Botanical journal of the Linnean Socic& (1987), 94: 385-397. With 19 figures

Anatomy of the vascular cambium of Acacia nilotica (L.) Del. var. telia Troup (Mimosaceae) in relation to age and season

MUHAMMAD IQBAL AND A. K. M. GHOUSE

Department of Botany, Aligarh Muslim University, Aligarh -202001, India

Received June 1985, revised manuscript accepted for publication April 1986

IQBAL, M. & GHOUSE, A. K. M. 1986. Anatomy of the vrrrculu cambium of Acacia nilotica (L.) Del. var. telia Troup (Mimouceae) in relation to age aud season. In the non-stratified cambium of Acacia ndolica var. lelk the majority of fusiform initials are multinucleate, a few having as many as eight nuclei. Their length increases down the stem from the apex, attaining a maximum in the old trunk and declining slightly near the base. The width of the initials exhibits similar variation. In the main trunk, fusiform initials, relatively short at the time of cambial reactivation (April), elongate steadily until July. There is a sharp decline in August/September, the cell length recovering during the winter. Seasonal variation in cell width is inconsistent. Ray cell initials, on the other hand, do not vary much in size. They divide more frequently in the older stem, adding to the size of rays. In young shoots, short and uni- to biseriate rays are most abundant, whereas tall and multiseriate rays dominate the cambial surface in the trunk region throughout the year, with their minimum population in the early phase of cambial activity and the maximum during peak activity. The overall proportion of fusiform initials in the cambial cylinder initially increases with age, from young shoots towards the base, and later becomes more or less constant in the trunk region. Here it remains noticeably high during the active growth period and relatively low for the rest of the year.

ADDITIONAL KEY WORDS:-Cambial structure - ontogeny - seasonal impact.

CONTENTS

Introduction . . . . Material and methods . . Observations . . . .

General description . Ontogenetic variations Seasonal variations .

Discussion. . . . . Acknowledgement. . . References. . . . .

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385 386 388 388 39 I 392 393 396 396

INTRODUCTION

The bark and wood of most trees are important from medicinal and economic points of view and both are derived from the vascular cambium. These derivative tissues are dependent in quality and quantity on the structural and

385 0024-4074/87/030385 + I3 $03.00/0 0 1987 The Linnean Society of London

386 M. IQBAL AND A. K. M. GHOUSE

functional patterns of their mother meristem. The vascular cambium of different taxa varies in form and function and depends for its activities on the generic constitution and physiological phenomena of the plant as well as on environmental factors (Philipson, Ward & Butterfield, 1971). So far, the cambium of temperate trees has been studied more widely than that of tropical species, and only in the recent past has the cambium of tropical trees attracted the attention of certain workers. However, most of the studies have concentrated on comparative cambial morphology, whereas the structural changes which the meristem undergoes with age and with seasonal climatic variation are yet to be fully explored (Iqbal & Ghouse, 1985a).

MATERIAL AND METHODS

Blocks of 3 cm2 tangential surface, containing bark, cambium and sapwood, were collected from the main stem of nearly 50 year old, healthy trees of Acacia nilotica (L.) Del. var. telia Troup, (Mimosaceae). Fortnightly collections were made from the main trunks of three trees about 1.5 mm above the ground, one block from each of the four sides, for three consecutive years to study seasonal structural variations. The study was made at Aligarh (27"53'N, 78"4'E) in the monsoon belt of the Great Gangetic Plain of North India. For ontogenetic observations, samples were obtained from eight different height levels along the main stem (Tables 1 & 2). Further processing of the material, including fixation, sectioning, staining and mounting, was done as described previously (Iqbal & Ghouse, 1985b) using tannic acid, ferric chloride as stain and Canada balsam as mountant.

T o determine the mean cell size, 100 cells per sample were arbitrarily selected and measured and the range as well as means for length and width for each

Table 1. Abundance of ray initials of varying width in relation to the girth of stem axis. Means are based on readings from 720 microscopic fields, with ranges

in paren theses.

