IAWA Bulletin n.s., Vol. 11 (4), 1990: 379-391
STRUCTURE OF WOOD AND CAMBIAL VARIANf INTHE STEM OF DALBERGIA
PANICULATA ROXB.
by
M. N. B. Nair and H. Y. Mohan Ram Department of Botany, University
of Delhi, Delhi 110 007, India.
Summary The wood of Dalbergia paniculata is
unique as it consists of concentric layers of broad xylem,
alternating with bands of nar row phloem. This anomaly results
from the periodic formation of successive cambia in the secondary
phloem. Some phloem paren chyma cells dedifferentiate to form a
discon tinuous ring of cambium. Such parenchyma cells have higher
succinate dehydrogenase activity than the neighbouring cells of
sec ondary phloem. The newly differentiated cambial layer
functions bidirectionally, and its products give rise to xylem
internally and phloem externally. The phloem along with cambium
present internal to the newly form ed xylem becomes
included.
The wood is diffuse-porous and the inter vessel pits are vestured.
The phloem has well differentiated sieve tube members and com
panion cells. Key words: Cambial variant, successive bi
directional cambia, vestured pits, included phloem.
Introduction Wood produced by over a dozen species
of Dalbergia is commercially sold by the gen eral name rosewood
and is highly valuable (Anonymus 1979). However, the wood of
Dalbergia paniculata is unfit for use. The plant is widely
distributed throughout south em, central and western India. It
also extends northwards to the Siwaliks. The tree is erect and can
attain a height of up to 20 m. The bark is smooth and greenish
white. Among Dalbergia species the stem structure of D. paniculata
is quite unusual. Gamble (1902), Brandis (1906) and Rao and
Purkayastha (1972) reported that in this species broad concentric
masses of wood alternate with
narrow layers of soft tissue. These authors observed that planks
cut from the old trees often disintegrate due to separation in the
region of soft layers. Hill (1901) examined cross sections of a
dried trunk of Dalbergia paniculata and noted that the narrow soft
layer in wood was phloem, with well-dif ferentiated sieve tubes
and a certain amount of cambium. He was unable to study the exact
mode of development of the anomaly because the material available
to him was dry. As far as we are aware, there is no detailed
anatomical study of the wood of this species. A few reports
(Anonymus 1952; Thothathri 1987) indicate that the wood of D.
paniculata is used as firewood, for construction of houses and in
the manufacture of musical instruments. However, the latter uses
seem most unlikely. Thothathri (1987) is of the opinion that D.
paniculata cannot be con sidered as a separate species. He has
merged it with D./anceolaria and has given it the rank of a
subspecies. The objective of our work w.as to examine the structure
of the wood, trace the function of the cambial variant, production
of phloem, and its inclusion in the wood.
Materials and Methods Material for study was collected from
Dangs forest of Gujarat State, India. Wood samples were taken from
branches (1 to 25 cm in diameter) and the main trunk (45 cm in
diameter) from different felling sites. They were fixed immediately
in FAA (formalin: acetic acid: 50% ethanol; 1: 1 : 9) and sec
tioned on a sliding microtome, stained in 0.05% toluidine blue 0 in
0.1 M phosphate buffer, pH 6.8 (O'Brien et al. 1964), safra nin 0
and fast green FCF (Sass 1958).
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380 IAWA Bulletin n.s., Vol. 11 (4), 1990
Table 1. Wood characteristics of Dalbergia paniculata.
Vessel member length (11m)
Vessel member diameter <11m)
8.93
1136.74
127.30*
84.00*
9.42*
130.40*
79l.00*
l.70+
0.90+
l.35+
deviation
48.00
40.00
4.33
72.80
15l.00
0.21
0.22
0.24
Thickness of the bark at the time of initiation of first successive
cambium 2.83+ 0.30
Distance from the normal cambium where the phloem cells first
dedifferentiate into cambium (mm) 0.85+ 0.16
* = mean of 100 readings; + = mean of 25 readings.
