Download Radial secondary growth and formation of successive cambia and

Botanical Journal of the Linnean Society, 2008, 158, 30–40. With 16 figures
Radial secondary growth and formation of successive
cambia and their products in Ipomoea hederifolia L.
(Convolvulaceae)
KISHORE S. RAJPUT*, VINAY M. RAOLE and DHARA GANDHI
Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda,
Vadodara – 390 002, India
Received 29 September 2007; accepted for publication 17 March 2008
Ipomoea hederifolia stems increase in thickness using a combination of different types of cambial variant, such as
the discontinuous concentric rings of cambia, the development of included phloem, the reverse orientation of
discontinuous cambial segments, the internal phloem, the formation of secondary xylem and phloem from the
internal cambium, and differentiation of cork in the pith. After primary growth, the first ring of cambium arises
between the external primary phloem and primary xylem, producing secondary phloem centrifugally and secondary
xylem centripetally. The stem becomes lobed, flat, undulating, or irregular in shape as a result of the formation
of both discontinuous and continuous concentric rings of cambia. As the formation of secondary xylem is greater
in one region than in another, this results in the formation of a grooved stem. Successive cambia formed after the
first ring are of two distinct functional types: (1) functionally normal successive cambia that divide to form
secondary xylem centripetally and secondary phloem centrifugally, like other dicotyledons that show successive
rings, and (2) abnormal cambia with reverse orientation. The former type of successive rings originates from the
parenchyma cells located outside the phloem produced by previous cambium. The latter type of cambium develops
from the conjunctive tissue located at the base of the secondary xylem formed by functionally normal cambia. This
cambium is functionally inverted, producing secondary xylem centrifugally and secondary phloem centripetally. In
later secondary growth, xylem parenchyma situated deep inside the secondary xylem undergoes de-differentiation,
and re-differentiates into included phloem islands in secondary xylem. © 2008 The Linnean Society of London,
Botanical Journal of the Linnean Society, 2008, 158, 30–40.
ADDITIONAL KEYWORDS: cambial variant – included phloem – internal cambium – intraxylary cork –
interxylary phloem – reverse cambium.
INTRODUCTION
During the course of evolution, plants have evolved
different modes of secondary thickening for mechanical support and safety of vessels for hydraulic conductivity. From an evolutionary viewpoint, it has been
accepted that normal cambium is ancestral in dicotyledons and that the origin of variant types has very
probably been phylogenetic, as they represent an
alternative solution to the production of secondary
tissues (Carlquist, 1988). Of these, the formation of
successive cambia is most common and is usually
found in climbing species, whereas, in trees, it is
*Corresponding author. E-mail: [email protected]
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either rare or almost non-existent. Although its occurrence has been reported in some species (Schenck,
1893; Pfeiffer, 1926; Chalk & Chattaway, 1937;
Metcalfe & Chalk, 1950; Philipson & Ward, 1965;
Philipson, Ward & Butterfield, 1971; Carlquist, 1988;
Rajput & Rao, 1998, 1999; Rao & Rajput, 1998, 2000)
these species are restricted to only a fraction of
dicotyledonous taxa. In all the reported cases, the
development of successive cambia varies in its origin
from species to species. Successive cambia usually
form secondary xylem centripetally (towards the
centre of the stem) and secondary phloem centrifugally (towards the periphery of the stem). In Ipomoea
hederifolia, however, the development of successive
cambia occurs from the parenchyma cells located four
© 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 30–40
CAMBIAL VARIANT IN IPOMOEA HEDERIFOLIA L.
to six cells away from the phloem produced by previous cambium. However, cambia developing from conjunctive tissues show a functionally reverse nature,
which has not been reported so far in any of the
plants showing variant secondary growth.
The formation of successive cambia has been known
for a long time and tends to occur in different
members of Convolvulaceae (Metcalfe & Chalk, 1950;
Pant & Bhatnagar, 1975; Lowell & Lucansky, 1986;
Carlquist & Hanson, 1991). Various workers have
studied the stem and wood anatomy of Convolvulaceae, the most important contributions being those
of Metcalfe & Chalk (1950), Mennega (1969), Lowell
& Lucansky (1986), and Carlquist & Hanson (1991).
