination of the development of the buds and their location with respect to the soil surface. B. Seedlings (2 years) Eight potted 2-year-old seedlings raised at the Forestry Research Station at Muhos (26 °05'N, 64 ° 52'E) were used. Four of these had been decapitated to a height of 5 em at the beginnning of the growing season, so that they already had sprouts at the beginning of the experiment. The seedlings and sprouts were allowed to grow in a laboratory under uncontrolled moisture, illumination and temperature conditions. Illumination followed the normal rhythm during the period 5. 7.-5. 8. 1982, except that the additional lighting was adjusted to ensure a day length of 18 h. The number of buds at the base of each plant was counted at the beginning of the experiment and a drawing was made of the base with the positions of the buds marked on it as accurately as

Dormant Basal Buds

57

possible. The dormant or growing state of the buds following decapitation was assessed at a·day intervals and intervention in the form of removing a piece of the tip of the bud was also tested in a few cases. At the end of the experiment the stems were cut at the level of the buds and the vascular connections examined under a stereo microscope. C. Stumps The 162 stumps of Betula pubescens were taken at random from ten birch stands at Muhos, and represented both peat and mineral soils, trees of differing age (15-20 years and approx. 40 years) and trees of both seed and sprout origin. The material was selected in order to obtain information on growth from the moment of felling up to the fifth growing season of the new sprouts, although with principal attention paid to sprouting during the first growing season. The first stumps were collected immediately after felling, and the remainder at intervals of two weeks to a month. The stumps carrying shoots of a year or more in age were collected on just two occasions, in spring and autumn. The ground level was marked exactly on each stump before collection. D. Macroscopic Examinations The bark was removed from the stumps and a sample of the bases of the sprouts after softening in water. Finally each stump was split and cut into slices at the buds. The vascular connections were then examined in the smoothed surfaces of the sections under a stereo microscope in order to determine the manner of bud initiation and the bud type. E. Light Microscopy Eight wood samples were taken from the areas of single and clustered buds for accurate determination of the point of bud initiation. These were softened by boiling (JANE 1975) and cut into slices of thickness 10 pm on a Historange microtome. Sections of size approx. 1.5 X 1.5 cm were taken systematically at 40 pm or 90 pm intervals from the surface of the wood to the pith and mounted on microscope slides with Haupt's adhesive (KUPILA-AHVENNIEMI 1977) and stained with safranin (JOHANSEN 1940). Whole basal buds, either dormant or after bursting, were taken from the stumps or sprouts, fixed in FAA (KUPILA-AHVENNIEMI 1977), embedded in paraffin wax, cut into slices of approx. 10 pm and stained with toluidine blue (SAKAI 1973). The anatomy was documented by drawings. F. Scanning Electron Microscopy The surface structures of the sprouts and dormant buds were studied using a JEOL JSM-35 scanning electron microscope. The FAA-fixed material was dehydrated in an alcohol gradient, dried by the Critical Point method, mounted on SEM adapters with glue or double-sided adhesive tape and coated with gold.

Bud Structure and Inferred Development Patterns A. Terminology The terms applying to buds were classified by reference to the earlier nomenclature (see RINNE 1985, p. 9, table compiled from the literature) and were then revised on the basis of the morphological and anatomical observations made here. There are in principle two types of bud: those initiating at the growing point and those appearing on the older, already differentiated tissues. They may then differ further in their later development, giving rise to a range of types, as depicted in Table 1. B. First Buds on Seedlings The cotyledons of Betula pubescens emerged from the seed a week after sowing (Fig. 1A) and the axillary buds became visible at the bases of these oval-shaped, hairless seed leaves about two weeks after germination (Fig. 1 B). The plant then began to develop its foliage leaves (some juvenile leaves first) and grew in the normal way (Figs. 1 C, D and E). All the buds initiated in the first growing season were of the axillary type, their location being determined by the leaf arrangement. No adventi-

--

from the apical meristem

later, in already differentiated tissues, usually callus

II Adventitious buds

from mer is tern of primary dormant basal bud

2. further up trunk

secondary

-

irregular, in various organs

in axils of prophylls or foliage leaves

in axils of primary dormant bud scales

in axils of juvenile leaves at base of tree

variable

remain at bud stage for some time

remain at bud stage for some time

remain at bud stage for some time

to vascular connection of primary dormant basal bud to pith

only to place of origin

to branch

to sprout or branch, depending on position

to pith

to sprout

to sprout

to pith

~

0.-

'"

H

'"d '"d

q

;.-

Pi

primary

-

from apical meristem at seedling stage only

to branch of stem

Vascular connection

r-

usually develop in following growing season

Development

1. dormant basal buds

usually in the axil of each leaf

from the apical meristem of the stem throughout development of plant

I Axillary

Dormancy

Dormant buds

Position

Origin

Bud type

Table 1. Classification of bud types on trees

Ol 00

59

Dormant Basal Buds

c. A.

Fig. 1. A, B: Young seedlings of Betula pubc8cen8. C: Seedling at age 2 weeks. Axillary buds are seen at the bases of the cotyledons. Initiation of the first foliage leaves has begun. D: SEM micro· graph of axillary buds to the cotyledons when the plant has two foliage leaves. E: SEM micrograph of the same type of buds when the plant has 4 foliage leaves.

