Flora (1990) 184: 197-208 VEB Gustav Fischer Verlag Jena
The Importance of Stem Anatomical Characters for the Systematics of the Genus Potamogeton L. GERHARD WIEGLEB Fachbereich 7 Biologie, Universitiit Oldenburg, Oldenburg, BRD
Abstract A historical outline of the use of stem anatomical characters in Potamogeton taxonomy is given. The most important characters for the distinction of taxa (stelar type, sclerenchyma, endodermis, interlacunar bundles, subepidermal bundles, and pseudohypodermis) are listed and possible character states are defined. The variability within one taxon is discussed with reference to P. nodosus. Even though considerable variations can be found this variation follows a regular pattern within clearly defined limits. An identification key to the broad-leaved species of Potamogeton is presented using the characters discussed above. The relation between supposed taxonomic affinity and stem anatomy is discussed. In some cases there are subsections showing similar anatomical patterns. In some cases there is a differentiation on species level. In other cases the situation is not yet clear. Keywords: Identification key, stem anatomy, Potamogeton, subsections, variability
Introduction The importance of stem anatomical characters for the systematics of the genus Potamogeton has been controversially discussed for a long time. During the course of time stem anatomy was used for four different purposes: 1. 2. 3. 4.
Grouping of species within the genus. Recognition of hybrids. Recognition of new species. Identification of herbarium specimens.
Pioneer work was done by RAUNKIAER (already in 1896, later summarized 1903). RAUNKIAER'S main interest was in the first named aspect. As he always took into consideration the total morphology of the plant, his groups were defined by a mixed set of anatomical and morphological characters. FISCHER (1907) successfully used RAUNKIAER'S characters both for identification of specimens and description of new taxa. HAGSTROEM (1916) broadened the empirical basis and developed a more elaborate anatomical nomenclature. He mainly stressed the importance of stem anatomical characters for recognizing new taxa. HAGSTROEM partly went too far in his appreciation of stem anatomy, particularly with respect to the recognition of hybrids. Further data were contributed by MIKI (1937) and OGDEN (1943). The latter one developed an elaborated identification key of North American species. OGDEN'S terminology is the basis of the treatment in TOMLINSON (1982) and the present paper. With OGDEN, emphasis clearly shifted to identification. BERTON (1978) and MARKGRAF (1981) presented keys of the European species both starting from the tradition of RA UNKIAER. The key of MARKGRAF is more detailed with emphasis on species distinction, while BERTON'S key more carefully deals with the similarities within species groups. SYMOENS et al. (1979) used stem anatomy for the distinction between two very similar African taxa. This is still the only successful application to a special taxonomical problem. TUR
(1982); HAYNES (1985) and ROWECK & SCHUTZ (1988) referred to stem anatomy among other characters. On the other hand GRAEBNER (1908) and ST. JOHN (1925) explicitely doubted the taxonomical usefulness of stem anatomy. ASCHERSON & GRAEBNER (1907); FRYER & BENNETT (1915); FERNALD (1932); DANDY (1937) and CASPER & KRAUSCH (1980) did not use these characters. A key question is the variability within the entity studied, be it the species group, the species or even the individual plant. HAGSTROEM surly underestimated the variability within species, even though in his work many examples of variability can be found. TOMLINSON (1982); KADONO (1983) and KADONO & WIEGLEB (1987) described further examples of infraspecific variability. The aim of the present paper is to elucidate some of the old problems by means of a new anatomical technique. The treatment is restricted to the broad-leaved species ofthe genus. The stem anatomical differentiation within the small-leaved species is more restricted (HAGSTROEM 1916).