Height Maximum from width base (no. of Uniseriate Biseriate Triseriate Tetraseriate Multiseriate (m) cells (%I (YO) (Yo) (YO) ( O/O )

8.8

8.6

8.4

6.4

5.3

4.0

I .8

0.7

46 (43-48) 40

(37-42) 12

( 10- 14) 13

( 10- 14) 7

(4-8) 8

(5-10) 10

(7-12) 8

(5-9)

41

40.5 (37-44)

23 (20-24)

13 (12-15)

8 (5-10)

10 (8-12)

9

(38-48)

(8-1 2) 13

( 12-15)

12 ( 10-1 4)

19 ( 1 7-20)

37 (34-40)

19 ( 16-2 1)

19 ( 1 7-2 I )

10 (8-13)

8

16 (5-9)

(13-18)

1 (0-2)

1.5 (0-2)

26 (21-28)

31 (28-34)

30 (28-31)

31 (20-32)

10 (8-12)

23 (21-26)

0

0

2 (1-4)

24 (20-26)

36 (34-40)

41 (36-43)

63 (60-65) 40

(38-43)

VASCULAR CAMBIUM OF ACACIA NILOTICA 387

Table 2. Abundance of ray initials of varying heights in relation to the girth of stem axis. Means are based on readings from 720 microscopic fields, with ranges

in parentheses

Hright from base ( m )

8.8

8.6

8.4

6.4

5.3

4.0

I .8

0 . 7

Maximum height (no. Short

of cells) ( Y O )

96 79

82 73

74 45

68 42

95 45

80 39

70 34

75 42

(76-82)

(70-77)

(43-48)

(40-46)

(42-48)

(35-4 1)

(30-36)

(38-45)

Medium Moderately i 04,) tall ( Y o )

Tall (04)

12 (9-14)

20 ( I 7-22)

40

35 (30-36)

36 (34-40)

30 (28-35)

26 (24-28)

28

(37-45)

(24-30)

7 (4-9)

6 ( 4-9 )

13 (11-15)

21 (18-24)

15 ( I 3-18)

21 (20-24)

27 (23-29)

18 (15-21)

2 (0-3)

1 (0-2)

2

(7-12) 13

( 12- 15) 12

(10-13)

Table 3. Seasonal variation in the relative abundance of ray initials of varying width. Means are based on readings from 720 microscopic fields, with ranges in

parentheses

Month

January

February

March

April

M a y

J U l l C

J u l y

August

Septrmber

()clobrr

November

Drcrtnbrr

Maximum (no. of cells)

7

7

6

5

6

8

7

7

7

7

6

6

Uniseriate )

Biseriate ( Y o )

8 (4-11)

10 (6-13)

7 (4-12)

15 (10-17)

13 ( 10-1 7)

12 ( 10- 15)

7 (4-10)

I I (8-15)

10 (7-15)

7 (4-11)

8 (4-1 I )

8 (5-1 1)

13 (10-17)

12 (10-16)

10 (5-13)

16 (14-20)

20 (16-23)

21 (20-23)

12 (9-15)

13 (10- 15)

7 (4-12)

5

1 1 (8-14)

12 (8-16)

(3-8)

Triseriate (?o)

13 (10-16)

15 (12-17)

16 (13-20)

18 ( 14-22)

19 (14-21)

21 ( 19-24)

I 1 (9-15)

8 (4-10)

7 (4-10)

7 (5-10)

14 (12-1 7)

12 (9-14)

Tetraseriate ( Y O )

Multiseriate i O/O )

20 ( 17-24)

21 ( 18-24)

24 ( 19-26)

22 (20-25)

21 (19-25)

23 (20-25)

17 (14-20)

14 ( 12-1 7)

13 (10- 15)

16 ( 1 1-20)

25 (20-28)

22 (20-26)

46 (39-49)

42 (38-46)

43 (38-45)

29 (28-33)

27 (24-29)

23 (20-25)

53 (48-58)

54 (5 1-58)

63 (60-68)

65 (6 1-68)

42 (40-46)

46 (42-51)

388 M. IQBAL AND A. K. M. GHOUSE

Table 4. Seasonal variation in the relative abundance of the ray initials of varying height. Means are based on readings from 720 microscopic fields, with

ranges in parentheses

Mximum height (no. Short

Month of cells) (%)

January 70 40 (37-45)

(34-40) February 68 36

March 78 33

April 75 45 (30-35)

May

June

July

August

September

October

November

December

70

70

70

68

60

70

55

65

(40-49) 42

(40-4) 34

(30-37) 28

(25-32) 30

(26-35) 33

(30-35) 44

(40-49) 40

(38-46) 42

Medium (Yo)

26 (21-30)

(25-34) 22

( 19-25) 36

(32-40) 33

(30-40) 32

(30-40) 23

(20-27) 20

(1 7-25) 18

( 15-22) 29

(25-35) 25

(20-31) 28

28

Tall 0 0 )