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Fig. 2. Development of the basal buds during dormancy (SEM micrographs). A: bud aged 1 year, with one pair of scales, B: aged 2 yrs., with two whorls bf scales, C: aged 3 years, D: aged 4 years.

tious buds were observed on any part of the plant. All the buds were aerial in location, and it was only occasionally that the lowermost buds on a seedling came close to the ground due to bending of the hypocotyl.

c. Dormant

Basal Buds on Adult Trees

In contrast to the situation in seedlings, the dormant basal buds on the adult trees occurred chiefly below ground level, and while those above the ground were dark in colour, dry and partially retracted into the bark, the underground ones were large, soft and light brown in colour. These were not structural differences, however, but merely consequences of the respective moisture conditions. The buds were not located in accordance with the leaf arrangement, but were irregularly distributed, having increased in number with the growth of the tree. Thus

61

Dormant Basal Buds

B

A

4mm

Fig. 3. A: A dormant basal bud about 20-years-old, B: A dormant axillary bud.

B Fig. 4. A: A bud cluster on the surface of the bark. B: Vascular connections to the buds revealed beneath the bark. The dark streak is a tunnel produced by maggots.

the same stump could feature dormant basal buds of quite different size, and therefore age, located side by side. The age of a bud can be roughly determined by counting the layers of scales, as the acquisition of a new layer would seem to be an annual event during dormancy (Fig. 2). The bud alters in shape from round to oval and eventually triangular as it grows and ages. The scales originate from stipules, the laminae remaining undeveloped (Fig. 2A). The young scales were green and covered by large numbers of unicellular hairs, but became suberized with age and shed these hairs. The scales may alter in structure as the bud prepares for winter. These dormant basal buds differed markedly from the axillary buds which initiate on the adult tree each year (Fig. 3), the latter being narrow and triangular in shape and possessed only a few scales.

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Fig. 5. Cross-section of a vascular connection. a = pith of bud stem, b = secondary xylem of the bud, c = tangentially sectioned xylem of the parent tree, d = splits in the tissue, e = vessel.

The stele, which constitutes an important part of the dormant basal bud, is buried in the xylem of the stem and extends as far a.s the point of initiation of the bud. Thus a bud on a sapling develops its stelar connection at the same rate as radial growth proceeds in the stem, remaining visible at the base of the tree even when this has reached a considerable age. Similarly the stelae stand out clearly as sharp spikes on the surface of the wood when the bark and its attached buds have been removed (Fig. 4). The stelar connection of the bud contains xylem elements oriented perpendicular to the secondary xylem elements in the stem. The bud undergoes an growth in thickness annually at its base, at the point where its cambium is still functional, whereas the part of the stele buried in the stem cannot grow. The stelar connection, which may also be referred to as the xylem or stem of the bud, corresponds in structure to a stem which has undergone secondary radial growth but which no longer possesses

Dormant Basal Buds

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A'

I'

Fig. 6. Branching of basal buds on a seedling. A: Bud with two retarded leaves at its base. A': schematic diagram of a detached profyll. B: SEM micrograph of a prophyll at the base of a bud.

cambium, phloem or cortex. The stem of the bud is circular in cross-section and about a millimetre in diameter (Fig. 5). The age of the bud can also be judged from the thickness of its stem. D. Clustering of Buds Where the one-year-old Betula pubescens seedling had only a few individual buds close to the base of the stem, 2-year-old plants even had clusters of buds, and older trees, with very many more basal buds, had these predominantly located in clusters. Thus the increase in the numbers of buds is a consequence of the formation of clusters in the course of time as the tree grows. 1. Initiation of Bud Clusters in Seedlings The basal axillary buds of the seedlings were seen to have developed into clusters by the end of the first growing season, and 72 % of the buds in the axils of the cotyledons were found to have burst, though arrested in their growth, compared with only about 1 % of those associated with foliage leaves. A cluster of two or three buds of differing size was observed in places where buds had burst and then returned to the dormant state before they had grown appreciably in length. A heterogeneous group of this kind sometimes arose without the buds having to burst visibly. 1-2 small leaves could be seen at the bases of these buds at an early stage (Fig. 6), but their differentiation remained incomplete, with the lamina relatively narrow and the pe-

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A2

Fig. 7. Seedlings of Betula pubescens after one growth season. AI, 2: The axillary buds to the cotyledons have burst and returned to dormancy, and this has led to new secondary buds at their bases. BI, 2: The axillary buds to the cotyledons have burst and died. Only new buds are found at the base. 01, 2: The axillary buds to the cotyledons have remained dormant. (The stem of one of the plants has bent).

tiole absent entirely. Only the leaf base structures with their stipules were normal. Once these leaves dropped, secondary dormant buds became visible in their axils, . i.e. at the base of the main bud or primary dormant basal bud. The formation of a cluster of this kind may be interpreted as indicating that the primary bud has been activated and has begun to branch out, but that this process has been arrested at once. The leaves appearing at the base of the primary bud may be interpreted organologically as prophylls for this branch, as they are located on either side of the bud, in which case the secondary buds are axillary to these prophylls. The axillary buds to the cotyledons may sometimes grow further than this, but the resulting branch nevertheless soon dies off. In this case a cluster of two, three or more basal buds of similar size, a "homogeneous group", will form. Here again the new secondary buds are axillary to leaves retarded in their growth, i.e. to the scales or juvenile leaves. It is also possible, however, for the axillary buds to the coty-