Material and Methods The study is based both on fresh material and material taken from herbarium specimens. Fresh material of the following taxa was collected in the surroundings of Oldenburg: P. alpinus BALBIS, P. gramineus L., P. lucens L., P. nodosus POIRET, P. perfoliatus L., P. polygonifolius POURR., P. x decipiens NOLTE, P. x spathulatus SCHRADER, and P. x sterilis HAGSTROEM. The following broad-leaved taxa were studied on herbarium specimens from the own private collection (at Oldenburg): P. coloratus HORNEMAN, P. distinctus A. BENNETT, P.fryeri A. BENNETT, P. oakesianus ROBBINS, P. praelongus WULFEN, P. wrightii MORONG, P. x malainoides MIKI, P. x nitens WEBER (incl. P. x nipponicus MAKINO), P. x schreberi FISCHER, P. x sparganifolius LAESTAD, P. x undulatus WOLFGANG, P. x yamagataensis KADONO et WIEGLEB, and P. x zizii KOCH ex ROTH. Additionally, P. berchtoldii FIEBER, P. crispus L., P. maackianus A. BENNETT, P. ochreatus RAOUL, P. octandrus POIRET and P. trichoides CHAM. et SCHL. were available. In the course of studying P. distinctus, P. wrightii and P. nodosus from the following herbaria (A, AAV, ABD, B, BGK, BM, BP, BREM, BRI, CANB, CGE, E, F, GAT, GOET, L, LD, M, MICH, MO, NY, P, PH, TAl, TI, V, VC, VPNG, VPS, WU, Z, ZT) also P. amplifolius TUCKERMAN, P. cheesemanii A. BENNETT, P.ferrugineus HAGSTROEM, P. illinoiensis MORONG, P. linguatus HAGSTROEM, P. montevideensis A. BENNETT, P. parmatus HAGSTROEM, P. pulcher TUCKERMAN, P. schweinfurthii A. BENNETT, P. suboblongus HAGSTROEM, P. sumatranus MIQUEL, P. thunbergii CHAM. et SCHL. and P. tricarinatus MUELLER et BENNETT could be studied. Also specimens of undescribed species from New Guinea and the Pacific area were included. For the study of stem anatomy short pieces of the stems were cut off the specimens. In fresh material and material from the own collection an internode from the upper part of a flowering vertical shoot was investigated. In case of foreign herbarium specimens often only the lower part of any kind of shoot could be studied in order not to destroy the specimens. In case of P. alpinus, P. gramineus and P. lucens cross-sections of various shoot types from one individual plant (shoot complex, WIEGLEB & BRux 1989) were studied. The fresh material was proceeded directly as described below. The herbarium material was first soaked in an ethanol/glycerol mixture for 3 days. It was then imbedded in hard plastic (Technovit 7 100 and 3040, Fa. Kulzer). Cross sections of 5 !lm were cut with a microtome (Fa. Reichert-lung) and stained with toluidine blue according to Sakai. Photographs were obtained using a Polyvar research microscope at magnifications 100 to 250 X .
Results Characters of Potamogeton stems OGDEN (1943) used five groups of characters for distinction among species, the type of the stele, the shape of endodermis cells, the presence of interlacunar and subepidermal bundles, and the presence of a pseudohypodermis. In the course of the present study it could be shown that also the development of the stelar sc1erenchyma is a differentiating character. In Fig. 1 a general scheme of a cross-section through a Potamogeton stem is shown, displaying all possible variations of the important characters. The most complex character is the construction of the stele. There are 4 basic types which are connected by intermediate forms:
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Stem Anatomical Characters of the Genus Potamogeton
TYPE OF STELE prato
absent/1 - 4 layers
SUBEPIDERMAL BUNDLES present I absent
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Fig. I. General scheme of the stem of a vegetative shoot of Potamogeton showing the taxonomically important characters and character groups.