20 (16-24) 26

(2C30) 29

(25-36) 1 1

(9-15) 19

(16-25) 24

36

36 (30-40) 39

(35-44) 19

(16-22) 22

(20-25) 18

(20-27)

(34-40)

14 ( 10-18) 10

(8-13) 16

(14-18) 8

(6-10) 6

(5-10) 10

(7-15) 13

10-16) 14

10-16) 10

(8- 16) 8

(5-10) 13

11-18) 12

(38-45) (24-35) ( 15-25) (10-14)

month or position in the stem was determined after pooling the readings from all relevant samples collected during the years of study. The independent sets of values for individual trees in different years did not show any significant differences. In tangential longitudinal sections, the heights of the cambium rays were categorized as short (1-15) cells, medium (16-30 cells), moderately tall (31-45 cells) and tall (over 45 cells), and the widths as uniseriate, biseriate, triseriate, tetraseriate and multiseriate (Tables 1-4). Estimation of the relative proportion of component cambial initials was made with the help of camera lucida drawings of the initials following the method of Ghouse & Iqbal (1975).

OBSERVATIONS

General description

The vascular cambium of Acacia nilotica develops first in fascicular and then in interfascicular regions to form a complete ring. The cambium is non-stratified comprising both fusiform and ray initials. The radial walls of fusiform initials are thicker than tangential ones and have a beaded appearance, especially in dormant cambium, due to alternate wall thickening and primary pit fields (Fig. 2). Most fusiform initials possess several nuclei, sometimes as many as eight. The majority of the initials had one to five nuclei per cell, mostly spherical, or fusiform in shape (Figs 7, 9, 12 & 15). Some nuclei showed signs of

VASCULAR CAMBIUM O F ACACIA NILOTICA 389

Figures 1-8. Cambium in tangential longitudinal section showing relatively thin and smooth cell walls (Fig. 1) during active phase and thick beaded walls (Fig. 2) during dormant phase; fusion of two adjacent rays (Figs 2 & 3); segmentation of fusiform initial lying contiguous to rays (Figs 3 & 4); split of ray due to intrusion of adjacent fusiform initial (Figs 5 & 6); and apical elongation of fusiform initials (Figs 7 & 8). Multinucleate condition of fusiform initials is apparent especially in Figs I , 6 & 7. All Figures to scale bar in Fig. 6.

degeneration. Their contents appeared to degenerate before the nuclear membrane. Generally, the spherical nuclei first elongated to become spindle- shaped with filiform ends and later experienced a complete necrosis, usually starting from the elongated tips. The nuclei generally were smaller and darker during the dormant phase than in the active phase of cambium.

Following pseudotransverse anticlinal division of a fusiform initial, the daughter cells, generally unequal in size, undergo apical elongation which determines the length of tapering ends of the initials. Fortnightly observations

390 M. IQBAL AND A. K. M. GHOUSE

Figures 9-16. Cambium in tangential longitudinal section showing development of rays at the tips of fusiform initials (Figs 9-12); initiation and development of rays laterally from fusiform initials (Figs 13-15); and complete conversion of fusiform initial into uniseriate ray (Fig. 16). Multinucleate condition is distinct especially in Figs 12 & 16. All Figures to scale bar in Fig. 15.

throughout the year revealed the mean length of fully grown initials in the trunk region to be approximately 336 pm. Of this, about 54 pm was occupied by each of the tapering ends of the initial. Just after the pseudotransverse division, oblique ends of the daughter initials measured nearly 24 pm. These cells had to elongate approximately 1.6-2.4 times their initial length to acquire the size of the precursor cell. In some cells the tip was forked or bent (Figs 7, 8) showing that apical intrusive growth had occurred. The growing fusiform initials sometimes intruded the ray bodies, often dividing them into smaller entities (Figs 5, 6).

VASCULAR CAMBIUM OF ACACIA NILOTICA 39 1

Fusiform initials and ray cell initials are not necessarily constant. New ray cell initials originate from fusiform initials through transverse segmentation or by cutting off their lateral or terminal segments (Figs 9-16). The mean anticlinal and periclinal diameters of ray cell initials varied from 12 to 15 pm and 14 to 18 pm respectively during a growth year. Sometimes, two or many adjacent groups of ray cell initials, the cambium rays, fused together to form tall and broad composite bodies (Figs 2, 3). The population of ray cell initials increased through their multiplication or by segmentation of neighbouring fusiform initials (Figs 3, 4). Concomitantly, splitting of tall cambium rays into smaller units took place owing to the intrusion of fusiform initials into the ray, or by the elongation of a ray cell to form a fusiform initial.