However, most of these studies are based on dried
wood samples, with fewer studies on wet preserved
samples. D’Almeida & Patil (1945, 1946) described
the xylem structure of some of the species of Convolvulaceae, but did not pay much attention to variant
secondary growth. Similarly, Pant & Bhatnagar
(1975: 64) concentrated their studies on the development of medullary bundles (interxylary phloem) in
two species of Argyreia, but paid little attention to the
pattern of secondary growth.
Ipomoea is the largest and most diverse genus of
the family, comprising about 500 species (Angiosperm
Phylogeny Group website, version 8, June 2007,
http://www.mobot.org/MOBOT/research/APweb), generally characterized by a vine growth habit
(McDonald, 1992). Ipomoea hederifolia L. has been
introduced worldwide by cultivation because it has
ornamental flowers, and is naturalized in the palaeotropics (Lowell & Lucansky, 1986: 382), but most wild
and escaped cultivated species are noxious weeds
(Lowell & Lucansky, 1990: 232). Although the vegetative anatomy and morphology of I. hederifolia have
been studied previously (Lowell & Lucansky, 1986),
most attention has been paid to the structural details
of the xylem and phloem. In the present study, we
report certain additional features which have not
been described previously for this species. These data
contribute to a better understanding of the taxonomy
of I. hederifolia, add to the existing knowledge of vine
anatomy, and reveal a new type of cambial variant
that has not been reported previously in any species
of Ipomoea.
MATERIAL AND METHODS
Ten to 15 segments of the main stem [from young
(3 mm) stem to the maximum thickness (22–40 mm)
available; 40–60 mm in length] were collected from
ten plants of I. hederifolia (Convolvulaceae) growing
in Panch Mahal forest (north-eastern Gujarat), Saurashtra region (Rajkot, western Gujarat), and Dediapada forest (Gujarat State) (i.e. ten plants from all
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three areas). These samples were collected three
times (October, December, and March), fixed immediately in formalin–acetic acid–ethanol solution, dehydrated with a tertiary butanol series, and processed
by the routine paraffin embedding method (Johanson,
1940). Thick/woody stem samples were also sectioned
on a sliding microtome. Transverse, radial, and tangential longitudinal sections, 12–15 mm thick, were
obtained with a rotary and sliding microtome, and
stained with a combination of safranin and fast green
(Johanson, 1940).
To obtain the length and width of vessel elements,
fibriform vessel elements, and xylem fibres, small
pieces of xylem adjacent to the cambium of each
successive ring were macerated with Jefferey’s
solution (Berlyn & Miksche, 1976) at 55–60 °C for
24–36 h and stained with safranin to study the
general morphology and size of the vessel elements
and fibres. One hundred objects were chosen randomly to obtain the mean and standard deviation.
Important results were microphotographed with a
Leica trinocular research microscope.
RESULTS
MORPHOLOGY
OF THE STEM
Morphologically, the stem portion adjacent to ground
level and not in contact with any supporting object
remains circular in outline and shows distinct concentric rings of cambia. However, the portion away
from the ground and in contact with the supporting
object shows differential cambial activity, ultimately
resulting in a lobed pattern. The number of lobes is
not constant and may vary for a given plant. The
stem is often deeply furrowed, flattened, broadly
lobed, or cylindrical. The change in the shape of the
stem may be correlated with cambial variants and an
unequal production of secondary vascular tissues in
the stem. It has been observed that a lobed pattern
forms as a result of increased cambial activity on the
opposite side of the stem to the support.
DEVELOPMENT
OF VASCULAR CAMBIUM
A normal vascular cambium arises as a continuous
cylinder between the external phloem and metaxylem. The cambium functions in a normal way, as for
other dicots, for a short time and produces several
layers of secondary phloem centrifugally and secondary xylem centripetally. The secondary phloem
consists mainly of sieve tubes, companion cells, and
phloem parenchyma cells, whereas the secondary
xylem consists of vessels, fibriform vessels, fibre
tracheids, and xylem ray parenchyma cells. This
cambium remains functional for a definite period of
time, producing 5–6 mm of secondary xylem, which
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K. S. RAJPUT ET AL.
