DEVELOPMENTAL ANATOMY OF THE STEM OF WELFIA GEORGII,
IRIARTEA GIGANTEA, AND OTHER ARBORESCENT PALMS:
IMPLICATIONS FOR MECHANICAL SUPPORT
PAUL M. RICH
Made in United States of America
Reprinted from American Journal of Botany
Vol. 74, No. 6, June, 1987
Copyright 1987 by the Botanical Society of America, Inc.
Am. J. Bot. 74(6):792-802. 1987.
DEVELOPMENTAL ANATOMY OF THE STEM OF WELFIA GEORGII,
IRIARTEA GIGANTEA, AND OTHER ARBORESCENT PALMS:
IMPLICATIONS FOR MECHANICAL SUPPORT1
PAUL M. RICH2
Harvard University, Harvard Forest, Petersham, Massachusetts 01366
ABSTRACT
Changes in stem anatomy with radial position and height were studied for the arborescent palms Welfia
georgii, Iriartea gigantea, Socratea durissima, Euterpe macrospadix, Prestoea decurrens, and Cryosophila
albida. Vascular bundles are concentrated toward the stem periphery and peripheral bundles contain more fibers
than central bundles. Expansion and cell wall thickening of fibers within vascular bundles continues throughout
the life of a palm, even in the oldest tissue. Within individual vascular bundles, the fibers nearest the phloem
expand first and fiber cell walls become heavily thickened. A front of expanding fibers moves outward from the
phloem until all fibers within a vascular bundle are fully expanded and have thick cell walls. Peripheral vascular
bundles differentiate first and inner bundles later. In the stem beneath the crown, vascular bundles and ground
tissue cells show little or no size increase, but marked cell wall thickening during development for Welfia georgii.
Beneath the crown, diameters of peripheral vascular bundles increase more than twofold for Iriartea gigantea,
while diameters of central bundles do not increase. In Iriartea stems, ground tissue cells at the periphery elongate
to accommodate expanding vascular bundles and cell walls become thickened to a lesser degree than in fibers;
central ground tissue cells elongate markedly, but cell walls do not become thickened; and large lacunae form
between central parenchyma cells. For Iriartea, Socratea, and Euterpe, sustained cell expansion results in limited,
but significant increases in stem diameter. For all species, sustained cell wall thickening results in dramatic
increases in stem stiffness and strength.
THERE IS CONSIDERABLE variation among palm
species in the capacity for sustained stem thickening
during height growth (Schoute, 1912; Schatz et al.,
1985; Rich, 1985, 1986; Rich et al., 1986). Some
species, such as Welfia georgii, Prestoea decurrens,
and Cryosophila albida have little or no capacity for
sustained stem thickening. Other species, such as
Iriartea gigantea, Socratea durissima, and Euterpe
macrospadix have pronounced capacities for stem
diameter increase during height growth. Stiffness
and strength are concentrated toward the stem
periphery and base of palms. Palms also have a major
capacity to increase the stiffness and height growth
(Rich, 1987). Herein, I characterize developmental
anatomy in palm stems as it relates to mechanical
support. I examine anatomical basis for sustained stem
thickening and strengthening during height growth. I
focus on the Costa Rican palm species Welfia georgii
H. A. Wendl. ex Burret and Iriartea gigantea H.A.
Wendl. ex Burret, and also include comparisons with
Socratea durissima (Oerst.) H.A. Wendl., Euterpe
macrospadix Oerst., Prestoea decurrens (H.A.
Wendl.) H.E. Moore, and Cryosophila albida Bartlett.
The outer portion of palm stems consists of a
narrow cortex that is composed of unspecialized
ground parenchyma and fibrous or fibrovascular
strands. In some palms, especially small arecoid palms
(e.g., Prestoea decurrens), the epidermis is
permanent; however, in most palms, the epidermis is
sloughed off. In palms that shed their epidermis, a
superficial protective layer forms. Some palms have
an etagenmeristem of the outer ground parenchyma, a
zone of cell division without permanent initials
(Philipp, 1923) that produces suberized or lignified
secondary tissue; while other palms form the
protective layer by suberization and sclerosis of the
1
Received for publication 21 April 1986; revision
accepted 24 September 1986.