Dormant Basal Buds

65

ledons to remain dormant (Fig. 7), so that no cluster of buds is formed. Axillary buds to foliage leaves were not observed to form clusters. 2. Initiation of Bud Clusters in Adult Trees In order to determine the mechanism governing the initiation of bud clusters, these were studied from a series of tangential sections in which the xylem strands were visible in cross-section. The vascular elements lying around the groups of bud traces, and particularly between the strands in a bud cluster, were seen to have bent over and been intersected in a number of directions (Fig. 8B). The tangential tissue of the parent tree showed a normal orientation slightly further away from the bud cluster (Fig. 8A), and there was often a rift between the normal woody tissue and the deviantly oriented xylem elements of the bud cluster. The vessels had sometimes extended well beyond their normal size around the cluster, but had been compressed somewhat between the bud traces so that the tissue there was abnormally dense. Other elements, especially the tracheids and pith rays, also formed spiralstructures in this area (Fig. 8B). One example of the anatomical findings may be quoted here to illustrate the main stages in the initiation of bud clusters (Figs. 9A-C). This particular cluster consisted of eight buds of similar size at the base of a young tree of seed origin grown on peatland. Each bud had its own vascular trace at the surface of the wood, and these traces then remained distinct close to the pith, although nearer together. A connection with an older bud which had undergone radial growth was found only relatively close to the pith. This bud (X) had itself burst and died, as seen from the presence of brown phloem and cortex at its tip (Fig. llA). The vascular traces of all eight new buds were connected to the leaf gaps of this bud (Fig. 9C), which would seem to have been the primary dormant basal bud, since its point of origin was close to the pith. This implies that the eight buds in the cluster must have been secondary dormant basal buds that had originated in the axils of the primary bud scales during the early seedling stage, their increase in number being due to branching. Both anatomically and in terms of outward appearance, the buds on all the stumps may be said to be of the dormant basal type, while the initiation of new buds, i.e. branching, cannot be regarded as an accident, but would normally seem to require bursting of the initial bud, although a lower level of activation may also suffice. Dead buds which had burst were to be found in the majority of the clusters. A burst bud would seem to die only at its tip, while its base remains alive. The first stage in the initiation of a bud cluster consists of the activation or bursting of the parent bud, whereupon new buds are initiated in the axils of its scales. Even though the original bud may die, the new secondary ones remain alive in an apparently dormant state, since they are extending their vascular traces to keep pace with the radial growth of the stem. It is for this reason that they are no longer to be found at the base of the burst bud but in a group on the surface of the bark. It is sometimes possible many years later to see the dead tip of a sprout surrounded by the stelae of young buds in the secondary xylem (Fig. lOA). Once sufficient time has elapsed, however, all signs of the initial bud disappear (Fig. lOB), it having become buried entirely within the trunk as a result of secondary radial growth. Similar branching may also affect a secondary dormant basal bud after some time, thus causing the clusters of buds to increase in size as the tree ages and grows radially, and corrrespondingly reducing the numbers of single buds. Although the clusters tend to increase in size, the num bers of buds can also diminish for some reason, and the buds may disappear from view entirely. There were no seedlings completely lacking in buds in the present material, but there were three such stumps. 4a

Flora, Bd. 179

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Fig. 8. A: Tangential section of the normal wood of a pubescent birch. B: Tangential section through a bud cluster, showing the cells between the bud stems. The elements are disorganized and the cells tightly compressed. a = pith rays, b = vessels, c = tracheid, d = elements in spiral .structure.

67

Dormant Basal Buds

1mm

1mm

Fig. 9. Principal stages in the formation of a cluster of eight buds. 1-8 vascular connections of the secondary dormant basal buds, x = vascular tissue of the primary basal bud. For details, see text.

-

Fig. 10. A: Bud cluster, with the dead tip of the burst primary bud in the centre. B: Cluster of several secondary dormant basal buds. The dead primary bud has been enclosed by the wood of the tree. SEM micrographs.

Dormant Basal Buds

69

Fig. 11. Reduction in numbers of buds on a stump due to the development of ligneous knots. A: Vigorous branching of buds and elevation from the stump surface. B: This abnormal growth causes the buds to form a spherical tumor. C: Although the site of the bud cluster is now obviously occupied by a ligneous knot, the vascular connection to the cluster has not yet severed. D: Independent knots with no vascular connection. The buds will gradually die off.