Figures 1–6. Transverse section of stem showing origin of cambium in Ipomoea hederifolia. Fig. 1. Parenchyma cells
external to the phloem showing irregular divisions (arrow) that lead to the formation of a wide meristematic band.
Arrowhead indicates the initiation of cell division in the parenchyma cells external to the phloem. Fig. 2. Repeated
periclinal divisions in a wide meristematic band differentiating into radially arranged cambial cells (arrowheads). Note
the recently developed sieve tube element (arrow). Fig. 3. Development of new cambial segment (showing reverse
orientation) from the conjunctive tissue located at the base of the secondary xylem. Arrow and arrowhead indicate position
of secondary xylem and phloem, respectively. V, vessel. Fig. 4. Enlarged view of Figure 3 showing reverse orientation of
secondary xylem (arrow) and phloem (arrowhead). Fig. 5. Recently differentiated cambium from the secondary cortex
showing secondary xylem in upper right-hand corner (arrow) and, on the left, differentiating cambial segment (arrowhead). Fig. 6. Outermost two successive rings of secondary xylem showing normal secondary xylem and phloem. Scale
bars: Figs 1–5, 75 mm; Fig. 6, 225 mm.
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later ceases to divide. Up to this stage, the protoxylem
remains unchanged, but clusters of internal phloem
increase slightly in size as a result of the differentiation of additional pith cells into sieve tube elements.
The cortex consists of discontinuous bands of perivascular fibres, large thin-walled parenchyma cells,
secretory structures, and druse-containing cells.
A second ring of cambium develops from the axial
parenchyma cells at a distance of about three to
six cell layers outside the phloem produced by the
previous cambium. During the development of new
cambium, one or two parenchyma layers undergo
repeated divisions and result in the formation of fiveto six-layered wide bands of meristematic cells. Initially, these cells are arranged irregularly, but further
periclinal divisions lead to a radial arrangement
(Figs 1, 2). Cells in the centre of this band differentiate into vascular cambium, whereas the remaining
cells on either side of the newly formed cambium
differentiate into conjunctive tissues centripetally,
and the outer cells into secondary cortex. These conjunctive tissues form the site for the origin of reverse
cambium (Figs 3, 4). The newly developed cambium
from the axial parenchyma is functionally normal,
producing secondary xylem towards the centre and
secondary phloem towards the periphery (Figs 5, 6).
Each functionally normal successive cambium follows
a similar pattern of development. The first ring of
cambium forms a continuous band of xylem. In the
second ring onwards, the differentiation of conducting
elements of both the xylem and phloem remains
restricted to certain segments of the meristem, with
the remainder forming thin-walled conjunctive tissue
on both the outer and inner sides.
The mature stem is composed of five to six successive rings of xylem alternating with phloem (Fig. 7).
In these rings of secondary xylem, functionally
normal cambium is produced. In most of the rings,
there remains another discontinuous ring of secondary xylem, which is produced by functionally reverse
cambium and has a reverse orientation, i.e. secondary
xylem arranged centrifugally and secondary phloem
centripetally (Figs 7, 8). However, these cambia do
not form a complete ring, but rather are in the form
of small segments present only on the lateral side of
the flat stem. In some of the stem segments, tangentially arranged bicollateral vascular bundles are also
observed. The xylem is diffuse and porous with indistinct growth rings, and is composed of both wider
vessel elements and narrow fibriform vessels, which
are indistinguishable in transverse view. Vessels are
mostly solitary with a simple perforation plate on
their slightly oblique to transverse end walls (length,
237–476 mm; width, 134–260 mm). The alternate bordered pits on the lateral walls are oval to elliptic or
oblong (diameter, 7–10 mm). Tyloses are common in
the wider vessels in all successive rings and are
mostly thin walled.
As in other members of Convolvulaceae, fibriform vessels are also recorded in I. hederifolia.
Fibriform vessels are similar to imperforate tracheary
elements, except for the occurrence of a small subterminal perforation plate near each end of the cell. It is
difficult to distinguish fibriform vessels in transverse
view, and their dimensional details are studied either
in longitudinal sections or in macerated material. The
length and width of fibriform vessel elements vary
from 478 to 518 mm and 28 to 34 mm, respectively.