A few samples were also fixed in parafor maldehyde-glutaraldehyde
in 0.1 M caco dylate buffer pH 7.1 for one hour at room
temperature (20-25°C). The blocks were trimmed to 2 mm 3, gently
aspirated and again left in the fresh fixative for 4 hr in a
refrigerator (4 ± 1°C). The samples were washed in the same buffer
five times at 10 minute intervals. Post-fixation was done in 2%
osmium tetroxide in 0.1 M cacodylate buffer pH 7.1 for 10-12 hr at
4 ± 1°C. After five washes in the same buffer, the materials were
stained in 2% aqueous uranyl acetate for 30 minutes. After
dehydration in acetone series, infiltration and embedding were car
ried out in Spurr's low viscosity embedding medium (Spurr 1969).
Sections of I-211m thickness were cut on a JB4 Dupont micro tome
using glass knives. The sections were spread on 10% acetone, fixed
on the slides by heating to 60-80°C over a hot plate and stained
with methylene blue-azur II, basic
fuchsin (Humphrey & Pittmann 1974) and toluidine blue 0
(O'Brien & McCully 1981). Materials were prepared for scanning
electron microscopy as described earlier (Nair 1987).
For localisation of enzymes, 10 cm long pieces of logs were sealed
in polythene bags immediately after cutting and transported in a
flask containing ice to the laboratory. Fresh sections were cut on
a sliding microtome, and were collected in cold 0.1 M phosphate
buffer, pH 7.6 for succinate dehydrogenase (E.C. 1.3.99.1) activity
and in 0.1 M acetate buffer pH 4.5 for acid phosphatase (E. C.
3.1.3.2) activity according to the methods suggested by Bancroft
(1975). Controls for enzyme reaction were maintained by incubat
ing the sections after boiling them in distilled water for 5
minutes or by avoiding the sub strate in the incubating
medium.
Measurements of wood elements were made in the sections as well as
from macer ated materials. Vulnerability (mean vessel
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Nair & Mohan Ram - Cambial variant in Dalbergia paniculata
381
member diameter divided by mean vessel frequency) and mesomorphy
indices (vul nerability multiplied with mean vessel length) were
calculated according to Carlquist (1977).
Results The wood is pale yellowish white and
heartwood is absent. Broad concentric bands of wood alternate with
the narrow layers of soft tissue (Figs. 1, 28d). The dimensions of
vessel members and other wood characteris tics are given in Table
1. The wood is diffuse porous (Figs. 1, 2). Axial parenchyma is
paratracheal banded (Figs. 1-3). Rays are uniseriate to multi
seriate (Fig. 4), mostly ho mocellular and rarely heterocellular
(Fig. 5). Vessels are solitary, in radial multiple or rare ly in
clusters (Figs. 1, 2). The perforation plates are transverse to
oblique (Fig. 6) and simple (Fig. 7). There is a negative correla
tion between vessel member length and ves sel member diameter
(Fig. 29).
Legends of Figures 1-22:
The inteIVessel pits are bordered, alternate and vestured (Fig. 8).
They belong to type 2i recognised by Nair and Mohan Ram (1989).
Large vestures are present on the margin of the outer pit
apertures, whereas small ves tures are seen towards the pit
annulus (Fig. 8). Vestures are present in the inner pit aper tures
also (Fig. 9). However, they are absent in the parenchyma cells and
fibres. Occa sionally tyloses are obselVed in the vessels (Fig.
11).
The contact cells (parenchyma cells con tiguous with vessels) may
also have bordered pits on the wall facing the vessel member (Fig.
10). Certain axial parenchyma cells and fibres are chambered and
have large crystals in each chamber (Fig. 12).
Anomalous stem structure The wood contains wavy and
anastomos
ing layers of phloem tissue (Figs. 1, 13, 14, 28d). Each ring of
phloem is accompanied by
(text continued on page 386)
Fig. 1. Stem portion showing included phloem, x 70. PH = phloem. -
Fig. 2. CS of wood, x 70. - Fig. 3. Magnified views of CS of wood,
x 130. - Fig. 4. TLS of wood showing uniseriate to multiseriate
rays, x 210.
Fig. 5. RLS of wood showing heterocellular rays, x 210. - Fig. 6.
TLS showing vessel with extractives, x 110. VL = vessel- Fig. 7.
Vessel member with simple perforation plate, x 190. PP =
perforation plate. - Fig. 8. SEM showing vestured inteIVessel pits
as viewed from outer surface of the vessel, x 2700. - Fig. 9. SEM
of inner pit aperture with vestures as viewed from inside the
vessel, x 3000. V = vestures. - Fig. 10. Contact cells with
half-bordered pits, x 380. - Fig. 11. CS of wood showing tyloses in
the vessel, x 210. T = tyloses. - Fig. 12. TLS of wood showing
crystals in axial parenchyma and fibre (arrows), x 380.
