Effects of acute ionizing radiation on the xylem fibers of Quercus alba and Liquidambar styraciflua

Radiation Botao,, 1969, Vol. 9, pp. 331 to 339. Pergamon Press. Printed in Great Britain.

EFFECTS OF ACUTE I O N I Z I N G R A D I A T I O N ON T H E XYLEM FIBERS OF QUERCUS ALBA AND LIQUIDAMBAR STYRACIFLUA* J. R. H A M I L T O N a n d A. H. C H E S S E R

Division of Forestry, West Virginia University, Morgantown, W.Va. and U.S. Tariff Commission, Washington, D.C., respectively.

(Received in revisedform 6 August 1968) quantitative histological examination was conducted of selected cambial derivatives in the stems of two angiospermous species (Liquidarabarstyraciflua and Quercus alba) which grew in an area surrounding an air-shielded reactor. During a two-year period absorbed doses ranged between background and 35,000 rads for Q. alba and were background and 14,900 rads for L. styraciflua. Radiation greater than 3500 fads had marked effect on the morphology of cell increments during the time radiation occurred. The libriform fibers produced during irradiation in Q. alba were shorter and possessed thinner wails than the controls. Similar, but less distinct, effects were observed on fiber tracheids in L. styraciflua following dosages of 14,900 rads. Observations made in one year subsequent to irradiation indicated that the production of fibers of normal size occurred in trees which returned to an apparently normal gross morphological condition. Abstract--A

R ~ s m m ~ - - O n a r6alis~ un examen quantitatifdes ddriv~s du cambium des tiges de deux esp6ces d'Angiosperme (Liquidambarstyracilua et Quercusalba) qui croissent aux alentours d'un r6acteur prot6gd par air. Pendant une p6riode de deux ans les doses absorb~es se situent entre le fond permanent et 35.000 rads pour Q. alba et entre le fond permanent et 14.900 rads poir L. styradflua. Toute dose de rayons sup~rieure ~ 3500 rads exerce un effet marqu~ sur la morphologie cellulaire pendant la p6riode d'irradiation. Les fibres du liber produites pendant 1'irradiation de Q. alba sont plus courtes et poss~dent des parois plus minces que les t6moins. Des effets similaires mais moins distincts ont 6t~ observ6s pour les fibres des tracMides chez L. styradflula apr~s des doses de 14.900 rads. Des observations r~alis~es un an apr~s irradiation indiquent que des fibres de tailles normales sont produites d a m les arbres qui apparemment sont globalement revenus ~ leur condition morphologique normale. Z u s a m u m e n f a s s u n g - - E i n e quantitative histologische Untersuchung wurde durchgefiihrt an ausgew~ihlten Derivaten von Kambien der St~mme zweier Angiospermenspecies (Liquidambar styraciflua und Quercus alba), die in einem Areal wuehsen, das einen yon Luft abgeschirmten Reaktor umgab. Die in einer Periode yon zwei Jahren absorbierten Dosen lagen fiir Q. alba zwischen dem Background und 35.000 rad und waren fiir L. styraciflua Background und 14.900 rad. *Research conducted under the auspices of the U.S. Atomic Energy Commission in co-operation with the University of Georgia (Contract No. AT- (40-1)-2905 and West Virginia University (Contract No. AT- (40-1)3400. Approved for publication by the Director of the West Virginia Agricultural Experiment Station as Scientific Paper No. 979. 331

332

j. R. HAMILTON and A. H. CHESSER Strahlung tiber 3500 rad hatte eine merkliche Wirkung auf die Morphologie des Zellzuwachses w/ihrend der Zeit der Bestrahlung. Die w~ihrend der Bestrahlung gebildeten Holzfasern waren bei Q. alba ktirzer und hatten diinnere W~inde als die Kontrollen. Ahnlich, aber weniger ausgepriigt, war die Wirkung von 14.900 rad auf Fasertracheiden yon L. styraciflua. Ein Jahr nach der Bestrahlung erhaltene Befunde ergaben, dass in den B/lumen, die oftensichtlich wieder zu einem insgesamt normalen morphologischen Zustand gelangt waren, Fasern normaler Gr6sse gebildet wurden. INTRODUCTION