Xylem rays are mostly uniseriate, but biseriate rays
are also frequent. Ray cell walls are thick and lignified, measuring about 1.2–1.4 mm. Conjunctive tissues
are mostly thin-walled, unlignified parenchyma,
forming a band of six to ten cells in each radial file
and providing a site for the origin of reverse
cambium.
ORIGIN
OF FUNCTIONALLY REVERSE CAMBIA
The second ring of cambium is functionally bidirectional, as in other dicotyledons that form successive
rings of cambia. After the formation of 15–20 xylem
elements by this cambium, the parenchyma (conjunctive tissue) adjacent to the inner side of xylem derivatives undergoes periclinal divisions and results in the
© 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 30–40
CAMBIAL VARIANT IN IPOMOEA HEDERIFOLIA L.
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Figures 7–10. Transverse view of secondary xylem in the stem of Ipomoea hederifolia. Fig. 7. Successive rings of
secondary xylem alternating with phloem. Note the discontinuous segments of xylem in the second ring (from lower side)
showing xylem with reverse orientation (also see Fig. 8). Fig. 8. Enlarged view of small portion from Figure 7 showing
xylem with reverse orientation. Fig. 9. Origin of functionally reverse cambium (arrowhead) from the conjunctive tissue
at the base of the xylem produced by previous cambium. Arrow indicates recently differentiated secondary phloem. Note
the relatively large sized conjunctive tissue in the left corner. V, vessel. Fig. 10. Xylem showing the junction between two
opposite xylem regions. Note the conjunctive tissue between the two opposite xylem regions. Scale bars: Figs 7, 8, 225 mm;
Figs 9, 10, 75 mm.
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development of a cambial zone three to four cells wide
(Figs 3, 4, 9). This cambium is functionally abnormal
(reverse), producing xylem towards the periphery
and phloem towards the centre (Figs 4, 10). Before the
cessation of cambial cell division in the second normal
cambium, the reverse cambium produces around
20–25 xylem derivatives and 8–10 phloem elements.
The development of further reverse cambia follows a
similar pattern. Structurally considerable variations
are also observed in the secondary xylem of functionally reverse cambia. It is mostly composed of very
narrow fibriform vessels, fibre tracheids, and axial and
ray parenchyma cells. Wide vessels are rare or absent.
They are mostly solitary with a simple perforation
plate on their slightly oblique to transverse end walls.
In comparison with normal secondary xylem and
phloem, smaller amounts are produced, measuring
about 6–9 mm (xylem) and 3–5 mm (phloem).
DEVELOPMENT
OF INTRAXYLARY PHLOEM AND CORK
Intraxylary or internal phloem consists of strands of
phloem between the periphery of the pith and inside
the primary xylem of the stem. An unexpected finding
of the present study in I. hederifolia is that cambial
action occurs at the inner face of intraxylary phloem
(Figs 11, 12). This meristem (referred to here as
internal cambium) divides bidirectionally and forms
conducting elements of xylem and phloem in the
opposite direction, i.e. secondary xylem towards the
periphery and secondary phloem towards the centre.
The activity of this cambium is evident not just by the
crushing of protophloem, but also by the repeated cell
divisions in the meristem (internal cambium) and by
the differentiation of secondary xylem and intraxylary
phloem (Fig. 12). In mature stems, the amount of
xylem and phloem produced by the internal cambium
is relatively greater. Therefore, it exerts a pressure on
the pith and results in its complete obliteration
(Figs 11, 13). This cambium originates first on those
sides of the stem that do not touch the support. Later,
it develops on the other side of the stem; however,
only two segments, which are formed at the beginning, produce significant amounts of secondary xylem
and phloem; the remaining segments produce a
limited amount of secondary xylem and phloem. The
xylem produced from this cambium is composed of
fibre tracheids, wider vessels, narrow (fibriform)
vessels, and axial and ray parenchyma. Similarly, the
phloem consists of sieve tube elements, companion
cells, and axial parenchyma cells. The length and
width of the vessel elements remain more or less
similar to those of the vessel elements produced from
functionally normal and reverse cambium.
