In this article we will discuss about the life cycle of gnetum.
The Sporophyte of Gnetum:
The plants are mostly lianes (G. ula), which climb or trail over other vegetation, often reaching the tops of the tallest trees. A few species are, however, small trees (G. gnemon, G. costatum) and shrubs. The leaves are simple, large, oval, entire, exstipulate, leathery, and with short petioles.
They show pinnately reticulate venation resembling the foliage leaves of dicotyledons, and are arranged on the stem in opposite-decussate pairs.
The branches are of two kinds:
i. One of limited growth or dwarf shoot, and
ii. The other of unlimited growth or long shoot.
In climbing species the foliage leaves are borne on dwarf shoots only, while those on long shoots are usually reduced to scales. In case of a tree (G. gnemon), the differences between these shoots are very much reduced.
In transverse section of a young stem of Gnetum, whether belonging to the arborescent types (G. gnemon) or to the climbing ones (G. ula), shows a somewhat circular outline. The epidermis in single-layered and made up of rectangular cells, whose outer walls are thick and heavily cuticularized; sunken stomata are present. The cortex is many-layered and consists of polygonal or rounded parenchyma cells with chloroplasts.
Lying within the cortical tissue a number of scattered sclerotic cells or sclereids are found, which in little older stems form a conspicuous irregular ring. This is commonly known as the ring or zone of spicular cells. The endodermis as well as the pericycle, however, cannot be clearly differentiated. The vascular bundles, varying in number, are arranged in a circle, and are collateral and open in nature.
The phloem consists of sieve-tubes, phloem parenchyma and well- developed phloem fibres. The xylem is endarch and made up mainly of tracheids and a few vessels or trachea. The presence of true vessels is an angiospermic character found in Gnetum.
Broad and long, conspicuous medullary rays separate the vascular bundles from one another. The pith is more or less circular and consists of thin-walled parenchyma cells. In somewhat older stems, however, the cells of the pith lying towards the vascular bundles become lignified.
Secondary growth in the stems of Gnetum takes place in the normal fashion. In the case of lianes, however, though secondary growth commences normally, during later stages, secondary cambium arises at different regions within the deeper layers of the cortex. As a result, anomalous rings of secondary vascular bundles (either complete or incomplete) are produced successively.
A transverse section of a young root of Gnetum exhibits the following structure – the epiblema is single-layered. The cortex is composed of a large number of layers made up of conspicuous starch grain-containing polygonal cells; within these cortical cells numerous fibres can be observed.
Below the cortex there is an endodermis, which encloses within it a few-layered pericycle. The vascular bundles are diarch with a rather poor amount of primary xylem. The pith is also inconspicuous. Secondary growth is of the normal type, some of the xylem elements in a mature root contains starch grains.
A cross-section of the leaf of Gnetum seems to differ in no essential feature from that of a dicotyledon. The epidermal layers are provided with a thick cuticle. Numerous stomata occur entirely on the lower epidermis. The mesophyll consists of a single layer of upper palisade cells, and a well-developed lower spongy tissue.
Within the latter stellately branched sclereids occur, particularly near the lower epidermis. A large number of latex tubes and fibres are present within the mesophyll tissue, more conspicuously in the midrib region of the leaf.
In the midrib the vascular bundles are arranged forming a distinct curve. In some species a curved ring of sclereids is formed outside the phloem zone, whose cells are arranged in regular rows. Above the phloem there lies the xylem consisting of tracheids, vessels and xylem parenchyma.
In Gnetum the microsporophylls and megasporophylls form different strobili and the plants are mostly dioecious.
Staminate (or Male) Strobilus:
Staminate strobilus is a cone-like structure formed by the aggregation of several pairs of decussate bracts on, a slender axis. It is axillary or terminal, solitary or catkin-like cluster. The bracts are connate throughout and look like small cups.
In the axils of these bracts end surrounding the axes, numerous microsporophylls forming staminate flowers develop. Each staminate flower consists of a stalk, bearing at its tip, two anthers, each with a single, microsporangium, and invested at the base by two connate bracts (the perianth).
The presence of the perianth is a striking angiospermic character. The flowers mature in an acropetal order. Our knowledge about the development of microsporangium and microspores (pollen grains) is still very incomplete. Within the microsporangium gradually sporogenous cells become differentiated, and from these ultimately, by reduction division, spore tetrads are formed.
The outer wall of the anther consists of a single layer of cells, and the tapetum is formed from the outer sporogenous tissue, and not from the wall. As the microsporangia mature, all the cells between the microspores and the epidermis break, and the microspores are liberated.
Ovulate Strobilus:
The structure of the ovulate strobili resembles, in general, that of the staminate strobili, but are compound, since the mega-sporangia or ovules are borne on the secondary axis. The ovules or mega-sporangia arising in the axils of connate bracts. These are five or six whorls of ovules surrounding the axis, arid with five to seven ovules in each whorl. Sometimes the axis is terminated by a single ovule.
Though there are numerous ovules, only a few attain maturity. Each ovule is invested with two integuments; the inner one becomes prolonged into a micropylar tube, which is a characteristic feature of the genus.
The nucellus has all the characteristic features of the gymnosperms, consisting of a heavy mass of sterile tissue overlying the megaspore mother cell. It is quite probable that in some cases more than one megaspore mother cells may be organized within a single ovule.