Thanks go to Peter Ashton, William Bossert,
Elizabeth Burkhardt, Thomas Givnish, Ghiselle
Hernandez, Shawn Lum, Monika Mattmüller, Thomas
McMahon, Leda Muñoz, Lynn Phipps, Mauricio
Quesada, P.B. Tomlinson, the late Martin
Zimmermann, and the staff of the Organization for
Tropical Studies. Research was supported by the
Atkins Garden Fund of Harvard University, the Jesse
Noyes Foundation through the Organization for
Tropical Studies, a Fulbright Graduate Fellowship,
and National Science Foundation Doctoral
Dissertation Improvement Grant BSR84-13187.
2
Current address: Mailstop K495, HSE-12; UC
Los Alamos National Laboratory; Los Alamos, NM
87545.
792
June, 1987]
RICH – PALM STEM ANATOMY
793
TABLE 1. Characteristics of palms collected a
Ht
Hb
Hcr
DBH
Rt
C
Lf#
LfL
Species
m
m
m
m
m
m
#
m
Welfia georgii
6.7
2.4
1.9
0.187
0.0
3.6
15
4.6
W. georgii
13.0
8.0
7.9
0.154
0.0
3.5
27
6.2
W. georgii
19.0
17.0
16.7
0.169
0.0
2.3
13
3.9
Iriartea gigantea
6.7
2.6
1.8
0.087
0.3
1.8
6
3.1
I. gigantea
9.0
3.5
2.5
0.087
0.3
2.3
8
3.4
I. gigantea
17.2
14.1
12.6
0.197
1.3
3.3
6
3.9
I. gigantea
26.5
23.9
23.0
0.200
1.9
2.3
5
3.0
Socratea durissima
17.3
13.9
12.5
0.143
1.7
2.3
7
2.6
Euterpe macrospadix
20.6
18.7
17.7
0.115
0.7
2.1
6
2.7
Prestoea decurrens
10.2
7.5
6.7
0.079
0.1
1.8
6
3.2
Cryosophila albida
4.5
2.7
2.2
0.069
0.4
2.2
17
2.5
a
Key to symbols: Ht : height to top of crown (to highest point of highest expanded leaf). Hb: height to bottom of
crown (to divergence of lowest leaf from stem). Hcr: height to bottom of crown shaft (to bottom of lowest leaf base).
DBH: stem diameter (at 1.3 m or above stilt roots, whichever is higher). Rt: height of above-ground roots. C: crown
radius (mean of projected radius in four compass directions). Lf#: number of leaves in crown. LfL: mean leaf length
(from divergence of leaf from stem to leaf tip).
existing ground parenchyma (Weisse, 1897; Floresta,
1905; Philipp, 1923; Tomlinson. 1961). The inner
portion of the stem consists of a wide central cylinder.
The outer region of the central cylinder has a high
concentration of vascular bundles, and individual
vascular bundles, in turn, have a bundle sheath mainly
composed of fibers (Tomlinson, 1961). The outer
region of the central cylinder is of major importance
for support. The inner region of the central cylinder
has fewer vascular bundles, and individual bundles
have little fibrous tissue in the bundle sheath. Ground
tissue of the central cylinder consists of parenchyma,
which in many species becomes thick walled and
lignified with age. Many palms that undergo marked
sustained stem thickening form prominent lacunae, or
air canals, between expanding parenchyma in the
central stem (Tomlinson, 1961, 1964, 1979;
Tomlinson and Zimmermann, 1967). A lacunose
ground parenchyma appears to be primary in origin
and is common, especially in palms of wet places
(e.g., Metroxylon).