At the end of the growing season (i.e. February–
May), concomitant with the increase in the amount of
intraxylary phloem and xylem, the pith cells collapse
and undergo obliteration (Figs 11, 15), followed by
formation of an empty cavity in the centre. Pith cells
towards the cavity and adjacent to the internal
phloem undergo periclinal division and differentiate
into phellogen/cork cambium. Initially, the cork
cambium develops opposite the internal cambial arcs
that are functionally more active (Figs 15, 16). Thereafter, it originates opposite other small arcs of the
internal phloem cambium, which are functionally less
active. The formation of more and more internal
phloem and cork pushes the pith cells towards the
centre, crushing the pith cells. Some of the parenchyma cells adjacent to the cork cells also accumulate
phenolic compounds and form a boundary around the
cork cells (Figs 15, 16).
DEVELOPMENT
OF INCLUDED PHLOEM
Concurrent with or slightly later than the development of the successive ring of cambium, small isolated islands of included phloem arise from the xylem
parenchyma (Figs 13, 14). However, the parenchyma
cells situated deep inside the secondary xylem
undergo radial and tangential divisions, followed by
differentiation into isolated islands of included
phloem (Fig. 14). The cellular composition of these
islands is similar to that of normal phloem, and
consists of narrow sieve tubes, companion cells,
phloem parenchyma, and, occasionally, drusecontaining cells.
DISCUSSION
Convolvulaceae is currently regarded as containing
56 genera with 1625 species. Of these, Ipomoea is the
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CAMBIAL VARIANT IN IPOMOEA HEDERIFOLIA L.
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Figures 11–16. Transverse view of mature stem showing internal phloem, included phloem, and cork cambium in the
pith region. Fig. 11. Pith region of the mature stem showing secondary xylem and phloem differentiated from the internal
phloem cambium (arrows). Note the vessel elements towards the protoxylem and phloem towards the centre of the pith.
Fig. 12. Enlarged view of the pith in a relatively young stem showing distinct cambium (arrowhead) and vessel element.
V, vessel. Fig. 13. Arrowhead showing included phloem islands in the secondary xylem. Fig. 14. Enlarged view of the
secondary xylem showing included phloem islands (arrowhead). Fig. 15. Pith region of the mature stem showing
secondary xylem and phloem differentiated from the internal phloem cambium. Dark black bands in the centre represent
obliterated pith cells. Fig. 16. Enlarged view of the pith region showing the formation of cork. Arrowhead shows cork
cambium with distinct nucleus and cambial cells arranged in radial files. Also note the heavy accumulation of phenolic
compounds (PC) in crushed pith cells. Scale bars: Figs 11, 15, 75 mm; Figs 12–14, 16, 225 mm.
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largest and most diverse genus of the family, comprising about 500 species (Angiosperm Phylogeny Group
website, version 8, June 2007, http://www.mobot.org/
MOBOT/research/APweb). Generally, it is characterized by a vine growth habit (McDonald, 1992: 262).
The stem and wood anatomy of Convolvulaceae has
been studied by different workers, with the most
important contributions being those of D’Almeida &
Patil (1945, 1946), Metcalfe & Chalk (1950), Mennega
(1969), Pant & Bhatnagar (1975), McDonald (1981),
Lowell & Lucansky (1986), Carlquist & Hanson
(1991), and McDonald (1992). These studies have
shown that most of the genera form successive
cambia, and that intraxylary phloem is a characteristic feature of the family. Although I. hederifolia has
been studied previously (Lowell & Lucansky, 1986),
we report some additional features which have not
been described before.
The stem shape in I. hederifolia is a result of the
bidirectional activity of the original cambium and
successive rings of cambia, with the production of
increased vascular tissues on opposing sides of the
stem. This unequal production of secondary vascular
tissues is correlated with the development of the
cambial variant (Lowell & Lucansky, 1986: 386). The
location of increased cambial activity on opposing
sides of the stem is associated externally with those
sides of the stem that do not touch a support. Carlquist (1975) reported that stems of lianas are often
asymmetric in cross-section because of the addition of
vascular tissues in relation to twining support. A
similar correlation between the position of the
cambium and stem support has also been established
in I. versicolor (Scott, 1891) and I. quamoclit (Lowell
& Lucansky, 1990: 242). Based on the primary and
secondary growth, the developmental anatomy of
I. hederifolia can be classified into five stages. Two are
in primary growth: (1) the bicollateral bundle stage,
and (2) the cambium-like meristem stage. Three are
in secondary growth: (3) normal cambial stage, (4) the
anomalous cambial stage, and (5) the supernumerary
cambial stage (Lowell & Lucansky, 1986: 387).