When these were split, however, bud traces and dead buds could still be seen within the wood. The numbers of basal buds on the old trees varied greatly, the material being found to include both budless stumps and ones with very large numbers_ There would seem to be various reasons for the former situation, one of which, according to our observations, may be excessive branching, which can lead to the formation of ligneous knots without buds if growth proceeds more rapidly in the xylem of the cluster than in the parent stem (Fig. ll). The limited material studied here unfortunately does not allow us to demonstrate convincingly whether or not the lack of buds and the possession of large numbers of them are to be regarded as successive manifestations of the same phenomenon. 3. Experimental Induction of Bud Clusters Since the final outcome of the major event involved in the formation of bud clusters, the bursting and subsequent death of a bud, is destruction of the growth point, an attempt was made to induce this event by removing the tips from dormant buds on the bases of 2-year-old seedlings. Experiments were also made with a double action involving removal of the bud tip and the growing point of the stem. These two si5

Flora, Bd. 179

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Fig. 12. SEM micrograph of a young manipulation-induced secondary dormant basal bud possessing large numbers of trichomes.

tuations were examined alongside the spontaneous development of bud clusters in untreated seedlings in the course of one growing season. Removal of the primary bud tips from otherwise intact seedlings did indeed cause the development of clusters, the secondary buds emerging within 1-3 weeks of removal (Fig. 12) and remaining in a dormant state in the manner of the original ones. Unlike the basal buds, the axillary buds associated with foliage leaves did not react when treated in the same way, and did not develop clusters. A couple of small leaves grew up in exceptional cases (bud 5, Fig. 13), but no branch development took place. The axillary buds already contained leaf primordia; whereas the dormant basal buds

71

Dormant Basal Buds

I;",

/\ 4 .':' \ J

• f,) 3 : .... ,

~1

.~2

'" ~ .

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.

::.;

/

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o 3 Fig. 13. Formation of clusters from buds treated in various ways. A: Decapitated buds are indicated with arrows. 1-4 dormant basal buds. 5-7 ordinary axillary buds. B: 11 d later both the decapitated dormant basal buds had developed new secondary buds, and the axillary buds had grown two leaves. C: The situation after 18 d. Two more new buds had developed beside the primary dormant basal buds, but axillary ramification had come to an end. D: Cross-sections through the stem at the bud clusters. For details, see sext.

obviously had only bud scales or primordia of leaves in which the lamina will fail to develop. The dormant basal buds not treated in the above manner did not gain any new buds in the course of this experiment, even though the lowermost ones already had some secondary buds from the previous growing season (Fig. 13A). These early dormant basal bud clusters were located on opposite sides of the stem and were shown by 5*

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Fig. 14. A: SEM micrograph of a basal bud bursting after some years of dormancy. Light-coloured, scales are seen around the tip. B: SEM micrograph of a bud after bursting, with extended internodes. The stem carries the bud scales, stipules, while the tip shows the first primordia of juvenile leaves.

their vascular traces to have originated from axillary buds to the cotyledons (Fig. 13D). Where the apex of the seedling was also severed, a number of new growth points were induced in the axils of the primary dormant basal bud scales, developing directly into sprouts, i.e. branching occurred without any discernible secondary budding phase. Usually only one sprout from each bud cluster thrived, the others being delayed in their development. Thus these cases of experimental bud cluster formation support the theory derived from observations on existing clusters in adult trees. E_ Bursting of Dormant Basal Buds and Development of Young Sprouts The dormant basal buds were seen to become activated rapidly upon felling of the tree, becoming sticky on the surface and paler in colour at the tip (Fig. 14A). Some had begun to burst within two weeks, although the majority did so about a month

Dormant Basal Buds

73

Fig. 15. SEM micrographs of early sprout development. A: Vertical section through an activated primary basal bud in the process of sprouting. The arrow indicates a new secondary bud in its lower part. B: Higher-magnification view of the secondary bud. The arrow denotes a bud scale. C: Base of the new sprout, showing four secondary dormant basal buds.

after felling. No further bursting was recorded after that time, evidently because the developing sprouts were beginning to dominate over the still dormant buds. Even so, it was apparent that there were live dormant basal buds which were still able to become activated at the beginning of a growing season several years after felling of the parent tree, although there were few such cases, in none of which was the bud capable of developing a shoot. Of those buds that did burst at the beginning of the first growing season, only a certain number developed normally in terms of internodal extension and formation ofthe first pair of juvenile leaves (Fig. 14B). At the same time as the dormant basal buds began to develop into sprouts the meristematic cells in the axils of their scales divided, giving rise to new buds (Figs. 15A and B). These secondary dormant basal buds, the purpose of which is to ensure sprouting in the future should the present shoot be damaged or broken off, appear at the base of the rapidly growing sprout immediately it has shed its bud scales (Fig. 15C). The new buds usually appeared in the axils of the lowermost bud scale layers, normally on the outer edge of the sprout.

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Fig. 16. Germ-like, etiolated underground sprouts lacking in chlorophyll and with retarded laminae. B: An aerial sprout with green leaves.

The buds which had burst beneath the ground on account of the absence of light were pale in colour, brittle in structure and germ-like in appearance, and the laminae of their leaves remained undeveloped, unlike those of the aerial sprouts (Fig. 16). The etiolated sprouts originating deep in the ground had to grow up through compacted soil horizons and between stones and roots, and were thErefore frequently bent in appearance and had suffered damage to their growing tip. In such cases the axillary buds had begun to develop into branches and the secondary dormant basal buds into second-order sprouts. The customary outcome is then a bush-like growth composed of etiolated branches (Fig. 17). Only in exceptional instances did a bud located deep in the ground grow into a viable sprout. The sprouts which set out from ground level or above developed branches during their first growing season due to bursting of their normal axillary and basal buds (Fig. 18A), this ramification being basitonic, leading to a bush-like mode of growth. The spontaneous appearance of second-order sprouts may be regarded as a disadvan-

75

Dormant Basal Buds

tern Fig. 17. Multiple ramification in an underground sprout.