THE effect of ionizing radiation on differentiating vascular tissues has been of interest for some time. Early studies of the hypocotyl region of X-rayed herbaceous seedlings noted an increase in xylem, a corresponding reduction in pith cells and a decrease in cell size in Helianthus;~ e) delayed formation of the xylem ring and more compactly arranged tissues with smaller vessels in Zinnia and Euphorbia ;(11) and a reduced xylem ring accompanied by greater variability in fiber cell diameter and wall thickness in Linam.~) More recently some interest has been expressed in the effects of ionizing radiation in arborescent species. The organization of developing apical meristems in Quercus, Pinus and Taxus; ~s.9) the distribution of annual increment in P. rigida;02) and width of annual increment in Quercus(7) spp. have been reported to be influenced by ionizing radiation. In addition anomalous development of vascular tissues, either in the stems or leaves, has been observed in P. rigida, P. echinata, P. monophylla and Sequoia gigantia as a result of chronic or acute radiation exposures. (x,5,1o)

Results of quantitative histological studies of the characteristics of secondary xylem fibers in Q. alba L. and Liquidambar styraciflua L. which were produced during exposure to acute irradiation are given in this report.

MATERIALS A N D M E T H O D S

The selection of study trees was restl'icted by availability of suitable material but was made so as to provide a sample of the greatest possible range of irradiation doses in an oak-hickorypine forest surrounding an air-shielded reactor at the Georgia Nuclear Laboratories, Dawsonville, Georgia. Twenty-four Q. alba trees from five locations and six L. styraciflua trees from one location were selected. Five Q. alba and four L. styraciflua in a nearby unirradiated forest served as controls. Nearly all of the selected trees were located on plots established by McGinnis for observations of forest litter production.(7) A wide distribution of ages was represented, and as a result, the trees ranged between 14 in. and 20 in. dia. at 4-5 ft above ground level. Dosimetry data indicated(4) that total absorbed doses of a mixed g a m m a and

Table 1. Absorbed doses of n,ixed gamma and slow-neutron radiationfor Q. alba L. and L. styraciflua L. Error in dosage measurement --k 12 per cent Treatment No.

No. of trees

1 2 3 4 5 Control

5 4 5 5 5 5

6 Control

6 4

Absorbed dose June 1959, August 1960, rads rads

Total dosage, rads

Q. alba 15,000 4000 1550 1200 700

20,000 5800 2000 1600 800 background

35,000 9800 3550 2800 1500

7000 background

14,900

L. s~rac~ua 7900

32

31

30

nltiation of r e a c t o r ]ctivity June 1959

59

FIG. 1. X y l e m of Q. alba in cross section. The anatomical changes associated with acute irradiation may be noted midway of the latewood of the 1959 increment, as a reduction in libriform fiber wall thickness and an increase in latewood vessel size. The reduction in increment width in 1961 and 1962 probably reflects the declining physiological condition of the tree ( × 75). R.B.f.p. 332

EFFECTS OF IONIZING RADIATION ON XYLEM FIBERS slow-neutron flux ranged from background levels to 35,000 rads ( ± 12 per cent) which occurred principally in two bursts--in June, 1959 and August, 1960 (Table 1). Selected trees were sampled in August, 1962, by extracting an increment core from the bole at 4.5 ft above ground level. Interest centered on six specific annual increments in each t r e e ~ three preceding irradiation (1956, 1957, 1958), two during irradiation (1959, 1960) and one subsequent to irradiation (1961). Accurate cross dating, which was essential for this study, was accomplished by using patterns of successively wide and narrow increments prior to irradiation as a key. Radiation effects were quantitatively assessed by measuring the total length, double wall thickness and tangential lumen diameter of libriform fibers in Q. alba and of fiber tracheids in L. styraciflua. I n the remainder of this paper the libriform fibers and fiber tracheids will be referred to generally as fibers. The three dimensional characteristics will be referred to as length, wall thickness and lumen diameter. Thin transverse sections, cut from intact core samples and stained with safranin and fast green, were used for measurement of transverse fiber dimensions. Wood macerations, prepared from excised portions of annual increments, by treatment with equal parts peroxide and acetic acid were used in fiber length determinations. Fifty measurements of the length, wall thickness, and lumen diameter of fibers were made in each year using a projection microscope and calibrated rule. In order to analyze the effect of irradiation on the fibers of Q. alba nine separate analyses of variance---one for each combination of fiber length, wall thickness and lumen diameter and two irradiated years (1959, 1960) and one year following irradiation (1961)--were made. For each analysis a mean of the fifty measurements for each year in each tree was obtained and subsequently treated as a single observation (Table 2). Prior to computing the analyses of variance the data for each tree were adjusted, in an attempt to remove inherent tree differences, by subtracting the means of the fiber characteristics for the three years preceding irradiation from