The course of vascular bundles in palm stems has
been mapped and studied from the point of view of
hydraulic architecture, the morphological basis of
water transport (Zimmermann and Tomlinson, 1965,
1966, 1972, 1974; Zimmermann, 1913, 1983; Sperry,
1985). Mechanical architecture, the morphological
basis of support, has been studied for understory
palms (Chazdon, 1984, 1986) and for Cocos nucifera
(Richolson and Swarup, 1977; Sudo, 1980; Killmann,
1983), primarily to assess the suitability of coconut
stems for timber and paper pulp. Specific gravity and
associated strength properties have the greatest values
toward the periphery and base of the trunk in coconut.
Schoute (1912), expanding upon the work of earlier
authors, surveyed the occurrence of sustained stem
thickening in palms and examined the anatomical
basis for sustained diameter increase. Waterhouse and
Quinn (1978) found that sustained stem thickening in
Archontophoenix cunninghamiana is due to cell
enlargement and possibly division in the ground
tissue. Work of these last three researchers included
direct measurement of stem diameter increases within
individuals over time.
MATERIALS AND METHODS – Research was
conducted at the Organization for Tropical Studies La
Selva Biological Station, Costa Rica, as described
elsewhere (Chazdon, 1985; Hartshorn, 1983). From
forest near La Selva, I collected stem tissue from three
specimens of Welfia georgii, with heights measured
from the ground to the highest expanded leaves of 7,
13, and l9 m (Table 1). I collected similar samples
from four individuals of Iriartea gigantea, with
respective heights of 7, 9, 17, and 27 m. In addition, I
collected one individual each of Socratea durissima,
Euterpe macrospadix, Prestoea decurrens, and
Cryosophila albida. For collections of all six species,
two sets of trunk discs (approximately 2 cm thick)
were cut from each of three to five height intervals
along the trunk. One set of discs was subdivided into
cubes (each approximately 2 cm per side) that
corresponded to two to five radial positions and then
stored in the fixative formalin-acetic acid-alcohol
[FAA] for anatomical study. After transport to the
United States, anatomical material was transferred to
70% ethanol. The second set of discs was subdivided
into cubes to allow examination of the distribution of
specific gravity. In addition, 1 m long sections of
794
trunk from the three to five height positions were
collected for tests of tissue stiffness and strength.
Transverse and some longitudinal sections of stem
tissue were prepared for each height and radial
position within each collected palm. The hardest tissue
was soaked in concentrated hydrofluoric acid for two
to three weeks to remove silica and soften the tissue.
A sliding microtome was used to cut most sections.
Freehand sections were prepared for the very soft
tissue from the central stem of Socratea and Iriartea.
Microscope sections were assembled into a collection
that includes unmounted sections stored in ethanol and
permanently mounted sections, both unstained and
stained with alcian green and safranin.
Within transverse sections at each height and
radial position, I measured diameter of vascular
bundles (along both major and minor axes), number of
vascular bundles per unit area, proportion of fiber
cells in vascular bundles, diameter of ground tissue
cells (along major and minor axes), and diameter of
intercellular lacunae or air spaces. Diameter of
vascular bundles and ground tissue cell length and
width were calculated as the mean of ten
measurements. Number of vascular bundles per unit
area was calculated as the mean concentration of
bundles within ten microscope fields (each field 0.15
cm2). Proportion of fiber cells in vascular bundles was
calculated as the mean of ten estimates, to the nearest
10%, of the proportion of fiber area per total vascular
bundle area. For a developmental sequence of
peripheral stem tissue at breast height (1.3 m) in
Welfia and Iriartea, fiber cell wall thickness was
calculated as the mean of ten measurements. I
calculated 95% confidence intervals about each mean
for each quantitative characteristic. Quantitative
characteristics were plotted to examine differences
between peripheral and central tissue, changes along
the length of the stem, and inferred changes at breast
height during development.