With the same overall composition of the cambium, the size, abundance, and proportion of component initials varied considerably with respect to age and climatic conditions.

Ontogenetic variations

Having developed from procambial cells, the cambial initials exhibit dimensional and proportional changes as the plant ages. Cambial sections obtained from eight different positions in the stem axis showed that the length of fusiform initials increased down the stem until it attained a maximum in the trunk region. It declined slightly near the base (Fig. 17). The length varied from nearly 187 pm in young shoots to 327 pm in the main trunk (a 75% increase). The width of the initials followed a similar trend but with a much narrower range (14-16 pm). The length of tapering cell ends also increased (43-69 pm) with the age of the stem.

Ray cell initials did not undergo any significant change in their individual dimensions but multiplied vigorously in older parts of the stem, thus increasing the volume of cambium rays. Most of the rays were relatively tall (up to 100 cells) and broad (up to nine cells) in old stems. Uniseriate and biseriate rays were most abundant in young shoots, collectively constituting about 80-85% of the total ray population. In older branches, triseriate and tetraseriate rays dominated and in still older stems multiseriate rays were most abundant, up to 63% in the trunk region (Table 1).

0@ A a

@-0 r@ J I I I I I I 1 8.8 8.6 8.4 6.4 5.3 4.0 1.8 0.;

Stem heighl (m)

Figure 17. Variation in the length of fusiform initials in relation to the height of the plant axis.

392 M. IQBAL AND A. K. M. GHOUSE

Figure 18. Camera lucida drawings showing variation in the tangential area occupied by fusiform and ray initials in young (A) and old (B) stems. F = fusiform initial, R = rays.

Similarly, the relative abundance of short, medium, and tall cambium rays differed at different levels of the stem. Short rays, which dominated all along the axis, were most abundant (up to 79%) in young shoots, followed by the medium rays which were outnumbered by tall rays in the trunk region (Table 2) .

T o cope with the increasing stem girth, fusiform initials divided by pseudo- transverse anticlinal septation. Most daughter initials repeated the phenomenon after having attained their normal size, a few produced ray cell initials or were gradually eliminated. The ray cell initials also divided, few of them transformed into fusiform initials. As a result of these developmental changes, the relative proportion of fusiform initials and ray initials changed, the former constituting about 86% of the total tangential area of the cambial cylinder in the young shoots and 70% in the old trunk (Fig. 18).

Seasonal varialions

Dimensional and proportional changes also occurred in the cambium in relation to climatic changes. Within a year, fusiform initials in the trunk region constituted 69-85y0 of the total tangential area of the cambial cylinder. Their proportion was noticeably higher ( 78-85y0) during the active growth period (June-September) than for the rest of the year (69-70%, Fig. 19B).

During the active phase, periclinal divisions were frequent for most of the year, but anticlinal divisions usually dominated during September and October, the later part of the growth period. Depending on the extent of post-divisional cell elongation, the mean length of the tapering ends of the fusiform initials varied from 45 to 63 pm, and that of the entire cell, including the ends, from 294 to 385 pm during a year. Their mean width varied from nearly 15 to 18 pm. The initials were short in April and gained in length until July. After a sharp decline around August/September, the previous range of magnitude was regained during winter (Fig. 19A). The length of tapering ends alone also exhibited a similar variation in relation to change of season. No regular pattern of variation was identified for the width of the initials.

VASCULAR CAMBIUM OF ACACIA NILOTICA 393

r

L M J

Months

S O N D

Figure 19. Variation in the length of fusiform initials (A) and in relative tangential area of cambium occupied by fusiform initials (8) in different months of a calendar year.

The frequency of ray initialslcambium rays of diverse width and height differed periodically. The multiseriate rays, as observed in the trunk region, outnumbered others all the year round with their minimum (23-29y0) in the early part of the active phase (April-June) and the maximum (53-65%) during the vigorous growth period (July-October). Tetraseriate rays formed about 13-25% of the total number of rays. The uni-, bi- and triseriate rays were only 7- 15%, 5-2 194 and 7-2 1 o/o respectively, with a maximum frequency ( 15-2 1 yo) during April (Table 3).

The short and medium rays varied in abundance from 28 to 45 and 18 to 36% respectively, the variation being rather inconsistent. Tall rays, including both moderately tall and tall types, combinedly constituted about 19-50y0 of the ray population with their minimum in April-May and maximum around July-September (Table 4).