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Fig. 18. Ramification in sprouts originating from different positions with respect to the ground surface. A: Aerial sprout in which the axillary buds and some basal buds have burst. B: Initially underground sprout in which the axillary buds have burst but the basal buds remain dormant. C: l-year-old plant of seed origin in which only the axillary buds of the cotyledons (arrow) have burst, but with no further development.

tage, as these retard growth in the main shoot and exhaust the bud supply required for the next coppicing. Those sprouts which succeeded in developing from below the ground did not ramify so actively, as usually only their normal axillary buds burst in the first growing season (Fig. 18 B). The pattern of ramification in vegetative sprouts is in any case quite different from that found in seedlings, since with the exception of the cotyledons the axillary buds of the latter almost always remain dormant (Fig. 18C). These differences may be attributed in part to the history of the buds in seedlings and sprouts, in the sense that all of the former are primary in character, whereas the basal buds of the spouts are inevitably secondary, the products of one or more branching events.

Discussion The dormant basal buds of the Pubescent Birch are in effect axillary buds, so that the sprouts arising from them may be likened to branches. The differences lie in the fact that there is something in the development of these particular axillary buds that causes prolongation of their dormant phase and enables the resulting sprouts to grow up into independent trees rather than remaining at the level of branches. These developmental differences are evidently related in some way to the fact that some of the axillary buds produced in the tree's first growing season are located in a deviant manner compared with the remainder. The very first buds to appear on a seedling are not found in the axils of true foliage leaves but in those of cotyledons, and many of the subsequent buds also occur in connection with structurally deviant juvenile leaves. A corresponding situation in fact prevails regarding the axillary buds related to the prophylls of branches, which our observations suggest may also behave in the manner of dormant basal buds. These buds are described by CHURCH & GOD-

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MAN (1966) and CREMER (1972), but neither paper draws attention to the retardation in the development of the subtending leaves in connection with their deviant structure. In this case, however, they do not develop into sprouts but into epicormic shoots (KO:RMANIK & BROWN 1969). The buds found at the base of sprouts are also located in the axils of retarded leaves, i. e. the scales of dormant basal buds. It is interesting to note that the lignotubers of eucalypts appear to arise from similar leaf axils (CHATTAWAY 1958). The basal buds of seed and sprout-origin trees are capable of remaining "dormant" for many years, their vascular connections growing while they are resting, maintaining contact with the primary xylem throughout the life of tree (ROTH & HEPTING 1943; FINK 1983). Although this vascular connection is not regarded as important for the vitality of a bud (MIKOLA 1942), it is an essential point that it does continue to grow from year to year, enabling the bud to remain on the surface of the wood. The dormant basal buds of a tree may be either visible on the surface of the bark or invisible, within the bark or buried beneath it. The majority of tree species capable of sprouting which have been investigated to date regenerate vegetatively via hidden basal buds (WILSON 1968; CARRODUS & BLAKE 1970; DEBELL & ALFORD 1972), and in this respect Betula pubescens is of a rarer type, since all its buds of this kind are situated on the surface of the bark. A dormant bud contains a growing point and some leaf primordia (WAREING 1969), which increase in number with age. Functionally, however, the bud is capable of growth immediately upon initiation, in order to replace some damaged part of the tree. The growth point and leaf primordia are surrounded in the case of the birch by bud scales which have developed from stipules (KRAMER & KOZLOWSKI 1979). Axillary buds at various heights on the stem do not show any distinguishable external structural differences in their first year, presumably differing only in a physiological sense, and the obvious structural differences emerge only as dormancy proceeds, when more scales develop on the basal buds alone. As observed in some other species(CHuRCH & GODMAN 1966), the basal buds of the birch normally acquire one new whorl of scales each year. Thus a rough estimate of the age of a dormant basal bud or the duration of its dormancy can be obtained by counting the number of scale layers. The scales are important for maintaining the activity of the bud, since their dense arrangement prevents evaporation, gas exchange or penetration of light to the growing point (POLLOCK 1953; PUKACKI et al. 1980). The function is not merely a mechanical one, however, as they have also been shown to serve as reservoirs of growthretarding substances (TINKLIN & SCHWABE 1970). Thus the activation resulting from removal of the bud tip may also be a consequence of a loss of bud scales, and therefore be based on a physiological interaction between tis'sues of different types. The increase in the numbers of bud-scales and the length of the bud during dormancy is clear evidence of growth. Thus it seems that the dormant basal buds of Betula pubescens are capable of apical growth, in contrast to the observations of MIKOLA (1942). A lack of apical growth was similarly observed by BARANOVA (1960) when studying the hidden buds of Eucalyptus. On the other hand, the 'buds of the white maple, which occur on the surface of the bark, are reported to grow for 1-2 weeks immediately after winter dormancy, forming a new layer of scales (CHURCH & GODMAN 1966). The unusual structural properties of dormant basal buds evidently result in part from this superficiality of their state of dormancy. Since they are in fact growing during this time, they must be undergoing cell division and many metabolic functions. They may therefore be said to be buds in a state of active dormancy. They have therefore been commonly regarded not as ordinary buds but as short shoots (WILSON 1968) or highly altered dwarfed branches (LIMING 1940).