Detailed measurements were made of physical
and mechanical properties of these same stems, as
described in detail elsewhere (Rich, 1987). Specific
gravity was measured as both fresh and dry weight per
unit fresh volume. Elastic modulus, an index of
stiffness (Gordon, 1978), was measured by milling
test beams of fresh stem tissue, clamping the beams
horizontally in a vise, measuring displacement as a
function of weight applied to the free end of the
beams, and using standard formulas for cantilevered
beams (Timoshenko, 1956) to make calculations
(Rich, 1987). Modulus of rupture, the strength at
which failure occurs, was measured by applying
weights to the milled beams until breakage occurred
and using standard formulas for cantilevered
beams (Timoshenko, 1956) to make calculations
[Vol. 74
(Rich, 1987).
RESULTS – In the following subsections, first I
present results concerning anatomical structure as it
varies with height and radial position within
individual palm stems. Then I present results
concerning developmental changes inferred by
comparison of the anatomy at breast height for stems
from different individuals ordered according to overall
height of the individual. For the sake of discussion,
vascular bundle "diameter" refers to the length along
the major axis in transverse section. Ground
parenchyma cell "length" and "width" refer to the
lengths of the major and minor axes in transverse
section, respectively. Length in transverse section may
or may not be the longest dimension of the cell,
depending upon the axial length of the cell. Individual
height refers to the overall height of an individual
palm, as measured from the ground to the height of
the highest expanded leaf. Individual height should
not be confused with height within an individual, the
latter making reference to a specified height position
within an individual stem. Characteristics of the palms
collected are shown in Table l.
Comparative anatomy within individual stems –
Mean vascular bundle diameter (along the major axis
in transverse section) decreases with increasing height
above the ground within peripheral stem tissue of a l9m-tall Welfia and shows little or no change with
height for central tissue (Fig. 1). In the same Welfia,
peripheral vascular bundles are 70% larger than
central bundles near the base of the stem (1,348 vs.
795 μm), whereas peripheral bundles are 305 larger
than central bundles just below the crown (887 vs. 676
μm). In transverse area, more than 90% of each
peripheral bundle consists of fibers, whereas 30-40%
of each central bundle consists of fibers. Peripheral
vascular bundles from near the crown are narrower
than bundles near the stem base. Leaf and
inflorescence traces are more abundant near the
crown. In Welfia, the mean concentration of vascular
bundles (# bundles/unit area) is about twice as much
in peripheral as compared to central tissue near the
stem base (50 vs. 23 bundles/cm2), and the mean
concentration just below the crown increases
markedly (131 vs. 89 bundles/cm2) for peripheral and
central tissue respectively (Fig. 2). In the same Welfia,
the mean length of central ground parenchyma cells in
transverse section is approximately twice that of
peripheral parenchyma cells (164 vs. 86 μm; Fig. 3.
Ground parenchyma cell size decreases with
increasing height position, with both central and
peripheral cells converging on mean lengths between
50 and 80 m. A 21-m-tall Iriartea shows similar
June, 1987]
RICH – PALM STEM ANATOMY
Fig. 1-7. 1. Vascular bundle diameter in transverse
section at successive heights within the stem of a 19m tall Welfia georgii. "Diameter" refers to length
along the major axis. 2. Distribution of vascular
bundles (#/cm2) at successive heights in the stem of
the same Welfia. 3. Ground parenchyma cell length in
transverse section at successive heights within the
stem of the same Welfia. "Length" refers to length of
the major transverse axis. 4. Vascular bundle diameter
in transverse section at successive heights within the
stem of a 27-m-tall Iriartea gigantea.5. Distribution of
vascular bundles (#/cm2) at successive heights within
the stem of the same Iriartea. 6. Ground parenchyma
cell length in transverse section at successive heights
within the stem of the same Iriartea. 7. Lacunae
diameter in transverse section at successive heights
with the stem of the same Iriartea. For all figures,
points represent the mean of ten measurements and
vertical bars indicate 95% confidence intervals.
patterns of the distribution and size of vascular
bundles and parenchyma cells. Peripheral vascular
bundles near the base of the stem are more than three
times larger than central bundles (2,255 vs. 553 μm),
while both peripheral and central bundles from just
below the crown converge on a mean of 850-950 μm
(Fig. 4). In transverse area, more than 90% of each
peripheral bundle consists of fibers, whereas less than
10% of each central bundle consists of fibers.