a) The proto type, with four (rarely 3) median bun4les, and 3(-5, 6) lateral ones (formula 3,4,3). b) The eight bundles type, to be divided into the subtype with two phloema (trio type of HAGSTROEM), with 2 median bundles one of which is still recognizable as derived from the joining of three independent bundles, and mostly 3 ( - 5) lateral bundles, and the eight bundles type s. str. with 2 median and 3 lateral bundles on each side (2,3,2). The stele is always more or less lobed in types a and b. c) The oblong type, in which the stele is not lobed. There are either 2 independent median bundles or one fused one, and there are 1 to 3 lateral bundles with varying degree of fusion. As the fusion of the lateral bundles is only partly correlated to the fusion of the median ones different combinations expressed by the formulas 3,1,3, 2,2,2, or 1,1,1, can occur. Thbre is a complete series of reduction from the most complex forms of the proto type to the 1,1,1 form of the oblong type. d) The four bundles type is usually of squared shape with either 4 equally large bundles or two "median" large ones and two "lateral" smaller ones (1,2,1). The four bundles type can be derived directly from the proto type. The sclerenchyma are usually present as bundle sheaths (or caps) around the bundles of the stele. These bundle sheaths can be more or less strong, and open to the sides or to the centre. The sclerenchyma can also be scattered in the mark, or very abundant filling most of the space between the bundles. The shape of the endodermis cells partly depends on the developmental stage of the plant. The main forms are the U-endodermis and the O-endoderrnis with varying degree of cell wall thickening. There are also mixed forms (O-U-endodermis), and forms without thickening of cell walls. The interlacunar bundles can be present or absent. If they are present they can be more or less developed in terms of cell number and internal differentiation. Also the number is variable and one, two or more circles of interlacunar bundles can occur.
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The subepidermal bundles can be either present or absent. They are usually small and may be reduced to one bast fiber only. Subepidermal bundles are only present if also a pseudohypodermis is present. If this is present, it may be 1- or many-layered. Variability In Fig. 2 the observed variability within P. nodosus (incl. P. indicus ROXBURGH) is documented. The most frequent stelar type found is the eight bundles type with two phloema, which has also been displayed by OGDEN (1943) based on a North American specimen. However, also proto type steles can be found with 4 or 3 median and 3 to 6 lateral bundles. In some cases, at the base of vertical shoots also an anomalous "8 bundles type" is found with only 2 lateral bundles. In this case the stele is lobed and thus does not belong to the oblong type. STELE:
0 0'0' 0
Fig. 2. Variability of stern anatomical characters in Potalllogeton nodosus POIRET. Endodermis: O-endodermis, rarely 0 (U)-endodermis; sclerenchyma: Invariant (similar Fig. \); interlacunar bundles: None, rarely a few small ones; subepidermal bundles: None, rarely a few; pseudohypodermis: Absent, rarely I-layered, incomplete b
The endodermis usually consists of a-cells. But also an a (U)-type is found in which the shape of the cell walls differs within one cross-section. The sclerenchyma is quite invariant, especially the pattern around the two median bundles with the trio bundle having mostly lateral sclerenchyma and the other one having a closed cap in the central direction. In the cortex there is only few variation. Rarely very faint interlacunar and subepidermal bundles are present. A pseudohypodermis may also be present but only in 1 cell layer and often not complete. In total one can say that the anatomy of the vegetative shoot modules is similar within one shoot complex. The number of vascular bundles in the stele may be reduced in below-ground parts, and slightly increased in axillary leafy shoots. In species with interlacunar bundles the number of these bundles is reduced in the horizontal shoots, but increased in the basal part of the vertical shoot. These findings are in accordance with the circumstances reported by HAGSTROEM (1916) and TOMLINSON (1982). This is of special importance when studying herbarium specimens. In case of fragmentary material it is not well recognizable which kind of shoot actually is studied. Often only one shoot is present. And most of all stems cannot be dissected in an optimal position. Grouping of species In Table I an identification key of the broad-leaved Potamogeton species studied is presented. The key represents a broadened version ofthe key of OGDEN (1943) including also European species and species from the southern hemisphere. The main characters used for primary differentiation are the stelar type and the presence and absence of interlacunar and subepidermal bundles. For a finer differentiation also sclerenchyma and endodermis are used. The characters used by MARKGRAF (1981) for the differentiation of the European species are also included. In some cases it is possible to key out a single species. In others there remain species groups that cannot be furthermore divided by anatomical characters alone. In many cases one must also be aware of aberrant forms of other species and spe'cies groups, making the judgement more difficult. In Table 2, variability within groups and among groups is presented. Each distinct type is illustrated by a photograph. The P. polygonifolius group comprises the three species P. polygonifolius, P. coloratus (see WIEGLEB 1989) and P. suboblongus (Fig. 3) which are very similar among each other but can
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Stem Anatomical Characters of the Genus Potamogeton
Table I. Key to the broad-leaved species of Potamogeton based on stem anatomical characters
1. Bundles of the stele usually free with 2 to 4 median bundles and 3 to 5 lateral bundles on each side. Bundles of the stele usually united to 3 t04 groups . . . . . . . . . . . . . . . . . .