333

the observations from corresponding trees (Table 2). As a result of this procedure, the analysis of the variance of mean differences was used to determine whether dosage level was a significant factor. In instances where the factor was significant, Duncan's New Multiple Range TestC3) was used to determine those which differed significantly. Three separate analyses of v a r i a n c e - - o n e for each of the fiber properties--were used to determine the significant effects of irradiation in L. styraciflua. As was the case in Q. alba, the fifty individual measurements for each year in each tree were averaged in order to arrive at a mean value which was subsequently considered as a single observation. Adjustment for the innate capabilities of each tree was made by computing a mean for the four control trees for each year and subtracting this from the observations for each irradiated tree for the corresponding year. As a result, the analysis of variance was computed on the basis of the differences between observations in irradiated trees and the mean of control observations for each tree and year. In the analysis the principal sources of variation examined were trees and years. I n instances where years was a significant factor, DUNCAn'S New Multiple Range Test (3) was used to determine those which differed significantly.

RESULTS

Effect on Q . alba At a site in the 1959 increment, judged to approximately coincide with the beginning of high level reactor activity, distinct changes occurred in the xylem of the Q. alba trees receiving the highest absorbed dose. The intraincrement location of the beginning of these changes varied from tree to tree. After initiation the prominent areas of thick wailed supportative ceils characteristic of the latewood of Q. alba were no longer apparent, presumably because of a reduction in fiber wall thickness, the ceils in the uniseriate rays widened tangentially and there was an increase in the diameter of latewood vessels. In subsequent years no typical latewood was observed (Fig. I). Radiation effects were less pronounced in most irradiated trees--even those receiving the

334

J.R.

H A M I L T O N and A. H. CHESSER

Table 2. Means of three fiber characteristics of O . alba for three years prior to irradiation (1956-1958), two years during irradiation (1959, 1960) and oneyear subsequent to irradiation ( 1961 )

Treatment No. and total absorbed dose, rads

1

Means for calendar year 1956-1958

1959

1960

1961

1.50

Fiber length, mm 1.44(- 0.06)

0 . 8 3 ( - 0-67)

0.78(-0.72)

1.33

1.41 (+0-08)

1.22(- 0.11)*

1.19(-0.14)

1.39

1.41(+ 0.02)

1.38(- 0.01)

1.43(+ 0-04)

1.25

1.23(- 0.02)

1.26(+ 0"01)

1.33(+ 0.01)

1.51

1.52(+0-01)

1.52(+0-01)

1.60(+ 0.01)

1.55

1.56(+ 0-01)

1.57(+ 0-02)

1.54(-o.ol)

(35,000) 2

(9800) 3

(3550) 4

(2800) 5 (1500) Control

1

11-0

Wall thickness (Ix.) 11.2 (+0.2)

9.0 ( - 2 . 0 )

10.4 ( 0 - 0 . 6 )

8.6 ( - 2 . 1 )

8.7 ( - 2 . 0 )

(35,000) 2

10.7

10.8 (+0.1)

(9800) 3

9.8

9.8

(0.0)

10.0 (+0.2)

9.6 ( - 0 . 2 )

9.9

9.8 ( - 0 . 1 )

I0.1 (+0.2)

1o.3 (+0.4)

1o.9

ll.O (+o.1)

lo.8 (-o.1)

1o.9

(o.o)

Control

10.8

10.9 (+0.1)

10.8

lO.8

(o.o)

l

6.8

Lumen diameter (ix) 6.9 ( + O ' l )

7"8 (+l.O)

7.1 (+0.3)

6.7

6.8 (+0.1)

7.3 (+0.6)

6.5 (-0-2)

6.5

6.9 (+0.4)

6.7 (+0.2)

6.8 (+ 0.3)

4.

6.9

7.3 (+0.4)

7.4 (+0.5)

7.4 ( + 0.5)

5 (1500) Control

6.6

6.6

(0.0)

6.7 (+0.1)

6.7 (+0.i)

7.2

7.3 (+0.1)

7.1 ( - 0 . 1 )

7.1 (-0.1)

(3550) 4

(2800) 5 (15oo)

(0.0)

(35,000) 2

(9800) 3

(3550)

(2800)

The differences between the means for the three years during and subsequent to irradiation and the means for the three years prior to irradiation are given in parentheses. *Any two means not connected by the same line differ at the 5 per cent level of probability. This test is only applicable to significant sources from Table 3.