Fig. 13. CS of stem showing interconnected included phloem layers,
x 70. - Fig. 14. CS of stem showing disorganised growth pattern in
wood, x 70. - Fig. 15. CS of wood with a portion of included
phloem, x 80. - Fig. 16. Cambium associated with the included
phloem, x 250. - Fig. 17. CS of phloem showing sieve plate and
protein (at arrow), x 510. - PH = phloem; SP = sieve plate.
Figs. 18-20. CS of bark showing dedifferentiation of phloem
parenchyma cells into cambium (arrows). - 18: x 140; 19: x 280; 20:
x 210. PH = phloem. - Fig. 21. The dedifferentiating cells showing
higher SDH activity as compared to neighbouring phloem parenchyma
cells (Fig. 22), x 580. - Fig. 22. Phloem parenchyma cells with low
SDH activity, x 580.
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"0 >
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.S Nair & Mohan Ram - Cambial variant in Dalbergia paniculata
383
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384 IAWA Bulletin n.s., Vol. 11 (4), 1990
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Nair & Mohan Ram - Cambial variant in Dalbergia paniculata
385
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386 IAWA Bulletin n.s., Vol. 11 1990
Figs. 23-25. CS of wood showing differentiation of xylem internally
and phloem externally, from the cambium developed in the secondary
phloem. - 23: x 120; 24: x 130; 25: x 70. PH = phloem; VL =
vessels; XY = xylem. -- Fig. 26. CS of wood showing pith flecks, x
110. - Fig. 27. LS of pith flecks in the wood, x 230. - PF = pith
flecks.
cambium internally (Figs. 15, 16). The pres ence of crushed cells
along the periphery of four to six outer rings of included phloem
indicate that the cambia accompanying them are active. Concentric
rings of sclerenchyma,
interrupted by rays, alternate with the other phloem elements. The
phloem consists of well-differentiated sieve elements with sieve
plates (Fig. 17) and companion cells. The ini tial secondary
growth of the vascular tissue
386 IAWA Bulletin n.s., Vol. 11 (4), 1990
Figs. 23-25. CS of wood showing differentiation of xylem internally
and phloem externally, from the cambium developed in the secondary
phloem. - 23: x 120; 24: x 130; 25: x 70. PH = phloem; VL =
vessels; XY = xylem. -- Fig. 26. CS of wood showing pith flecks, x
110. - Fig. 27. LS of pith flecks in the wood, x 230. - PF = pith
flecks.
cambium internally (Figs. 15, 16). The pres ence of crushed cells
along the periphery of four to six outer rings of included phloem
indicate that the cambia accompanying them are active. Concentric
rings of sclerenchyma,
interrupted by rays, alternate with the other phloem elements. The
phloem consists of well-differentiated sieve elements with sieve
plates (Fig. 17) and companion cells. The ini tial secondary
growth of the vascular tissue
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Nair & Mohan Ram - Cambial variant in Dalbergia paniculata
387
a
b
c
in Da/bergia panicu/ata is normal. However, the concentric layers
of phloem and xylem are developed by the activity of successive
layers of cambium formed in the secondary phloem. The initiation of
cambium is first observed at certain loci in the secondary phloem
in branches when their diameter reaches 2-3 cm.
Phloem parenchyma cells located about 850 Ilm away from the cambium
start divid ing periclinally to form files of cells simu lating
the cambium (Figs. 18-20). These cells show higher succinate
dehydrogemise activity (Fig. 21) than the neighbouring secondary
phloem cells (Fig. 22). The dedif ferentiation of parenchyma cells
extends tan gentially so that a discontinuous cambial ring is
formed (Figs. 20, 28a). This newly form ed cambium is
bidirectional and produces phloem externally and xylem internally
(Figs. 23-25, 28b-d). Thus the phloem present internal to the newly
formed xylem becomes included between the xylem layers (Figs.
23-25, 28c, d). The differentiating cambial derivatives indicate a
higher acid phospha tase activity than the neighbouring phloem
parenchyma cells.
