F.Y.B.Sc.
Sem-I, Paper No-II (2019-20)
Gymnosperm : Gnetum
Dr. R.P. Jadhav, GKG College, Kolhapur
1. Distribution of Gnetum:
Gnetum, represented by
about 40 species is confined to the tropical and humid regions of the world.
Nearly all species, except G. microcarpum, occur below an altitude of 1500
metres. Five species (Gnetum contractum, G. gnemon, G. montanum, G. ula and G.
latifolium) have been reported from India (Fig. 13.1). Gnetum ula is the most
commonly occurring species of India.
Gnetum species occur in India
in the following regions:
Gnetumula:
It is a woody climber
having branches with swollen nodes. It is found in Western Ghats near Khandala,
forests of Kerala, Nilgiris, Godawari district of Andhra Pradesh and Orissa.
Gnetum contractum:
A scandent shrub growing
in Kerala, Nilgiri Hills and Coonoor in Tamil Nadu.
Gnetum gnemon:
A shrubby plant found in
Assam (Naga-Hills, Golaghat and Sibsagar).
Gnetum montanum:
A climber with smooth,
slender branches, swollen at the nodes. It is found in Assam, Sikkim and parts
of Orissa.
Gnetum latifolium:
A climber found in Andaman
and Nicobar Islands.
2. Habit of Gnetum:
Majority of the Gnetum
species are climbers except a few shrubs and trees. G. trinerve is apparently
parasitic. Two types of branches are present on the main stem of the plant,
i.e. branches of limited growth and branches of unlimited growth. Each branch
contains nodes and intemodes Stem of several species of Gnetum is articulated
In climbing species the
branches of limited growth or short shoots are generally un-branched and bear
the foliage leaves. The leaves (9-10) are arranged in decussate pairs (Fig.
13.2). They often lie in one plane giving the appearance of a pinnate leaf to
the branch. The leaves are large and oval with entire margin and reticulate
venation as also seen in dicotyledons. Some scaly leaves are also present.
3. Anatomy of Gnetum:
i) Root:
Young root (Fig. 13.3) has
several layers of starch-filled parenchymatous cortex, the cells of which are
large and polygonal in outline. An endodermal layer is distinguishable.
Casparian strips are seen in the cells of the endodermis. The endodermis follows
4-6 layered pericycle. Roots are diarch and exarch. Small amount of primary
xylem, visible in young roots, becomes indistinguishable after secondary
growth.
Vessels have simple or small
multiseriate bordered pits. Some of the xylem elements have starch grains. Bars
of Sanio are generally absent in the vessels. Phloem consists of sieve cells
and phloem parenchyma.
ii) Young Stem:
The young stem in
transverse section is roughly circular in outline, and resembles with a typical
dicotyledonous stem. It remains surrounded by a single-layered epidermis, which
is thickly circularized and consists of rectangular cells. Some of the
epidermal cells show papillate outgrowths. Sunken stomata are present.
The cortex consists of
outer 5-7 cells thick chlorenchymatus region, middle few-cells thick
parenchymatous region and inner 2-4 cells thick sclerenchymatous region.
Endodermis and pericycle regions are not very clearly distinguishable. Several conjoint,
collateral, open and endarch vascular bundles are arranged in a ring (Fig.
13.5) in the young stem.
Xylem consists of
tracheitis and vessels. Presence of vessels is an angiospermic character.
Protoxylem elements are spiral or annular while the metaxylem shows bordered
pits which are circular in outline. The phloem consists of sieve cells and
phloem parenchyma.
An extensive pith,
consisting of polygonal, parenchymatous cells, is present in the centre of the
young stem.
iii) Old Stem:
Old stems in Gnetum show
secondary growth. In G. gnemon the secondary growth is normal, as seen also in
the dicotyledons. But in majority of the species (e.g., G. ula, G. africanum,
etc.) the anamolous secondary growth is present.
The primary cambium is
ephemeral, i.e., short-lived. The secondary cambium in different parts of
cortex develops in the form of successive rings, one after the other (Fig.