The tip of the nucellus becomes more or less disorganized and shallow (G. gnemon), and this is the only trace of the pollen chamber found in other gymnosperms. A deeper and narrower pollen chamber may be formed in some cases (G. africanum).
The megaspore mother cell, as usual by reduction division, forms four cells, each of which may become an embryo-sac; thus, when several megaspores begin to function, several gametophytes may be produced within a single ovule. Other details about the development of the embryo-sac are still incomplete.
The Gametophytes of Gnetum:
Male Gametophyte:
A young pollen grain of Gnetum is usually provided with a thick spiny exine and a thin intine. In both the commonly occurring species, G. ula and G. gnemon, Negi and Madhulata (1957) noted that after first nuclear division in the microspore or pollen grain, a small lens-shaped cell is cut off at one end.
This subsequently becomes rounded but neither divides any further nor it does take part in the development of the pollen tube. On the other hand, it degenerates in situ, and is, therefore, regarded as a prothallial cell. The sister nucleus, however, divides again giving rise to two nuclei, the tube nucleus and the generative nucleus.
This generative nucleus soon develops a cytoplasmic sheath around itself to form a generative cell. Thus, a mature pollen grain contains a prothallial cell, a generative cell and a tube nucleus, lying in close proximity.
At the time of germination of the pollen grain, which takes place inside the pollen chamber, the exine is cast off and the intine is pushed out in the form of pollen tube. The tube nucleus is the first to enter the pollen tube and is followed later on by the generative cell. The generative cell finally divides to form two male cells.
Female Gametophyte:
The development of female gametophyte in G. gnemon and G. ula is distinctly tetrasporic. So far as known Gnetum is the only gymnosperm showing this type of development of the female gametophyte, though such a condition can be frequently met with in a number of angiospermic plants.
At the end of meiosis in the megaspore mother cell, a four nucleate coenomegaspore is formed. Further development takes place with free nuclear divisions within the megaspore. A conspicuous vacuole appears in the central portion of the developing gametophyte and the nuclei lying in the peripheral layer of cytoplasm undergo further divisions; these number of divisions vary in different species.
In addition to a normally developing main gametophyte, there may be 2-4 accessory gametophytes (which are smaller in size and contain a few nuclei only). These accessory gametophytes are usually placed above the main gametophyte and ultimately undergo degeneration. No archegonium is present within the gametophyte which is an angiospermic character.
Fertilization in Gnetum:
After the entry of pollen tube into the female gametophyte, whose lower end has become partly cellular, one or more nuclei (3-8) at its upper end also become delimited by cell-walls. These groups of cells are usually present in the neighbourhood of the pollen tube. Out of each such group, one or rarely two cells behave as egg cells, which often become surrounded by one or two layers of minute and degenerated cells.
In all probability these inconspicuous cells provide nourishment to the egg and become used up at the time of fertilization. Only a male nucleus enters the egg, while its sheath is left outside. In cases where two eggs are present in the vicinity of the pollen-tube, double fertilization may result.
Gnetum is remarkable among gymnosperms in the fact that a typical gymnospermic endosperm, which is formed prior to fertilization, is lacking here. Madhulata (1960) reports in G. gnemon that while eggs are being differentiated, the female gametophyte remains in a free nuclear condition, and only when zygotes are produced in the micropylar region of the ovule, the first walls are laid down in the chalazal region producing in an upward direction.
Very rarely the cell-wall formation may take place simultaneously throughout the gametophyte. Sometimes walls may be laid down even when eggs are not differentiated within the gametophyte.
The New Sporophyte of Gnetum:
After fertilization, the zygote develops into the embryo. The details of the embryo development, however, are not clear and much confusion exists, which may be attributed to variations existing among the different species of the genus.
In G. gnemon, Madhulata (1960) has shown that a small tuber-like projection may be developed directly from one side of the zygote, or in some ovules the zygote divides forming a two-celled structure, each of which on germination gives rise to a short tube; frequently, only one of these two cells germinates.
In rare cases, instead of a single tube, two or three tubes are developed from a zygote. The particular tube which receives the zygote nucleus remains functional while the others become abortive. The tube containing the nucleus grows in a downward direction, pushes inside the endosperm, grows through intercellular spaces and finally gives rise to a number of elongated uninucleate cells by the formation of septa.
This is the primary suspensor tube or suspensor tube or pro-embryo tube. Usually, a small protuberance appears either above or below a septum. This, later on, becomes elongated forming a more or less tube-like structure, into which passes a nucleus. Thus, the primary suspensor tube usually becomes much branched.
After the formation of the primary suspensor tube is completed, a small cell is cut off at its tip which first divides transversely and then longitudinally to give rise to a tetrad of cells. From these apical cells by further divisions there appears a more or less globose mass of cells which constitute the embryonal cells.
Sometimes the primary suspensor itself undergoes divisions. Of the group of embryonal cells, those which are terminal in position develop into an embryo, while those which are situated behind the embryo become considerably elongated and divide further to give rise to a long secondary suspensor. This secondary suspensor helps in pushing the embryo deeper within the endosperm for obtaining better nutrition.
In some species, like G. ula, G. gnemon and others, polyembryony is frequently met with, which may originate in several ways.
A mature seed is usually elongated but may be slightly oval also. Its colour varies from green to red. The endosperm is very conspicuous and remains enveloped by three envelopes. The cotyledons are two in number.
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