335

EFFECTS OF IONIZING RADIATION ON XYLEM FIBERS highest dose--but measurable changes in the morphology of fibers did occur (Tables 2 and 3). Reductions in the length of fibers were noted in the trees in Treatments 1 (35,000 rads) and 2 (9800 rads) but the only significant changes were associated with the highest dosage. At this level the fibers in 1960 were less by 45 per

activity but were delayed until the following year.

Effect on L. styraciltua The

obvious tissue abnormalities

present

in Q. alba (Fig. 1) were not noted in L. s~raciflua, probably

because of lower absorbed

doses

Table 3. Analysis of variance of mean differencesfor threefiber characteristics of Q. alba L

Length

Source of variation

d.f.

Mean squares

Treatment Residual Total

5 23 28

0.0097 0.0038 0.0049

Treatment Residual Total

5 21 26

0.2941 0.0124 0-0666

23.72**]'

Treatment Residual Total

5 19 24

0'2801 0.0194 0.0737

14.44"*

F

2.55

Fiber dimension Wall thickness Mean d.f. squares F

d.f.

Lumen dia. Mean squares F

1959 5 23 28

0-0500 0.0905 0-0833

0.55

5 23 28

0.1282 0.0727 0-0826

1.76

1960 5 20 25

4.1821 0.8446 1.5121

4.95**

5 20 25

0-5734 0.2284 0.2974

2.51

1961 5 22 27

2.5765 0"9765 1.2728

5 22 27

0.2653 0.2345 0-2402

1"13

]'Significant at the 1 per cent level of probability. cent Dom preirradiation lengths. An even greater reduction in length was recorded in 1961 (Fig. 2). Wall thickness was also noted to be reduced to a significant degree by irradiation at the two highest levels but only during the 1960 increment (Tables 2 and 3). In this year as much as 19 per cent reduction from preirradiation widths was recorded (Fig. 2). It should be noted that the effects of Treatment 1 and 2 did not differ and that there was no effect in 1961 which was clearly radiation induced. Fiber lumen diameter did not exhibit any differences which could be attributed to irradiation. No significant changes in any of the fiber characteristics were measured during the year coinciding with the first period of reactor

(Table 1), but statistically significant changes in each of the cell characteristics were attributable to years in the analysis of variance (Table 5). During 1959 and 1960 the average length of L. slyraciflua fibers in exposed trees was significantly less than that in the control years as indicated by the range test of mean differences (Table 4) and there was no significant differences in length between increments of the two years. In 1961, fibers approached normal length (Fig. 3) but were still significantly shorter than in control years. In the three years prior to 1959, the average difference in fiber length between exposed and control trees was 0'03 mm. In 1959 and 1960 this difference increased by a factor of approximately four (Table 4). In 1960, the second year of irradiation, the difference in fiber wall thickness between

336

J . R . HAMILTON and A. H. CHESSER

1 900. 1700" I 500-

I OE z E ~

1.3001100090007000500IRRAD IATED CONTROL . . . . . .

12,00" 11.00'~

~

~o.oo.

_a~ a

800700-

:- . . . . . . .

_

*

-

~

6.001200-

"J

1100-

~W Z~ drn-(J~ ::3:1: O,F-

10 00" 8C~0_ 7 006 00-

1956"1958 (AVEI

1g~59

19'60

19'61

ANNUAL INCREMENT

FIG. 2. The means of three characteristics oflibriform fibers in Q. alba L. which absorbed 15,000 rads in 1959 and 20,000 rads in 1960 compared with control means.

exposed and control trees was 1.I t~ whereas this difference in the period 1956-1958 was only 0.4 9 (Fig. 3). The very small differences noted during 1959 and 1961 (Table 4) were not unlike the differences in the years preceding irradiation. Analysis of variance indicated that there was a significant effect of years on fiber lumen diameter (Table 5) but the relationship was not as distinct as was the case with other properties. From an examination of the differences in Table 4 and reference to Fig. 3 it is apparent that during the two years in which irradiation occurred this dimensional characteristic was more irregular than in previous years. For example: in 1959 the lumen diameter was smaller in irradiated trees than in control trees but in I960 the opposite was observed.