Peripheral vascular bundles from near the crown
795
Fig. 8-11. 8. Vascular bundle diameter in
transverse section at breast height for a developmental
sequence of three Welfia georgii stems arranged
according to individual height. 9. Distribution of
vascular bundles (#/cm2) at breast height for the
developmental sequence of Welfia stems. 10. Ground
parenchyma cell length in transverse section at breast
height for the developmental sequence of Welfia
stems. 11. Length/width ratio of ground parenchyma
cells in transverse section at breast height for the
developmental
sequence
of
Welfia
stems.
Length/width ratio is the length of the major
transverse axis divided by the length of the minor
transverse axis.
are narrower than bundles near the stem base. Again,
leaf and inflorescence traces are more abundant near
the crown. In Iriartea, the mean concentration of
vascular bundles near the stem base is about three
times as great in peripheral as compared to central
tissue (30 vs. 11 bundles/cm2), and the concentration
just below the crown increases markedly (248 vs. 54
bundles/cm2 for peripheral and central tissue
respectively; Fig. 5). In the same Iriartea stem, the
mean length of central ground parenchyma cells in
transverse section near the stem base is about twice
that of peripheral parenchyma cells (381 vs. 228 μm;
Fig. 6). Ground parenchyma cell mean length
decreases with increasing height position (to 132 vs.
61 μm for central and peripheral tissue respectively
just below the crown). Mean lacunae diameter of
central tissue decreases markedly with increasing
height, from 470 to 160 μm (Fig. 7).
The general anatomy within individual stems of
Socratea durissima, Euterpe macrospadix, Prestoea
decurrens, and Cryosophila albida is similar to the
anatomy of Welfia and Iriartea. These species have a
greater concentration of vascular bundles at the stem
periphery and peripheral bundles have more extensive
fiber sheaths than do central bundles. Peripheral
vascular bundles from near the crown are narrower
than bundles near the stem base, except in
796
[Vol. 74
Fig. 12-15 Transverse sections (stained with alcian green and safranin) depicting developmental
changes of stem tissue at breast height for Welfia georgii. x3l. 12. Peripheral stem tissue of a young
individual. 13. Peripheral stem tissue of an older individual. 14. Central stem tissue of a young individual.
15. Central stem tissue of an older individual. Ground tissue (G) consists of parenchyma cells. Vascular
bundles consist of the xylem (X), phloem (P), and bundle sheath (B). Slight sustained cell expansion occurs
in fiber cells of peripheral vascular bundles, apparently accompanied by a slight constriction of parenchyma
cells of the peripheral ground tissue. No change in cell size is evident in central
June, 1987]
RICH – PALM STEM ANATOMY
797
Cryosophila, where there is no difference. Leaf and
inflorescence traces are more abundant near the
crown. Vascular bundle concentrations also increase
near the crown. For these species, central parenchyma
cells are larger than peripheral parenchyma cells.
Parenchyma cell size decreases with height position
for all species except Cryosophila.
Developmental changes as inferred from
comparison between stems – Comparison of stem
tissue at breast height from individuals of different
heights of Welfia georgii suggests that peripheral
vascular bundle mean diameter at a given level
increases a small amount during height growth (from
1,137 to 1,348 μm), while central bundle mean
diameter does not change (about 800-850 μm; Fig. 8,
l2-15). Peripheral vascular bundle mean concentration
at breast height decreases with individual height (from
103 to 50 bundles/cm2). while central bundle mean
concentration remains constant (20-25 bundles/cm2;
Fig. 9). Central ground parenchyma cells do not vary
as a function of individual height, with mean lengths
of 150-l70 μm (Fig. l0). Peripheral ground
parenchyma cells at breast height appear to increase in
length and then decrease (from 106 to 135 to 86 μm
with increasing individual height). This small change
may only be reflective of individual variation;
however, there was microscopic evidence that ground
tissue cells in older peripheral tissue may be
constricted by expanding vascular bundles. The mean
length/width ratio of ground tissue cells at breast
height does not differ markedly among individuals of
different heights, and peripheral ground cells have
greater length/width ratios than central cells, with
length/width ratios of near 2.0 for peripheral cells and
near 1.3 for central cells (Fig. 1l).