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One of the essential properties of dormant basal buds is their location close to the soil surface, it being normal in this species for them to pass below the surface as the tree matures, since the hypocotyl remains relatively short even at the seedling stage under natural conditions (see HEIKINHEIMO 1915, illustration of a natural seedling). A further factor which may bring the basal buds in contact with the surface of the ground is contortion of the stem, which is a fairly regular event in some species (STONE & STONE 1954), but which seems to occur only sporadically in Betula pubescens. Other factors may be movements in the earth's crust, an accumulation of forest litter or the development of adventitious roots (STONE & CORNWELL 1968). The most advantageous position for these buds would seem to be just below the surface, where the less extreme conditions help to keep them alive but where they are not yet exposed to the many dangers encountered at deeper levels. Developmental hazards may be reflected in excessive clustering of the buds, slow growth of the sprouts and excessive ramification after felling, features which may also result from domination by burst aerial basal buds. In the case of Betula pubescens the number of buds begins to increase at the base of the seedling during the first growing season, while further up the plant single buds tend to persist, as in other species (STONE & CONRWELL 1968; BRUNKENER 1984). A suspicion was voiced several decades ago that buds may increase by lateral branching (HAHNE 1926), new buds arising in the axils of the scales of existing ones. There are also reports of the dichotomous branching of buds (KORMANIK & BROWN 1964, 1969) but little accurate research or documentation has been available. Moreover, branching, has been reported in buds further up the stem (CHURCH & GODMAN 1966; CREMER 1972), or in species in which the buds occur under the bark or buried within it (HAHNE 1926; KORMANIK & BROWN 1964, 1969). It is nevertheless possible that branching in the dormant basal buds of Betula pubescens, which occur on the surface of the bark, may be similar in principle to the lateral branching of axillary buds reported earlier, in spite of the fact that certain species having both visible and hidden dormant buds have been found to undergo branching of the buds only in the latter case (WILSON 1968). Not every instance of a new bud emerging close to an existing external one is proof of branching and secondary bud formation, of course, since the same may be achieved by a persistently active axillary meristem (CHATTAWAY 1958; CREMER 1972) or the occurrence of adventitious buds (FINK 1983). Where previous work has ignored the questions of how or why branching occurs in dormant buds, the present study provides an answer to the first question at least. Regarding the second, it would seem that the reason for this branching, in the case of Betula pubescens lies in some degree of interruption of the state of dormancy, although it is still not known what factors, internal or external, are responsible for this. The differences in degree seem to be connected with the age of the tree, since the first dormant buds at the base of a seedling or sprout, if activated without bursting, usually gives rise to heterogeneous bud clusters, new buds being initiated in the axils of profylls of the parent bud. The formation of small buds has also been observed. in other species, although usually at the bases of axillary buds higher up the stem. As in this case, the interpretation has normally been one of premature bud initiation ("second-order axillary buds" TUCKER 1963) in a "branch" which is still at the bud stage itself, or else the structures have been regarded as a part of the regular development of an axillary bud ("collateral buds" KORMANIK & BROWN 1969; "single bud scale covering three shoot primordia" BRUNKENER 1984). These reported cases would also seem to involve the early formation of axillary buds to profylls. The dormant basal buds of adult trees are usually able to emerge from dormancy more fully than those of seedlings, bursting spontaneously but not developing into