Fig. 28. Diagrammatic represen tation of initiation and func
tioning of successive cambia resulting in the formation of con
centric layers of phloem alter nating with xylem. CA = cam bium;
PH = phloem; X = xylem.
c
388 IAWA Bulletin n.s., Vol. 11 (4), 1990
lS0--t •
~ • • 100 • • ,
• :.s · , .. • .... SO • • • .8 • • .. .... • • E • .' . • • ... E
60 • • • -'0 r = - 0·5207 • ell
40 - • •• ell II) • e\. • ;.- p (%) = < 0 ·1 . " . •
20 <P = 100 .,. ..
0 I 0 20 40 60 SO 100 120 140 160 180 200
vessel member length in Jlrn
Fig. 29. Scatter diagram showing negative correlation between
vessel member length and vessel member diameter.
The newly formed cambial cylinder may be discontinuous because
contiguous second ary phloem cells do not invariably become
meristematic. Consequently the inner and outer phloem layers become
connected by patches of secondary phloem (Fig. 13). In such regions
reorientation and readjustments of cells occur during the increase
in stem girth (Fig. 14). Irregular patches of paren chyma
generally known as pith flecks are also found in the wood (Figs.
26, 27).
Discussion Several modes of cambial variants (ano
malous secondary thickenings) have been reported in dicotyledonous
stems and roots (Solereder 1908; Davis 1961; Balfour 1965;
Philipson & Ward 1965; Studholme & Phi lipson 1966; Basson
& Bierhorst 1967; Esau & Cheadle 1969; Dobbins 1971, 1981;
Wheat 1977; Mikesell 1979; Zamski 1979; Bailey 1980. Carlquist
1988). The origin and func tion of cambial variants (anomalous
cam-
bium) have been shown to vary in different species. Esau and
Cheadle (1969) traced the anomalous secondary growth in Bougain
villea to bidirectionally active cambium pro duced from the oldest
phloem cells. Cambial variants are frequently reported in lianas
(Met calfe & Chalk 1983; Wheeler et al. 1989). The term
'anomalous' is considered as a mis nomer as the included phloem
and other cam bial variants are of regular occurrence in the taxa
in which they are found (Wheeler et al. 1989). Carlquist (1988) has
recognised three major types of cambial variants: 1) successive
cambia; 2) a single cambium which produces interxylary phloem and
xylem internally; 3) "cambia that begin as single (or in a few
cases, multiple and simultaneous) nor mal cambia that produce
phloem externally and xylem internally and have or develop a
conformation other than cylindrical."
The alternating layers of xylem and phloem produced by successive
cambia have been referred to as concentric type of 'included
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Nair & Mohan Ram - Cambial variant in Dalbergia paniculata
389
phloem' or 'interxylary phloem' (Esau 1979; Metcalfe & Chalk
1983; Wheeler et al. 1989). Mikesell and Popham (1976) and
Carlquist (1988) suggested to restrict these terms (in cluded
phloem and interxylary phloem) to those situations in which a
single (normal) cambium alternately produces xylem and phloem
internally. We have used the term 'included phloem' in the present
study with reservation as each phloem layer is included between the
cambium from which it is pro duced and the xylem formed from the
next cambium. The classification of cambial vari ants is debatable
as it is generally based on topography rather than ontogeny.
Generally in plants characterised by the presence of successive
cambia, the primary thickening meristem is formed from cortical
parenchyma (Carlquist 1988). The further development of cambia is
from the unligni fied parenchyma produced externally by the
primary thickening meristem (Carlquist 1988). In the Papilionoideae
the anomalous cambium is generally produced from the un lignified
cortical parenchyma and pericycle (Solereder 1908). The successive
cambia in Dalbergia paniculata are formed by dediffer entiation of
phloem parenchyma cells and not from the cortical parenchyma. Each
cambium functions normally, producing phloem exter nally and xylem
internally.
Cambial variants may have arisen at dif ferent stages in evolution
for attaining woodi ness (Metcalfe & Chalk 1983). Usually spe
cies with winged or grooved stems produce cortical vascular
bundles. The interxylary phloem in Strychnos is believed to have
orig inated from ancestors with successive cambia (Joshi 1931).
Joshi (1937) proposed that the occurrence of secondary thickening
by suc cessive cambia in the Amaranthaceae and Chenopodiaceae is
an ancestral character but that this feature has been lost in some
species. Carlquist (1988) has suggested that succes sive cambia
and interxylary phloem have originated independently.