Comparison of stem tissue at breast height from
individuals of different heights of Iriartea gigantea
demonstrate that peripheral vascular bundle mean
diameter at this level increases more than two-fold
during height growth (from 1,062 to 2,255 μm), while
central bundle diameter does not change (about 500550 μm; Fig. 16, 22-25). With increased individual
height, peripheral vascular bundle mean concentration
at breast height decreases from 140 to 30 bundles/cm2,
while central bundle mean concentration decreases
from near 46 to 11 bundles/cm2 (Fig. 17). Peripheral
ground parenchyma cells at breast height increase in
mean length with individual height from to 85 to 228
μm, while central parenchyma cells increase in mean
length from 22.2 to 38.1 μm (Fig. 18). The mean
width of peripheral parenchyma cells remains
relatively constant between individuals of different
heights (51-60 μm), whereas central parenchyma cells
decrease somewhat in mean width with individual
height (from 104 to 16 μm; Fig. l9). The mean length/
Fig. 16-21. 16. Vascular bundle diameter in transverse section at breast height for a developmental
sequence of four Iriartea gigantea stems arranged
according to individual height. 17. Distribution of
vascular bundles (#/cm2) at breast height for the
developmental sequence of Iriartea stems. 18. Ground
parenchyma cell length in transverse section at breast
height for the developmental sequence of Iriartea
stems. 19. Ground parenchyma cell width in
transverse section at breast height for the
developmental sequence of Iriartea stems. "Width"
refers to the length of the minor transverse axis. 20.
Length/width ratio of ground parenchyma cells in
transverse section at breast height for the
developmental sequence of Iriartea stems. 21.
Lacunae diameter in transverse section at breast height
for the developmental sequence of Iriartea stems.
width ratio of both peripheral and central ground
tissue cells increases markedly with individual height
in Iriartea. Older parenchyma cells become quite
elongate, with length/width ratios near 5.0 in older
peripheral tissue and near 6.0 for older central tissue
(Fig. 20). In the same comparison across individuals
of different heights, mean lacunae diameter of central
tissue increases from 160 to 470 μm (Fig. 21).
As inferred by comparison of individuals of
different heights, fiber cell walls become markedly
thicker during development in both Welfia and
Iriartea (Fig. 26, 27, 30-33). Fibers nearest the
phloem become sclerified first. Individual fiber cells
increase in diameter, quite markedly in Iriartea, and
fiber cell walls become increasingly thick until the
entire lumen is occluded. Stem tissue stiffness
798
[Vol. 74
Fig.22-25. Transverse sections depicting developmental changes of stem tissue at breast height for Iriartea
gigantea. x 31. 22. Peripheral stem tissue of a young individual. 23. Peripheral stem tissue of an older
individual. 24. Central stem tissue of a young individual. 25. Central stem tissue of an older individual. Ground
tissue (G) consists of parenchyma cells with lacunae (L) forming in the central ground tissue. Vascular bundles
consist of the xylem (X), phloem (P), and bundle sheath (B). Sustained cell expansion occurs in fibers within
the bundle sheath of peripheral bundles and parenchyma cells of both peripheral and central ground tissue.
Central vascular bundles remain unchanged in size.
increases markedly with increases in cell wall
thickness (Fig. 28, 29). A front of maturing fibers
moves outward from the phloem to the outer bundle
sheath until all fibers are expanded and sclerified.