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sprouts. The result in this case is the formation of homogeneous clusters of secondary dormant basal buds equal in size. Although the spontaneous bursting of buds in intact plants is shown here to be of essential importance for the formation of bud clusters, there are few references to it in the literature. MIKOLA (1942) notes that all the time a specimen of Betula pubescens remains alive buds may "sometimes" burst and the base of the tree may carry sprouts a few centimetres long "from time to time", while LAITAKARI (1935) claims that the buds burst "often" and the dormant basal buds develop into sprouts even while the tree is alive. STONE & CORNWELL (1968), referring to one birch species which develops very large clusters of buds, note that "they seldom elongate actively in uninjured plants", whereas "some suckers often grow" under similar conditions in certain bush-like birch species in Japan (TABATA 1971). No connection has been pointed out previously, however, between spontaneous bursting and the formation of bud clusters, although KORMANIK & BROWN (1964) noticed the rise of new buds in the immediate vicinity of dead ones. Clusters of buds can develop in the Pubescent Birch which reach the size of tumours at times, corresponding to the lignotubers found in the eucalypts (CARTER 1929; CARRODUS & BLAKE 1970), which are regarded as adaptations to difficult conditions, on account of the nutrient reserves found in them (MULLETTE & BAMBER 1978). The bud clusters of the birch similarly seem to be adaptations to vegetative reproduction (LAITAKARI 1935; KOROVIN 1971; TABATA 1971), even though the present results show that the very large clusters do not serve this purpose. Evidence for this is provided by the close relationship between an excess and an absence of buds in adult trees, suggesting that a cluster may develop into a tumours growth which is no longer of any reproductive value if its buds branch too actively and their vascular connections to the main stem are broken. These observations support the assumptions of STONE & CORNWELL (1968) regarding the origins of tumours. No other authors propose any connection between tumours and bud cluster formation, and the former have been observed to originate either adventitiously (WELLENSIEK 1952; BALDINI & MOSSE 1956; DERMEN 1948, 1959), or from a dormant bud whose vascular connection has been severed by the force of the growing bark layer (MIKOLA 1942; FINK 1983). The bursting of buds, either spontaneously or upon coppicing is an indicator of their release from correlative inhibition, a phenomenon for which a number of theories exist, although none of them is entirely exhaustive (HILLMAN 1984). The retention of buds in a dormant state is regarded as being due principally to apical dominance, the major factor in which is probably auxin (WAREING & PHILLIPS 1981), although other hormones, and particularly their mutual proportions, are also important. The difference in behaviour between the basal buds and those further up the tree (e. g. growth and branching of the former during dormancy), the location of the former in the axils of retarded leaves and other morphological characteristics lead us to suspect that they may be physiologically distinct entities, and some authors do indeed attribute differing inhibition mechanisms to basal and ordinary axillary buds (CARPENTER & RODRIGUEZ 1971). The axillary buds of Betula pubescens do not usually burst until they have spent the winter in dormancy, and even after this their growth is controlled to ensure that the branches do not grow longer than the main stem (cf. BROWN et al. 1967: growth patterns). The present work indicates, however, that this holds good only for the ordinary axillary buds, the tree being unable to maintain anything like the same control over its basal buds. The superficial dormancy of these and their branching to form clusters serve as good indications of the incompleteness of this control. Hints have emerged here that it may be the apical meristem of the

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Fig. 19. Developmental cycle of dormant basal buds in the birch. The middle part of the diagram represents growth and development of the individual bud at the base of the tree. The clockwise chain then describes the development of a bud cluster ani the anti-clockwise chain sprouting. For details, see text.

basal bud that regulates the emergence of secondary buds and the main stem that regulates their bursting. Decapitation brings about many changes in the physiology of the stump (TAYLOR et al. 1982), implying above all a removal of apical dominance, whereupon the buds emerge from dormancy, become biochemically active and begin to grow. The majority of the buds were found to burst following decapitation, but only some of them developed into sprouts. One reason for the inconsistency of this new growth may be that some of the buds were located below the surface of the ground, since light is essential for the bursting of basal buds in some species (KHAYAT & ZIESLIN 1982), while in others it is not necessary for their bursting but does promote the growth of the sprouts (KRAMER & KOZLOWSKI 1979). In the present case the lack of light did not actually prevent bursting of the dormant basal buds but it did cause etiolation of the sprouts

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and eventually arrest their growth. A further reason may be correlative inhibition maintained by the developed sprouts, causing the other buds to remain dormant (WILSON 1968). Only the uppermost bud clusters appeared to grow into sprouts in the case of Betula pubescens. The difference in conformation between the sprouts and seed-origin shoots is evidently due to the lack of apical dominance in the former (BLAKE & CARRODUS 1970). The early development of a sprout is characterized by vigorous growth, which is suspected by BRUNKENER (1984) to lead to a disturbance in this dominance. Axillary buds have indeed been found to burst even in seedlings if their growth has been accelerated (RICHARDS & LARSON 1981), but the fundamental reason for the rapid early growth of sprouts is still not known, even though various possible explanations have been mentioned (ZAHNER & DEBYLE 1965; SHELDRAKE 1973; LEOPOLD & KRIEDEMANN 1975; JOHNSON 1979; BLAKE 1981). It is only later that growth in the sprouts slows down, the juvenile stage characteristics disappear and the regulation of bud development begins to function in a manner typical of the species.

Conclusions The following cyclic model may be proposed for the initiation, development and growth of dormant basal buds on seedlings, sprouts and adult trees of Betula pubescens. The basal buds begin to develop as axillary buds in the apical meristem of the shoot, and it is probable that in the case of the seedling will only the first buds - axillary buds to the cotyledons and early retarded ju· venile leaves - remain dormant and develop in the manner depicted in Fig. 19. These buds are able to form clusters at once, in their first growing season, whereupo~ the cluster is usually heterogeneous' containing a large bud and one or two small ones, and the new buds are organologically axillary to the profylls of the primary one. The buds may nevertheless remain in a state of active dormancy, growing individually at the base of the tree for some years (middle row, Fig. 19). Alternatively, they may burst spontaneously, causing secondary buds to form in the axils of the primary bud scales. The development of a burst primary bud is usually arrested fairly early on, having apparently become regulated by the tree itself, since it dies rather than producing a sprout (lower course in Fig. 19). This leads to the formation of a homogeneous duster of buds of similar size. The resulting secondary buds may then behave similarly, leading to further enlargement of the cluster. When the tree is damaged in some way, however, a different course of development is generally pursued, i.e. a primary or secondary bud bursts and grows into a sprout (upper course in Fig. 19), and those buds that would have formed the cluster had the primary one burst and died now remain dormant at the base of the resulting sprout. These may in turn either remain as individual buds as the sprout ages or from clusters of their own and will in turn be responsible for second-order sprouting should the sprout in question be decapitated again. It is common for the dormant basal buds of the birch to develop in the manner depicted above, and only the direction of development may be found to vary on occasions, e.g. sprouts may exceptionally develop on an intact tree for some reason, or else bud clusters may be found on the .stump of a felled tree, should the buds have failed to develop into sprouts.