2. (1) Proto-type stele (usually more than 8 free bundles with more than 2 median bundles - Trio-type stele (usually 2 median bundles and 3 (to 5) lateral bundles) . . . . . . . . .
3. (2) Interlacunar bundles usually absent, o-(u)endodermis, pseudohypodermis usually 1 cell layer - Interlacunar bundles usually present, pseudohypodermis 1 to 3 layers . . . . . . . . . . . . . .
4. (3) Subepidermal bundles usually present, stele distinctly lobed (P. polygonifolius group): P. polygonifolius, P. coloratus, P. suboblongus (subepidermal bundles partly lacking). See also: P. x spathulatus and other P. polygonifolius hybrids; species of the P. nodosus group. - Subepidermal bundles absent: P. pulcher. See also: P. nodosus. -
5. (3) Subepidermal bundles present . . . . . . . . . . . . . . . . Subepidermal bundles absent: P. amplifolius. See also P.ferrugineus.
6. (5) O-endodermis, scierenchyma in caps around stelar bundles: P. fryeri. P. linguatus. P. ferrugineus. See also:
P. tricarinatus. P. malaianus. P. natans. P. oakesianus. - U-endodermis, sclerenchyma scattered: P. praelongus. 7. (2) Interlacunar bundles always absent, subepidermal bundles and pseudo-hypodermis usually absent. - Interlacunar bundles or subepidermal bundles present, pseudo-hypodermis usually present . . . . . . .
8. (7) Stelar scierenchyma in caps and sheaths around bundles, o-endodermis, stele variable, all intermediate stages between trio and prototype: P. alpinus. and P. nodosus group (P. nodosus complex incl. P. indicus and P. distinctus). - Stelar scierenchyma few, trio bundle usually not marginal, but central with two phloema (P. peifoliatus group): P. richardsonii. P. peifoliatus.
9. (7) U-endodermis (P. natans group): P. natans. P.oakesianus. P. thunbergii. See also P. x sterWs. P. x sparganifolius and P. x schreberi. See also P. parmatus with only one ring of interlacunar bundles. See also P. illinoiensis. - O-endodermis. . . . 10 10. (9) Pseudohypodermis absent (P. epihydrus group): P. epihydrus. P. montevideensis. -
Pseudohypodermis present: . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. (10) Scierenchyma very strong, rings around both median bundles open at the sides (P. tricarinatus group):
P. tricarinatus complex. Very variable. partly proto-type stele, interlacunar or subepidermal bundles lacking. - Scierenchyma cap on the trio bundle open to the centre: P. malaianus incl. P. sumatranus. See also P. lucens. P. gramineus or P. illinoiensis. 12. (1) Stele more or less quadrangular, two median bundles, and two lateral bundles separated by strong scierenchyma strands, o-endodermis. interlacunar bundles usually absent, subepidermal bundles present (P. cheesemanU group): P. cheesemanii complex. Dubious species belonging to this group: P. sclerocarpus, P. drummondU. - Stele oblong, one or two median bundles and two to fourlateral bundles (oblong type) . . . . . . . . . . . 13
13. (12) Stele usually with 2 median and resp. 2 lateral bundles (2,2,2): P. illinoiensis. Very variable, also with proto- and triotype stele, o-endodermis or lacking subepidermal bundles. See also P. lucens and P. gramineus. - Stele usually with only one median bundle (1,1,1 or 2,1,2), interlacunar bundles present, well developed, subepidermal bundles present or absent, u-endodermis, rarely 0 (P. lucens group s. str.): P. lucens (with at least 2 rings of interlacunar bundles) incl. P. dentatus, P. gramineus inc I. P. sarmaticus, P. schweinfurthii. Variable species with occasional proto- and triotype steles. See also P. x zizii, P. x nitens and other P. lucens and P. gramineus hybrids.