DISCUSSION

Even though considerable variation in response was observed in trees of the same species which absorbed similar doses, the results of this study indicate an association between acute irradiation and abnormalities in the supportative cells of the two species studied. The difference in response between trees of the same species in the same treatments is typical of the natural variation commonly encountered when xylem characteristics are examined. Response differences between individual trees were most pronounced in Q. alba, especially in T r e a t m e n t 1. No change was detected in the 1959 increment in some individuals receiving this highest dosage, yet in others distinct abnormalities were observed in the latter portion of the increment. This

EFFECTS OF IONIZING RADIATION ON XYLEM FIBERS

337

Table 4. Means of three fiber characteristics of L. styraciflua for three years prior to irradiation ( 1956-1958), twoyears during irradiation ( 1959, 1960) and oneyear subsequent to irradiation (196 i ) T r e a t m e n t No. and total absorbed dose, rads

Means for calendar year 1956-1958

1959

1960

1961

1-73

1.82

Fiber length, m m 6

1-76

1-77

(14,900) Control Difference

1.79

1.88

1.85

1.89

0.03

0.11

0.12

0.07*

Wall thickness (Ez) 6 (14,900) Control Difference

10.3

10-5

9.6

10.4

10.7

10.8

10-7

10.9

0-4

0.3

1. l

0.5

11.5

11.8

13.2

12.7

11"7

12-8

12.5

13.4

0"2

1"0

- 0.7

0.7

L u m e n dia. (~.) 6 (14,900) Control Difference

*Any two means differences not connected by the same line differ at the 5 per cent level of probability.

Table 5. Analysis of variance of mean differencesfor threefiber characteristics o f L . styraeiflua L Fiber dimension Source of variation

Length d.f.

Wall thickness

L u m e n dia.

M e a n squares

F

M e a n squares

F

M e a n squares

F

Trees

5

0.1505

93"02"'1-

10.0678

46.18"*

15.8097

30-91"*

Years

5

0.0129

7.98**

1-0137

4.65**

2.1984

4.30**

Trees × years

25

0-0016

0.2180

0.5114

Total

35

0.2450

1.7388

2-9379

1"Significant at the 1 per cent level of probability.

338

J . R . HAMILTON and A. H. CHESSER

..4

1.90

I"

186 I

~'--~.~.. ~ ~ . ~

1 84'

zE

W~ .J

180 1.76' 1.72" IRRADIATED - CONTROL . . . . . . . . . 135130.

ZW W ~-

12.5-

52g

120-

c~

.

/ /

11 .5" 11.0 -

z ~

10.61

~)~ IO21

°

9.4

t

i

1956-1958 (AV)

i

i

1959 ANNUAL

1960 INCREMENT

i

1961

FIG. 3. The means of three chaa'acteristics of fiber tracheids in L. styradflua L. which absorbed 7000 rads in 1959 and 7000 in 1960 compared with control means. apparent anomaly possibly reflects the timing of irradiation with respect to increment development. It is likely that annual increments had been completed in some Q. alba stems by J u n e at the latitude of Dawsonville, Georgia. There is also the possibility that trees which were under stress as a result of some other environmental factor ceased cell multiplication upon initiation of irradiation. The latter alternative is not supported by a cursory examination of increment widths, for the 1959 increments were not notably reduced in size. The fact that significant quantitative changes were not scored in the 1959 increment reflects the influence of trees which did not contain the abnormal structure depicted in Fig. 1. On the average only slight changes were noted in the 1959 increment (Table 2). In all probability the August 1960 irradiation period occurred after the yearly increment had been completed, or very nearly so in the Q.

alba and toward the end of the yearly increment in L. styraciflua. As a result, the response to this late irradiation would also be most noticeable in the following year. Such was the case for fiber length in both species. This delayed effect cannot, however, be clearly said to be radiation induced because it cannot be distinguished from the effect of reduced availability of photosynthate as a result of defoliation and subsequent limited leaf production which was reported by McGinnis.(~) The abruptness of the onslaught of radiation induced changes, where readily discernible, suggest that the initial cause of these abnormalities is not a reduction in available carbohydrates associated with defoliation but possibly a disruption of the synthesis or transport of some regulator system such as IAA or a combination of both, as has been noted by GORDON.(4) However, changes in cell size in subsequent years in which irradiation occurs, may be influenced both by