Acknowledgements Thanks are due to all those people who helped in the work, and to Mr. MALCOLM HICKS, 1\1. A., for translating the manuscript into English.

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KUPILA-AHVENNIEMI, S_ (1977): Mikrotekniikan ty6ohjeita. Oulun yliopiston kasvitieteen laitoksen monisteita No.4. LAITAKARI, E. (1935): Koivun juuristo. Summary. The root system of birch. Acta For. Fenn. 41: 1~126. LEOPOLD, A. C., & KRIEDEMANN, P. E. (1975): Juvenility, maturity and senescence. In: LEOPOLD, A. C. & KRIEDEMANN, P. E. Plant Growth and Development: pp. 249~269. McGraw-Hill. New York. LIMING, F. G. (1940): Origin and growth of dormant buds in oaks. J. For. 38: 226. MIKOLA, P. (1942): Koivun vesomisesta ja sen metsanhoidollisesta merkityksestii. Referat: Uber die Ausschlagbildung bei der Birke und ihre forstliche Bedeutung. Acta For. Fenn. 50: 1~102. MULLETTE, K. J., & BAMBER, K. J. (1978): Studies of the lignotubers of Eucalyptus gummijera (Gaertn. & HOCHR.). III. Inheritance and chemical composition. Aust. J. Bot. 26: 23~28. POLLOCK, B. M. (1953): The respiration of Acer buds in relation to the inception and termination of winter rest. PhysioI. Plantarum. 6: 47~64. PUKACKI, P., GIERTYCH, M., & CHALUPKA, W. (1980): Light filtering function of bud scales in woody plants. Plant a 150: 132~133. RICHARDS, J. H., & LARSON, P. R. (1981): Morphology and development of Populus deltoides branches in different environments. Bot. Gaz. 142: 382~393. RINNE, P. (1985): Hieskoivun silmujen morfologiasta seka vesojen alkukehityksesta. Pro-grad\!. Department of Botany, University of Oulu. ROTH,E.R., & HEPTING,G.H. (1943): Origin and development of the oak stump sprouts as affecting their likelihood to decay. J. For. 41:.41: 27~36. SAKAI, W. S. (1973): Simple method for differential staining of paraffin embedded plant material using toluidine blue O. Stain Technology 48: 24 7~250. SHELDRAKE, A. R. (1973): The production of hormones in higher plants. BioI. Rev. 48: 506~559. STONE, E. L., & CORNWELL, S. M. (1968): Basal bud burls in Betula populijolia. Forest Sci. 14: 64--65. ~ & STONE, M. H. (1954): Root collar sprouts in Pine. J. For. 52: 487~491. TABATA, H. (1971): Root habit of japanese birches (Betula). Series of Biology IV: 130~138. TAYLOR, J. S., BLAKE, T. J., & PHARIS, R. P. (1982): The role of plant hormones and carbohydrates in growth and survival of coppiced Eucalyptus seedlings. PhysioI. Plant. 55: 421~430. TINKLIN, I. G., & SCHWABE, W. W. (1970): Lateral bud dormancy in the blackcurrant Ribes nigrum (L.) Ann. Bot. N. S. 34: 691~707. TUCKER, S. C. (1963): Development and phyllotaxis of the vegetative axillary bud of Michelia jU8cata. ArneI'. J. Bot. 50: 661~668. VALANNE, T. (1973): Germination experiments on the seeds of Betula species~Ann. Univ. Turkuensis, A. II. BioI.-Georg.-GeoI. 52. WAREING, P. F. (1969): Germination and dormancy. In: WILKINS, M. B. (ed.) Physiology of Plant Growth and Development pp. 506~644. McGraw-Hill, London. ~ & PHILLIPS, J. D. J. (1981): Growth and Differentiation in Plants. Pergamon press, Oxford. WELLENSIEK, S. J. (1952): Rejuvenation of woody plants by formation of sphaeroblasts. Proc. K. NederI. Akad. Wetensch. Amsterdam C. 55: 567~573. WILSON, B. F. (1968): Red maple stump sprouts: development the first year. Harward Forest Paper 18: 1~10. ZAHNER, R., & DEBYLE, N. (1965): Effect of pruning the parent root on growth of aspen suckers. Ecology 46: 373~375. Received February 28, 1986 Authors' addresses: A. KAUPPI and P. RINNE, Department of Botany, University of Oulu, Linnanmaa, SF - 90570 Oulu, Finland; A. FERM, Finnish Forest Research Institute, Kannus Research Station, SF - 69100 Kannus, Finland.