DISCUSSION

In most dicotyledonous species, the cambium has been reported to originate first within the provascular strands and then to spread tangentially to cover interfascicular areas, although a reverse pattern is found in some species such as Ricinus communis L. (Fahn, Ben-Sasson & Sachs, 1972). The condition in Acacia nilotica conforms to that in the majority of dicotyledons.

The mean fusiform initial length in A. nilotica, which ranges from 294 to 385 pm, is much shorter than that obtained by Bailey (1920) for non-storeyed fusiform initials (460-4400 pm) in a large number of dicotyledons. Instead, it is close to Bailey's measurements for storeyed fusiform initials (1 70-410 pm). This shortness of fusiform cells in the non-storeyed cambium of A. nilotica possibly

394 M. IQBAL AND A. K. M. CHOUSE

indicates their tendency towards phylogenetic advancement. Comparable ranges of cell length have been recorded in several other Indian trees (see Bartwal, Siddiqui & Iqbal, 1983; Ghouse & Iqbal, 1975; Ghouse, Yunus & Iqbal, 1976).

Some studies indicate that the length of fusiform initials increases with the age of the meristem, but having reached a certain maximum it usually becomes relatively stable (Bailey, 1923; Ghouse & Yunus, 1973). The situation in A. nilotica more or less conforms to the above trend as the cell length increases from apical shoots down the stem, although it decreases slightly in the samples obtained from the basal part of the trunk. This might be due to the proximity of the root system, as was suggested by Hejnowicz & Hejnowicz (1958) and Purkayastha, Krishnalal, Rameshrao & Negi (1974) with reference to the size variation of wood elements in Populus tremula L. and Michelia champaca L. respectively. Thus, the observations on A. nilotica do not wholly agree with those that describe a constant increase in the cell length through all the stem down to the base (Cumbie, 1969a; Ghouse & Hashmi, 1980), or an early stabilization of the cell length after an initial increase in young stems (Bailey, 1944; Butterfield, 1972; Cumbie, 1963, 1967, 1969b), or a gradual length increase followed by a slow decline near the base (Ghouse & Iqbal, 1977; Khan, Siddiqi & Khan, 1983). Many earlier works suggest that the mean length of fusiform initials in a certain growth ring increases from the tree top downwards, reaching a maximum at about one-third of the stem height from the ground, and then decreases towards the base (see Iqbal & Ghouse, 1985a; Philipson, Ward & Butterfield, 1971). In A. nilotica, the longest initials are relatively close to the base.

The length of fusiform initials is believed to be related partly to the frequency of pseudotransverse division and partly to the loss of the initials, the former producing a reduction in the cell length and the latter enhancing the elongation of adjoining cells (Iqbal & Ghouse, 1985a; Philipson et al., 1971). Hence, the shortness of fusiform initials recorded in the present study in young shoots may be attributed to the probable abundance of pseudotransverse divisions in fast- growing apical shoots, as was suggested by Wilson (1966) in the case of Pinus strobus L.

Measurements of the tapering ends of the fusiform initials suggest that the elongation capacity of the initials is possibly reduced near the base, causing a decline in cell length in a region where anticlinal divisions are generally less frequent and the cambium cylinder does not expand as rapidly as in young shoots. However, with reference to conifers, Bannan (1961, 1964) suggested that a high frequency of pseudotransverse divisions does not depress the cell length as much as might be expected, since the rapid elongation associated with frequent cell division and a continuous elimination of the too-short fusiform initials mitigate the effect of division. To him, the cell size variation was a result of inherent determiners rather than of the rate of anticlinal divisions.

Previous studies have shown that the cambial sturcture does not remain stable all the year round. Paliwal & Prasad (1970, 1971) and Paliwal et al. (1975) suggested some positive correlations between cambial activity and fusiform cell size in Dalbergia sissoo Roxb. and Polyalthia longifolia Benth. & Hook. respectively. The cell size, at its minimum before the start of radial growth, increases with the

VASCULAR CAMBIUM OF ACACIA NIL0 TICA

onset of cambial activity, attains a maximum after the activity has touched its zenith and then declines towards the end of the active period, all with certain occasional fluctuations. However, Yunus (1976) and Hashmi (1977) did not find any consistent correlation between cell size and cambial growth in both the above species. The fusiform initials of Acacia nilotica elongate almost consistently as cambial activity advances. Their mean length declines sharply around August-September, the time of wood formation, after which the mean length becomes restored to that characteristic of winter months. The sudden fall of mean cell length during the peak activity might be due to the onset of anticlinal division at that time (Bannan, 1950).