Vascular bundles nearest the stem periphery become
sclerified first and central bundles later. In Welfia the
ground tissue cells throughout the stem also become
sclerified. In Welfia, peripheral stem tissue is
June, 1987]
RICH – PALM STEM ANATOMY
distinctly more sclerified than central stem tissue and
there is a continuous decrease in vascular bundle
concentration, proportion of fibrous tissue per bundle,
specific gravity, stiffness, and strength from the stem
periphery to the center (Rich, 1987). In Iriartea, the
older fibers of peripheral vascular bundles become
infused with a dark substance, apparently lignin
containing dark tannins. As a result of this darkening
of the vascular bundles, the outer central cylinder of
older Iriartea appears black. Cell walls of the
peripheral ground tissue in Iriartea become sclerified,
but ground tissue of the inner central cylinder does not
become sclerified. In Iriartea there is a distinct
differentiation between the dark-colored, highly
sclerified outer central cylinder and the light-colored,
spongy inner central cylinder; there is an abrupt
transition from high values for vascular bundle
concentration, proportion of fibrous tissue per bundle,
specific gravity, stiffness, and strength in the outer
central cylinder to low values for these properties in
the inner central cylinder (Rich, 1987).
Though only a single stem was collected for each
of the species Socratea durissima, Euterpe
macrospadix, Prestoea decurrens, and Cryosophila
albida, it is possible to infer some developmental
patterns. Socratea and Euterpe show evidence of
sustained stem thickening, including enlarged vascular
bundles and parenchyma cells. Cryosophila and
Prestoea do not show evidence of sustained stem
thickening. All species have greatly thickened cell
walls of fibers and less extensively thickened
parenchyma cell walls, except Socratea, which does
not have thickening of central parenchyma cell walls.
Socratea has a stem structure quite similar to that of
Iriartea, with stiff black tissue forming at the
periphery and spongy central tissue with many large
lacunae. However, the sclerified region in stems of
Socratea is less extensive than that of Iriartea of
similar height. In both species the black appearance of
peripheral tissue results from darkening of vascular
bundles. Euterpe has greatly enlarged parenchyma
cells in the central stem and some lacunae.
DISCUSSION – It is not possible to follow
developmental changes within individuals because
such study would require impractically long periods of
time and anatomical studies require destructive
sampling. Patterns of development can be inferred by
comparison of individuals at different developmental
stages. All comparative studies of development face
the problem that phenotypic variation may confuse
results, especially when sample sizes are small.
799
Fig.26-29. 26. Fiber cell wall thickness in transverse
section at breast height for developmental sequences
of Iriartea georgii stems arranged according to
individual height. 27. Fiber cell wall thickness as a
function of individual height for Iriartea gigantea. 28.
Elastic modulus as a function of fiber wall thickness
in transverse section from stem tissue at breast height
for Welfia.29. Elastic modulus as a function of fiber
wall thickness for Iriartea. Elastic modulus increases
markedly with increased sclerification of the stem.
Arborescent palms are large and difficult to collect,
and consequently sample sizes are small. Even with
small sample sizes, distinct morphological changes
can be identified that are the result of development
and not phenotypic variation. Such is the case for the
observations that vascular bundles and ground
parenchyma cells expand markedly with development
in Iriartea. Increased sclerification of stem tissue with
development in both Welfia and Iriartea involves
morphologically distinct cell wall thickening and
lignification. The small increase in vascular bundle
diameter in Welfia and small decrease in ground tissue
cell size may be attributed to phenotypic variation;
however, the result is consistent with my finding that
taller individuals have slightly larger stem diameters
(Rich, 1985, 1986; Rich et al., 1986). Wessels Boer
(1968) reported that tall Welfia display sustained stem
thickening, though he also implies that this is
accounted for by expansion of ground parenchyma
cells. Wessels Boer found that cell wall thickening in
fiber cell walls is often accompanied by an increase in
fiber cell diameter, presumably also leading to an
increase in vascular bundle diameter.