EFFECTS OF I O N I Z I N G R A D I A T I O N ON XYLEM FIBERS a v a i l a b i l i t y of p h o t o s y n t h a t e a n d r a d i a t i o n i n d u c c d d a m a g e . I n instances where dosages were at a levcl sufficient to cause a r e d u c t i o n in foliage,~7) Q. alba in T r e a t m e n t 1 (35,000 rads), n o r m a l i n c r e m e n t p r o d u c t i o n was suppressed in subsequent years w h e t h e r or not i r r a d i a t i o n o c c u r r e d : for e x a m p l e , l a t e w o o d tissue d e v e l o p m e n t was m a r k e d l y restricted, the a m o u n t of e a r l y w o o d tissue was progressively r e d u c e d , a n d fibers were shorter t h a n n o r m a l i~l b o t h the 1960 a n d 1961 increments. O n the o t h e r h a n d , a r e t u r n to the p r o d u c t i o n of cells with n o r m a l dimensions in the y e a r following i r r a d i a t i o n was r e c o r d e d in trees noted b y MCCrINNIS(7) to have less severe foliage r e d u c t i o n ( T r e a t m e n t s 3 - 6 a n d controls, r a d i a t i o n < 9000 rads). These results suggest a transitory role of s u b l e t h a l exposures w h i c h cause no c o n t i n u i n g effect. S i m i l a r results have b e e n n o t e d in P. echinataO) in w h i c h tracheids of n o r m a l size were noted in the same i n c r e m e n t following acute i r r a d i a t i o n in J u n e . T h e d a t a suggest a difference in radiosensitivity of the two species. D i r e c t comparisons are not possible because equivalent a b s o r b e d doses are not a v a i l a b l e ; however, inferences m a y be m a d e from a c o m p a r i s o n of 1960 control a n d i r r a d i a t e d increments in Q. alba ( T r e a t m e n t 2, 9800 rads) a n d L. styraciflua ( T r e a t m e n t 6, 14,900 rads). Even though L. styraciflua received 50 p e r cent g r e a t e r i r r a d i a t i o n , l a r g e r differences in m e a n fiber wall thickness a n d l e n g t h were n o t e d in Q. alba. Based on the v a r i a b i l i t y of the L. styraciflua results it w o u l d a p p e a r t h a t exposure levels were n e a r the m i n i m u m r e q u i r e d to cause cellular a b e r r a t i o n s in this species. REFERENCES I. BRANDENBURG M. K., MILLS H. L., RICKARD

W. D. and SHIELDSL. M. (1962) Effects of acute gamma radiation on growth and morphology in Pinus monophylla Torr. and Frern. (Pinyon Pine). Radiation Botany 2, 251-263.

339

2. D'AMATO F. (1957) Fasciazioni caulinari e fiorali, sterilita ed altre rnodificazioni di sviluppo indotte dalla irradiazione cronica gamma da radiocobalto nel lino. Wuovo Giorn. bot. ital., N.S. 44, 1-8. Cited by J. E. GUNKEL (1961) Modifications of plant growth and development induced by ionizing radiations pp. 365-387 hi Encyclopedia of Plant Physiology, Vol. 15(2). Springer, Berlin. 3. DUNCAN D. B. (1951) A significent test for differences between ranked treatments in an analysis of variance. Virginia 07. Sei. 2, 171-189. 4. GORDON S. A. (1957) The effects of ionizing radiation on plants: biochemical and physiological aspects. Quart. Rev. Biol. 32, 3-14. 5. HAMILTONJ.R. (1963) Characteristicsoftracheids produced in a gamma and gamma-neutron environment. Forest Prod. o7. 13, 62-67. 6. JOHNSON E. L. (1926) Effects of X rays upon growth, development, and oxidizing enzymes of Helianthus annunes. Botan. Gaz. 82, 373-402. 7. McGINmS J. T. (1963) Effects of radiation from an air-shielded reactor on forest litter production, pp. 283-287. In V. Schultz and A. W. Klement Jr. (eds.), Radioecology, Reinhold, New York and Am. Inst. Biol. Sci., Washington, D.C. 8. MERICLE L. W., MERICLE R. P. and SPARROW A. H. (1962) Cumulative radiation damage in oak trees. Radiation Botany, 2, 276-271. 9. MmSCHEJ. P., SPARROWA. H. and ROGERSA. P. (1961) Effects of chronic gamma irradiation on the apical meristems of Pinus strobus and Taxus media. Am..7. Botany 48, 529. 10. TAYLOR F. G. Jr. (1968) Some effects of acute gamma radiation in giant sequoia seedlings. Radiation Botany 8, 67-70. 11. WOLCOTr E. (1936) Leaf and stem structure of some X-rayed plants. Univ. Colo. Studies A23, 223-232. 12. WOODWELLG. M. and MXLLERL. (1963) Chronic gamma radiation affects the distribution of radial increment in Pimts rigMa stems. Science 139, 222-223.