rarely be confused with other species except P. distinctus. P. alpinus, which was included here because of overall morphological similarity and intergradation (WIEGLEB 1988), does not fit completely. It shares the thin endodermal cell walls with the group, but the shape of the stele and the anatomy of the cortex is more similar to the P. nodosus group. Thus it may be justified to resurrect an own group (or subsection) for this species. 14 Flora. Bd. 184. 3
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Table 2. Homogeneity and overlap of species groups in Potamogeton (as defined in WIEGLEB 1988) with respect to stem anatomical characters (CC = central cylinder, IB = interlacunar bundles, SEB = subepidermal bundles) Species group (species studied)
Homogeneity within group
Overlap with other groups
P. polygonifolius group (P. polygonifolius, P. coloratus, P. suboblongus, P. alpinus)
high, except lack of SEB in P. alpinus
low, with P. nodosus group in case of lacking SEB
P. nodosus group (P. nodosus P. distinctus)
low, with P. polygonifolius group in case of SEB present
low, see also OGDEN (1943)
high, both with P. natans and P. lucens group
P. epihydrus group (P. montevideensis)
P. natans group (P. natans, P. oakesianus, P. thunbergii, P. parmatus)
high, minor differences in shape and frequency of IB and SEB
low, with group
P. tricarinatus group (P. tricarinatus s.l.)
low, especially as to IB and SEB
none, strong laterally open bundle cap around bundles in CC
P. cheesernanii group (P. cheesernanii s.I.)
low, especially as to SEB
in principle none (4-bundles type)
P. lucens group (P. illinoiensis, P. gramineus, P.lucens s.l., P. schweinfurthii, P. wrightii)
low, but similar variability patterns among species except P. wrightii
low, with P. amplifolius group, for P. wrightii with P. nodosus
P. perfoliatus group P. praelongus)
low, clear distinction on species level
low, sclerenchyma scattered or not developed, endodermis walls not thickened
P. amplifolius group (P·ferrugineus, P.fryeri, P.linguat us, P. amplifolius, P. pulcher)
In the P. nodosus group there is no great variability. In P. nodosus (Fig. 4) interlacunar bundles rarely occur, in P. distinctus subepid ennal bundles may be found. As to other characters P. nodosus (incl. P. indicus) and P. distinctus show a similar pattern of variation . One may doubt the independent status of P. distinctus also because of other reasons (WIEGLE B 1989a). The P. amplifolius group is obviously artificial. It shows the lowest degree of homogeneity and the highest degree of overlap. This is true both from the morphological and the anatomical point of view. P.fryeri (Fig. 5), P.lingua tus, and P.ferrug ineus often cannot be distinguished from species of the P. natans group. At least the two first named species should be transferred there. The group traditionally also includes P. pulcher which is anatomically more similar to P. nodosus. P. montevideensis (Fig. 6) was treated by HAGSTROEM (1916) within the Amplifolii but may be more closely related to P. epihydrus which has not been studied so far. The exact degree of variability within this group is unknown because of taxonomical problem s in South American taxa. The P. natans group, even after partial inclusion of the amplifolius group, is quite homogenous despite strong variation within species. P.oakes ianus is shown as an example in Fig. 7. P. parmatus was included here because of its obvious similarity to P. thunberg ii. The P. tricarinatus group consists of the collective species P. tricarina tus (incl. P. sulcatus A. BENNET T and P. tepperi A. BENNET T). The construction of the stele (Fig. 8) is highly characteristic because of the strong laterally open bundle sheaths. The plant is very fibrous and the tissue is sometimes destroyed when being cut. There is considerable variation in other characters of these plants that seem to be restricted to the Australian mainland and Tasman ia. It can be assumed that P. tricarinatus has to be split into several species.
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Stem Anatomical Characters of the Genus Potamogeton
Fig. 3. Cross section of the stem of P. suboblongus, Kirk 111903, B (P. coloratus group).