There are 45 species of Dalbergia in Bang ladesh, Burma, China,
India, Malaysia, Paki stan, Sri Lanka and Vietnam (Thothathri
1987). Wood structure of nine trees and five climbers/shrubs have
been described by Gamble (1902). The structure of wood has
been described in six timber species by Pear son and Brown (1932).
But only Gamble (1902) and Rao and Purkayastha (1972) noted
anomalous structure in Dalbergia paniculata. Hill (1901) proposed
that D. paniculata might have evolved from a liana and consequently
inherited and retained the anomaly. It is necessary to investigate
the stem structure of all the Dalbergia species to understand the
precise affinities of the species and to draw phylogenetic conclu
sions.
Fahn and Shchori (1967) have suggested that successive anastomosing
layers of xylem and included phloem are of adaptive value in
perennial desert plants. Dobbins and Fisher (1986) have suggested
that the living tissue within the xylem is advantageous to lianas
because it helps rapid and vigorous regener ation following
wounding and girdling. Ano malies such as multiple vascular
cylinders, convoluted or disjunct cambia, supernumer ary cambia,
wide rays, and abundant xylem parenchyma provide a structurally
efficient system for wound healing as the stems of lianas are under
high risk to bark injury, vas cular interruption and
girdling.
Carlquist (1988) has attempted to correlate cambial anomalies with
habit and ecology. He has suggested that the products of succes
sive cambia may provide channels for supply of photosynthates to
storage structures. In the family Onagraceae a few species that
have interxylary phloem show sudden flowering events (Carlquist
1988). It is presumed that to aid rapid formation of flowers and
fruits interxylary phloem provides many distribu tion channels for
efficient transport of photo synthates. It is also advantageous to
plants if the vascular tissues remain functional over a period of
years (Carlquist 1988). In Dalber gia paniculata alternating
layers of phloem produced by successive cambia remain func tional
as the cambium accompanying them also retains its activity.
Carlquist (1977, 1980, 1985, 1988) has proposed that low
vulnerability and meso morphy values indicate xeromorphic wood
with a capacity to resist water stress in the vessels. In Dalbergia
paniculata these values are very high (see Table 1), suggesting
their mesomorphic habitat. The relative importance
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390
of negative correlation between vessel mem ber length and vessel
member diameter in the conduction of water remains to be assessed
cri ticall y.
Dalbergia paniculata has been treated as a distinct species in
Indian and Burmese floras. In his recent taxonomic revision of the
tribe Dalbergieae of the Indian subcontinent, Tho thathri (1987)
has concluded that D. panicu lata cannot stand as a distinct
species on ac count of its similarity with D. lanceolaria. He
considers D. paniculata as a subspecies of D. lanceolaria.
Thothathri (1987) has also stated that D. nigrescens described from
Burma is con specific with D. paniculata and hence a synonym of D.
paniculata.
Anomalous structure is always of diag nostic value because of its
restricted occur rence (Metcalfe & Chalk 1983). Several ex
amples have been cited by Carlquist (1988) to indicate that the
presence of successive cambia is a significant criterion in
systemat ics. Absence of successive cambia was one of the reasons
for separating Bataceae from the Chenopodiaceae (Carlquist 1978,
1988). Occurrence of successive cambia lead to the removal of
Simmondsia from the Buxaceae (Bailey 1980; Cariquist 1982) and
Avicennia from the Verbenaceae (Studholme & Philip son 1966;
Zamski 1979).
The stem structure is markedly different in D. paniculata and D.
lanceolaria. Dalbergia paniculata is characterised by concentric
rings of 'included phloem' in the wood, whereas D. lanceolaria does
not show this feature. According to Gamble (1902) the wood of D.
nigrescens has normal structure.
In our view an important character such as anomalous stem structure
should be taken into consideration for taxonomic delimitation. We
therefore propose that D. paniculata be retained as a separate
species and not be merged with D.lanceolaria.
Acknowledgements This study was supported by the Univer
sity Grants Commission, New Delhi. The access to the scanning
electron microscope provided by the Regional Electron Micro scope
Facility, All India Institute of Medical Sciences, New Delhi, is
gratefully acknowl edged.
IAWA Bulletin n.s., Vol. 11 1990
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STRUCTURE OF WOOD AND CAMBIAL VARIANT IN THE STEM OF DALBERGIA
PANICULATA ROXB
Summary
Introduction