13.6). The first cambium cuts off secondary xylem towards inside and secondary
phloem towards outside. This cambium ceases to function after some time.
Another cambium gets
differentiated along the outermost secondary phloem region, and the same
process is repeated. In the later stages, more secondary xylem is produced on
one side and less on the other side, and thus the eccentric rings of xylem and
phloem are formed in the wood.
This type of eccentric
wood is the characteristic feature of angiospermic lianes. The periderm is thin
and develops from the outer cortex. It also possesses lenticels. The cortex
also contains chlorenchymatous and parenchymatous tissues along with many
sclereids.
In old stems the secondary
wood consists of tracheids and vessels. Tracheids contain bordered pits on
their radial walls while vessels contain simple pits. Transitional stages (Fig.
13.7), containing one to many perforations in the terminal part of the vessels,
are also seen commonly.
In tangential longitudinal
section (T.L.S) of the stem (Fig. 13.8), the wood xylem and medullary rays are
visible. Bordered pits on both the radial and tangential walls are present.
Medullary rays are either uniseriate or multiseriate and consist of polygonal
parenchymatous cells. They are boat-shaped (Fig. 13.8) and their breadth varies
from 2 to many cells. Sieve cells of the phloem contain oblique and perforated
sieve plates.
iv) Leaf:
Internally, Gnetum leaves
also resemble with a dicot leaf. It is bounded by a layer of thickly
circularized epidermis on both the surfaces. Stomata are distributed all over
the lower surface except on the veins. The mesophyll is differentiated
generally into a single-layered palisade and a well-developed spongy
parenchyma.
The latter consists of
many loosely-packed cells. Many stellately branched sclereids are present near
the lower epidermis in the spongy parenchyma. Many stone cells and latex tubes
are present in the midrib region of the leaf.
Several vascular bundles
in the form of an arch or curve are present in the prominent midrib region
(Fig. 13.9). A ring of thick-walled stone cells is present just outside the
phloem. Each vascular bundle is conjoint and collateral.
The xylem of each vascular
bundle faces towards the upper surface while the phloem faces towards the lower
surface. The xylem consists of tracheids, vessels and xylem parenchyma while
the phloem consists of sieve cells and phloem parenchyma.
4. Reproduction of Gnetum:
Gnetum is dioecious. The
reproductive organs are organised into well-developed cones or strobili. These
cones are organised into inflorescences, generally of panicle type. Sometimes
the cones are terminal in position.
A cone consists of a cone
axis, at the base of which are present two opposite and connate bracts. Nodes
and internodes are present in the cone axis. Whorls of circular bracts are
present on the nodes. These are arranged one above the other to form cupulas or
collars (Fig. 13.10). Flowers are present in these collars. Upper few collars
may be reduced and are sterile in nature in G. gnemon.
Male Cone and Male Flower:
The male flowers are
arranged in definite rings above each collar on the nodes of the axis of male
cone. The number of rings varies between 3-6. The male flowers in the rings are
arranged alternately. There is a ring of abortive ovules or imperfect female
flowers above the rings of male flowers.
Each male flower contains
two coherent bracts which form the perianth (Fig. 13.11). Two unilocular
anthers remain attached on a short stalk enclosed within the perianth. At
maturity, when the anthers are ready for dehiscence, the stalk elongates and
the anthers come out of the perianth sheath. In Gnetum gnemon a few (2-3)
flowers are sometimes seen fusing each other (Fig. 13.12).
Development of Male Flower
(Figs. 13.13, 13.14):
In very young cones,
certain cells below each collar become meristematic. They divide repeatedly and
form a small hump-like outgrowth. Certain cells on the upper side of this
annular outgrowth start to differentiate into the initials of the ovules. They
develop into abortive ovules which form the uppermost ring. The cells of the
lower side of this annular outgrowth form the primordium of male flower.