Successive supernumerary cambia are common in
woody species of Ipomoea, regardless of their growth
habit (Metcalfe & Chalk, 1950: 961; McDonald, 1981;
Lowell & Lucansky, 1986: 393, 1990: 243). In I. hederifolia, the first ring of cambium remains functional
for a definite period, and later ceases to divide. A new
ring of cambium is developed from the axial parenchyma cells at a distance of about three to six cell
layers outside the phloem produced by the previous
cambium. D’Almeida & Patil (1945, 1946) and Pant &
Bhatnagar (1975) have reported similar observations
in other species of Ipomoea and Argyreia. However,
the development of successive cambia of many plant
families generally arises external to the areas of
specialized vascular growth of normally positioned
cambium (Studholme & Philipson, 1966; Esau &
Cheadle, 1969; Lowell & Lucansky, 1986, 1990;
Rajput & Rao, 1999, 2000, 2002; Rajput, 2001, 2002).
Prior to the formation of new cambium, parenchyma
cells three to six cell layers away from the phloem
produced by the previous cambium undergo repeated
divisions and form a wide band of meristematic cells.
From this band, central cells differentiate into a new
cambium, whereas surrounding cells differentiate
into conjunctive tissues to the inside and parenchyma
cells to the outside of the newly developed cambium.
However, the parenchyma cells developed from the
wide band of meristematic cells outside the newly
developed cambium act as a site for new cambium in
the future (Rajput & Rao, 2000, 2002; Rajput, 2001,
2002: 226). All of the investigated species in which
successive cambia have been reported always produce
xylem towards the centre and phloem towards the
periphery. An unexpected finding of the present
study in I. hederifolia, however, is the development
of cambium that is functionally abnormal. This
cambium produces xylem centrifugally and phloem
centripetally. To our knowledge, this type of cambial
variant has not been reported previously in I. hederifolia. The information generated from this study can
be added to our previous report on Dolichos (Rajput,
Rao & Patil, 2006: 68).
An internal cambium arises in the mature stems of
I. hederifolia from parenchyma cells between the
primary xylem and internal phloem. It produces
secondary phloem centripetally and secondary xylem
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centrifugally. No mention is made of the development
of secondary xylem having a reverse orientation and
its differentiation from internal cambium in a previous study (Lowell & Lucansky, 1986). The development of internal phloem remains characteristic to
some families of dicotyledons, with Convolvulaceae
being one of them. This internal cambium is functionally bidirectional, producing secondary xylem centrifugally and secondary phloem centripetally. The
formation of such internal cambium, similar in origin,
position, and product, also arises in the pith region of
I. versicolor (Scott, 1891), Calystegia (Philipson et al.,
1971), I. quamoclit (Lowell & Lucansky, 1990: 244),
and Ericybe coccinea, Operculina palmerie, and Stictocardia benaviensis, members of the same family
(Carlquist & Hanson, 1991: 60, 62). This cambium
originates first on those sides of the stem that do not
touch a support. Later, it develops on the other side of
the stem, but only two segments of the cambium,
which are produced at the beginning, give rise to
more secondary xylem and phloem. The other segments are suppressed, with a limited amount of secondary xylem and phloem, giving an impression of
vascular bundles embedded in the pith region.
According to Carlquist & Hanson (1991: 89, 90), this
type of cambial action has not been reported previously in any species of Ipomoea. Thus, one more
genus can be added to the list of genera that produce
an interxylary phloem cambium which divides bidirectionally, forming secondary xylem centrifugally
and secondary phloem centripetally.