Anticlinal divisions, frequent around the end of the active growth period in A. nilotica, are pseudotransverse with strongly oblique dividing walls, at times so long as nearly to touch the end of the dividing cells. Such a wall is often laid down toward an end of the cell, producing daughter cells of unequal size. The cells thus produced elongate at their apices until they have attained an adult state. The apical intrusive growth is often evidenced by forking, bending or serration of the cell tips or by their intrusion into cambium rays.

In general, the fusiform initials, regardless of their size variation, are believed to be uninucleate as suggested by earlier workers. However, multinucleate fusiform initials have been reported in some species such as Solanum melongena L. (Patel, 1975), Psidium guajava L. (Ghouse & Khan, 1977), Delonix regia Raf., Mimusops elengi L., Polyalthia longfolia Benth. & Hook. (Hashmi, 1977) and a few members of the Verbenaceae (Ghouse et al., 1979). The present study reveals a similar condition in A. nilotica where as many as eight nuclei per cell can be seen. However, unlike Ghouse & Khan (1977), who recorded a decline in the ratio of polynucleate initials during the active phase compared to the dormant phase of the cambium in Psidium guajava, no regular correlation has been recognized between seasonal changes and frequency of polynucleate initials of A . nilotica. As in many other species, the nuclei appear to be larger and more lightly stained in the active than in the dormant cambium (see Iqbal & Ghouse, 1985a).

Gregory (1977) observed that the mean dimensions of ray cell initials in Acer saccharum Marsh. increased in size, slowly when the growth rate was low and rapidly when it was high, but there was little fluctuation in the number of rays per unit of tangential area. However, in many other species (see Iqbal & Ghouse, 1985a), including A . nilotica, the ray cell initials hardly undergo any dimensional change in relation to age, but they increase in number and form broader rays in older stems.

The rays in A . nilotica are mostly multiseriate and appreciably tall but without uniseriate wings, a tendency towards phylogenetic advancement. The development of ray initials from fusiform initials occurs by various ways already described (see Iqbal & Ghouse, 1985a; Philipson et al., 1971). Fusion and splitting of rays, as described by Barghoorn (1940a, b, 1941) are also common.

There are diverse reports on the proportion of ray initials in the cambial cylinder. It was estimated to be less than 10% in some species (Wilson, 1963, 1964; Butterfield, 1972), about 15-40y0 in several dicotyledons (see Iqbal & Ghouse, 1985a), and more than 50% (Bailey, 1923) or as much as 75% of the total cambial surface in certain extreme cases like Dillenia indica (Ghouse &

395

396 M. IQBAL AND A. K. M. GHOUSE

Yunus, 1974). In the present observations on A. nilotica, ray proportion is up to about 30% (Fig. 19B), a condition observed in a fairly large number of Indian tropical trees (Iqbal & Ghouse, 1985a).

The ratio of ray and fusiform initials also differs with age. In A. nilotica, the mean proportion of ray initials increases from young shoots (about 14%) basewards, becoming more or less constant (nearly 29%) in older parts of the trunk. A similar pattern was found in Dalbergia sissoo (Ghouse & Yunus, 1973). In Polyalthia longifolia (Ghouse & Hashmi, 1980), Bauhinia parvij7ora Vahl (Khan, Ahmad & Iqbal, 1981) and Sterculia urens Roxb. (Furqan & Ahmad, 198 1 ), however, the ray initial proportion increases more or less steadily from the tree apex down to the base. On the other hand, there is no consistent increasing or decreasing trend in Prosopis spicigera L. (Ghouse & Iqbal, 1977) and Citrus sinensis (L.) Osbeck (Khan et al., 1983), although it was relatively low in young axes and high in old trunks. Nevertheless, in no case is the ray proportion as low as 10%. The abundance of the different types of ray initials also varies with season. The multiseriate rays of Acacia nilotica, though most frequent throughout, form a relatively small proportion at the onset of cambial activity. The proportion increases sharply during the period of vigorous growth. The tetraseriate rays, second in abundance, exhibit an opposite trend. The frequency of other types of rays and the relative proportion of fusiform and ray initials fluctuate irregularly.

ACKNOWLEDGEMENT

The work was carried out under the award of a senior research fellowship of CSIR, New Delhi.

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