A central cushion of cells
develops by the repeated divisions in the male flower primordium. This cushion
gets surrounded by a circular sheath called perianth. The sheath-like perianth
encloses the central cushion-like mass only partially. With the development of
a depression or notch in the central mass two lobes differentiate and later on
develop into two anther lobes.
With the help of many
divisions the basal portion of this central mass of cells starts to
differentiate into a stalk. This stalk elongates and pushes the anther lobes
towards the outer side. Each anther lobe remains surrounded by an epidermal
layer and a few wall layers which enclose a microsporangium.The innermost wall
layer enclosing the sporogenous tissue is known as tapetum.
The sporogenous cells
become loose, contract, round up and change into the spore mother cells. In the
process of microspore formation the tapetum and two wall layers are used for
the developing microspores. The spore mother cells undergo meiosis and
ultimately the spore tetrads are formed.
The characteristic radial
thickenings develop in the epidermal cells. They help in the dehiscence of
microsporangium. The microspores are ornamented.
Female Cone:
The female cones resemble
with the male cones except in some definite aspects. A single ring of 4-10
female flowers or ovules is present just above each collar (Fig. 13.15). Only a
few of the ovules develop into mature seeds (Fig. 13.15B).
In the young condition,
there is hardly any external difference between female and male cones. All the
ovules are of the same size when young but later on a few of them enlarge and
develop into mature seeds. All the ovules never mature into seeds.
Ovule or Female Flower:
Each ovule (Fig. 13 16)
consists of a nucellus surrounded of three envelopes. The nucellus consists of
central mass of cells. The inner envelope elongates beyond the middle envelope
to form the micropylar tube or style. The nucellus contains the female
gametophyte. There is no nucellar beak in the ovule of Gnetum.
Stomata, sclereids and
laticiferous cells are present in the two outer envelopes. Madhulata (1960)
observed the formation of a circular rim from the outer epidermis,
however, observed the
formation of a second such rim at a higher level. The ovules in G. ula are
stalked.
Pollination:
Wind helps in carrying the
pollen grains up to the micropylar tube of the ovule. The micropylar tube
secretes a drop of fluid in which certain pollen grains get entangled and reach
up to the pollen chamber. The nucellus cells below the pollen chamber are full
of starch.
Fertilization:
The fertilization in
Gnetum has been studied only by a few workers. Vasil (1959) studied this
phenomenon in G. ula. At the time of fertilization, the pollen tube pierces
through the membrane of the female gametophyte just near to a group of densely
cytoplasmic cells. The tip of pollen tube bursts and the male cells are
released. One of the male cells enters the egg cell.
The male and female nuclei,
after lying side by side for some time, fuse with each other and form the
zygote. According to Swamy (1973), the only identifying features of the zygote
are its spherical shape and dense cytoplasm. Both the male cells of a pollen
tube may remain functional if two eggs are present close to the pollen tube.
In some cases the upper
portion may become cellular instead of the lower portion. Sometimes only the
middle portion may become cellular and in still other cases there may not be
any
The Embryo:
The embryo development in
several species of Gnetum has been studied by many different workers including
Lotsy (1899), Coulter (1908) and Thompson (1916), but the details put forward
by these wokers are highly variable.
Seed:
Gnetum seeds (Fig. 13.26)
are oval to elongated in shape and green to red in colour. It remains
surrounded by a three-layered envelope which encloses the embryo and the
endosperm. Outer envelope is fleshy, and consists of parenchymatous cells. It
imparts colour to the seed.
The middle envelope is
hard, protective and made up to three layers, i.e., outer layer of
parenchymatous cells, middle of palisade cells and innermost fibrous region.
The inner envelope is parenchymatous. Branched vascular bundles traverse
through all the three envelopes.
Germination of Seed:
Germination is of epigeal
type (Fig. 13.27). The cotyledons are pushed out of the seed. The hypocotyl
elongates, and this brings the cotyledons out of the soil. The first green
leaves of the plant are formed by the cotyledons. The first pair of foliage
leaves is produced by the development of plumule. A persistent feeder is
present up to a very late stage in the seed.
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