The occurrence of interxylary cork in the perennial
organs of some angiosperms has been reported previously in Epilobium and in several species of Artemisia
(Moss, 1934, 1936, 1940; Diettert, 1938; Moss &
Gorham, 1953; Ginzburg, 1964). Most of the work
performed up to 1940 was treated thoroughly by
Moss & Gorham (1953). This article listed around 40
species with interxylary cork and splitting of the
stem, and aimed to correlate various reports with the
hope of stimulating further research in this area. In
the present study in I. hederifolia, the development of
intraxylary cork is reported for the first time. To our
knowledge, this is the first report for Indian plants.
Previous workers have reported interxylary cork in
the secondary xylem, but in I. hederifolia it is formed
in the pith region and not as an interxylary cork.
Various physiological functions have been ascribed
to the formation of interxylary cork. Moss (1940: 768)
proposed that interxylary cork was formed as a protection mechanism against ‘damage from animals,
pathogens and wind blown soil’, and helped plants to
retard the loss of water and to restrict its upward
movement through relatively narrow wound zones.
Ginzburg (1964) added that, ‘apart from its being a
protection layer, interxylary cork also functions as a
separating layer in extremely dry habitats, such as
the Negev of Israel’. In I. hederifolia, the formation of
intraxylary cork may be associated with a similar
function, i.e. for protection, and requires further
study.
Later, ontogenetically, a foraminate pattern of
included phloem islands develops from the xylem
parenchyma of mature stems. The formation of included phloem islands by the differentiation of xylem
parenchyma, or centripetal differentiation of phloem
elements directly from the cambium, has been
reported in the stems of I. versicolor (Scott, 1891) and
I. quamoclit (Lowell & Lucansky, 1990: 241). In
I. hederifolia, parenchyma cells situated deep inside
the secondary xylem undergo differentiation into
included phloem islands; no direct differentiation of
phloem from the cambium towards the xylem side is
observed.
Structurally, the xylem produced from both functionally normal and reverse cambia shows considerable variation. The number of xylem elements
produced by reverse cambium is relatively less than
the elements produced by functionally normal
cambium. Xylem produced from reverse cambium is
mostly composed of fibriform vessels, fibre tracheids,
and axial and ray parenchyma cells. Vessels with
larger diameter are few or almost absent. Xylem
produced from normal cambium is composed of
vessels (both wide and fibriform vessels), xylem fibres,
and ray and axial parenchyma cells. An interesting
feature of I. hederifolia xylem is the presence of vessel
dimorphism, which has been reported in several
species of Convolvulaceae (Pant & Bhatnagar, 1975:
62; Carlquist & Hanson, 1991: 87). Vessel dimorphism
is a term that can be applied to the presence of wide
plus narrow vessel elements (Carlquist, 1981: 326;
Carlquist & Hanson, 1991: 74). Fibriform vessel elements are considered to be at least as effective as
tracheids in resisting the formation of air embolism in
vessels, because air embolism forms far less commonly in narrow vessels than in wide ones (Ellmore &
Ewers, 1985; Carlquist & Hanson, 1991: 74). Thus,
fibriform vessel elements, which are common in Convolvulaceae and other lianas, can form a subsidiary
conductive system with a degree of conductive safety
virtually as high as that provided by tracheids
(Ayensu & Stern, 1964; Carlquist, 1991: 151).
The occurrence of a thin-walled, unlignified band of
parenchyma (also called conjunctive tissue) followed
by every xylem ring in I. hederifolia may be associated with its scandent habit. One of the potential
advantages claimed for such a conformation is the
protection of vessels from damage during torsion in
stems of lianas (Schenck, 1893; Carlquist, 1975). This
explanation appears to be valid, because protection
from injury is presumably a better option than recov-
© 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 30–40
CAMBIAL VARIANT IN IPOMOEA HEDERIFOLIA L.
ery from injury. Water storage is another potential
function of the large parenchyma zone in a liana stem
(Carlquist & Hanson, 1991: 79).
ACKNOWLEDGEMENTS
One of the authors (KSR) is thankful to the Department of Biotechnology (DBT), Government of India
for financial support. The authors are also grateful to
both of the anonymous referees, Professor Stephen L.
Jury, and Dr S. K. Karandikar, The Maharaja Sayajirao University of Baroda (MSU) for their valuable
comments on the manuscript.
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