The individual fibers in Iriartea widen markedly
before cell walls thicken, accounting for all or part of
the increase in size of vascular bundles. Fibers may
also exhibit extension growth. Such growth would
800
cause increase in the size of the vascular bundles as
lengthening fibers intrude between other fibers. By
this token, one would expect older vascular bundies to
have more fibers in transverse section than younger
fibers. Wessels Boer (1968) found a slight increase in
[Vol. 74
number of fibers per sheath in Geonoma baculifera,
though he suggested that this may be the result of
limited sample size and individual variability.
Protoplasts and nuclei have been observed in even the
oldest thick-walled fibers of palms (Parthasarathy
June, 1987]
RICH – PALM STEM ANATOMY
and Tomlinson, 1967), though no tests have been
reported that demonstrate whether these nuclei are
living. This suggests that active fiber expansion and
sclerification is possible because fibers remain living
for extended periods of time in palms. Living fibers
have been reported for dicotyledonous trees
(Dumbroff and Elmore, 1977).
Because stem tissue is youngest near the top of
the stem and oldest near the base, it is tempting to try
to deduce the basic pattern of development for a
species from anatomical sequences from single stems.
Cells in old tissue are generally larger and more
sclerified than cells of young tissue. Schoute (1912)
and Waterhouse and Quinn (1978) caution against the
use of sequences from single stems because primary
changes also occur as a function of height in the stem.
For example, the length of vessel elements in Sabal
palmetto is shorter toward the crown (Tomlinson and
Zimmermann, 1967). I find that, even after fully
expanding, vascular bundles are smaller, more
abundant, and with a higher proportion per unit area of
leaf and inflorescence traces in tissue from higher in
the stem. Similarly, even after fully expanding, ground
parenchyma cells are also smaller in tissue from
higher in the stem. These represent primary
developmental differences. One could, for instance,
overestimate Welfia's capacity to increase stem
diameter on the basis of comparisons within
individual stems. The basic features of anatomical
changes with height growth are best deduced from
comparison of corresponding height and radial
positions for individuals of different overall heights.
These cautions not withstanding, it is possible to
deduce general patterns in the palms for which only
single mature stems were examined.
Socratea and Euterpe both show evidence of
substantial sustained stem thickening, whereas
Prestoea and Cryosophila show little or no evidence
of sustained stem thickening. These observations are
substantiated by studies of height and stem diameter
allometry (Rich, 1985, 1986; Rich et al., 1986).
Socratea, Euterpe, Prestoea, and Cryosophila all
show evidence of sustained cell wall thickening and
sclerification in fibers and to a lesser degree in
parenchyma cells, a pattern that appears universal
throughout palms, and that parallels dramatic
increases in stem stiffness and strength (Rich, 1987).
CONCLUSION AND SUMMARY – On the basis of
comparison of stem tissue from the same level in
individuals of different heights, Welfia georgii shows
a small increase in vascular bundle size in peripheral
stem tissue during height growth, in keeping with its
limited growth in stem diameter. Pronounced
801
sustained cell expansion in Iriartea gigantea accounts
for larger stem diameters of taller individuals.
Sustained expansion occurs in both fibers and ground
parenchyma cells and large lacunae are formed
between cells in the central stem. Sustained
sclerification, with major thickening of cell walls of
fiber cells, accounts for dramatic increases in stem
stiffness and strength for both species. Socratea
durissima and Euterpe macrospadix also show
evidence of stem thickening by sustained expansion of
both fibers and ground parenchyma cells. Socratea
durissima, Euterpe macrospadix, Prestoea decurrens,
and Cryosophila albida all show evidence of sustained
sclerification in fibers and parenchyma cells. Studies
of developmental anatomy of the palm stem
demonstrate the importance of secondary changes in
the stem below the crown. Sustained cell expansion
allows limited but significant stem diameter increase.
Sustained sclerification results in major increases in
stem stiffness and strength.
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