I Fig. 4. Cross section of the stem of P. nodosus, Hooker and Thompson s. n., L (P. nodosus group). 14*
Fig. 5. Cross section of the stem of P. fryeri, Kadono and Wiegleb 903, Oldenburg (P. amplifolius group).
Fig. 6. Cross section of the stem of P. montevideensis, Arechavaleta 2509, BREM (P. epihydrus group).
Stem Anatomical Characters of the Genus Potamogeton
Fig. 7. Cross section of the stem of P. oakesianus. Worbes 8186, Oldenburg (P. natans group).
Fig. 8. Cross section of the stem of P. tricarinatus, WaIter 2464, B, as P. tepperi (P. tricarinatus group).
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Fig. 9. Cross section of the stem of P. cheesemanii. Leland 137, F (P. cheesemanii group).
Fig. 10. Cross section of the stem of P. lucens. Wiegleb 1-0122. OLD (P. lucens group).
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Stem Anatomical Characters of the Genus Potamogeton
P. cheesemanii is a morphologically very variable species. It can easily be recognized by the 4 bundles type stele (Fig. 9) which can be developed in a way resembling the proto type (see also HAGSTROEM 1916). New Zealandish P. cheesemanii is always characterized by the presence of strong subepidermal bundles. The 4 bundles type also occurs in some Australian plants. These plants may represent undescribed Australian species of the P. cheesemanii complex. The species of the P. lucens group which are characterized by an oblong stele (P. lucens incl. P. dentatus HAGSTROEM, P. gramineus, P. illinoiensis, P. schweinfurthii) cannot be confused with others (Fig. 10). All species sometimes show also stelar development in direction to proto type steles. P. schweinfurthii has the weakest development of interlacunar and subepidermal bundles. P. wrightii is isolated in that group (WIEGLEB 1988). It has quite regularly an eight bundles type stele and always lacks subepidermal bundles. The overall shape of the cross-section is very similar to P. nodosus (particularly because of the similar pattern of stelar sclerenchyma), but the interlacunar bundles are well developed and the endodermis is mainly of the U-type. The P. perfoliatus group is strongly differentiated on species level, because both species differ by several characters. P. praelongus has a very complex anatomical construction similar to P. natans and P. lucens. P. perfoliatus is often characterized by the central position of one of the median bundles. However, both cannot be confused with other species because of the usually scattered sclerenchyma in combination with the almost lacking endodermal cell wall thickenings.
Conclusions These findings lead to the conclusion that a certain grouping is possible by means of stem anatomical characters. These characters must not be used in isolation but only combined with others. Sometimes there is a differentiation on the group (or subsection) level as in case of P. polygonifolius, P. nodosus, P. tricarinatus and P. cheesemanii group. Sometimes there is distinction on species level as among P. wrightii and the rest of the P. lucens group or between P. perfoliatus and P. praelongus. In some cases (P. amplifolius and P. natans group) a final judgement cannot be given because of lacking empirical data on the members of these groups. Recognition of new species and hybrids cannot be achieved by anatomical studies alone. The recognition of new species on the basis of anatomical aberrance would lead to a monothetic classification. In case of several specimens from New Guinea (LEACH & OSBORNE 1985) the suspicion as to the non-identity with P. malaianus (=P. wrightii) based on morphological characters was confirmed by anatomical studies (WIEGLEB 1990 c). The recognition of hybrids seems only possible in case of hybridization between species of two different anatomical groups, which, however, is a rare case. Such a case was described by KADONO & WIEGLEB (1987). If applied with care, however, the study of stem anatomy can be a useful tool for advancing hypotheses on the identity or non-identity of sterile or fragmentary herbarium specimens. It was recently used for the examination of type specimens of P. malaianus MIQUEL and P. sumatran us MIQUEL which led to the proposal of considerable nomenclatural changes (WIEGLEB 1990a, b).
Acknowledgements I thank G. LAMPEN (Oldenburg) for the technical assistance. I thank the curators of the above named herbaria for the loan of material and the patience in case of delayed return of specimens. Prof. Dr. G. W AGENITZ (Gottingen) critically read the manuscript. The study was partly funded by the DFG (Az Wi 647-312).
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