In this article we will discuss about the process of reproduction in gnetum with the help of diagrams.
Gnetum is dioecious, the fructifications are axillary (Fig. 18.1 A). The cones are lax to compact strobili.
Microsporangiate fructification (Fig. 18.2A) has short internodes and number of reduced and modified bracts which form a complete cup or collar around each node. In a microsporangiate strobilus (Fig. 18.2G), each vertical series consists of a non-functional ovule above and a series of functional microsporangia below.
In a fructification, there are 3-6 rings of 12-15 or more male flowers and above them is a single or more rings of 7-15 imperfect female flowers or abortive ovules. Each male unit (Fig. 18.2E, F) has two unilocular microsporangia on a stalk (antherophore or microsporangiophore) enclosed in a sheath of perianth. On maturity the stalk elongates and the microsporangia come out of the perianth (Fig. 18.2B).
Several variations of this normal process are on record. Quite common is the fusion of adjacent male units. At times, two or more sporangiophores are present in the same perianth cover. The number of microsporangia may also vary.
Double pollen grains are present in several species. Their origin is possibly due to non- separation of two or more cells of a tetrad. Like Ephedra and Welwitschia, there is no pollen kit in Gnetum.
Pollen which are devoid of the germpore, are shed at the 3-celled stage and germinate in pollen chamber having pollination drop formed by the degenerated cells of the nucellar tip, rich in sugar. During germination, the exine is cast off, pollen tube is formed, the spermatogenous cell moves into the tube and the prothallial cell degenerates. On division of the spermatogenous cell, two equal male cells are formed in G. gnemon and two unequal cells in G. africanum.
G. ula demonstrates a novel mode of development of male gametophyte (Swamy 1974). In this the microspore nucleus divides into a small cell and a large cell, which moves into the pollen tube and gives rise to male gametes.
The female fructification (Fig. 18.2C, D), at a young stage, also looks similar to that of male strobilus. Hence, it is difficult to distinguish between the two. However, with the growth the distinction becomes clear. In a female fructification a ring of 4-10 ovules (Fig. 18.2H) are present above each collar. To begin with, all the ovules look alike but later only a few grow to maturity. The upper few collars lack ovules and are sterile. There is complete absence of male units.
On the megasporangiate strobilus, the ovules are arranged in a single verticil above each bract cup.
In G. ula, an ovule is stalked whereas it is sessile or sub-sessile in other species. An ovule primordium lies on a cushion, and three envelops arise on it in a centripetal manner. Of these three envelops, only the inner one is integument (Fig. 18.4G). The first to differentiate is the outer envelope, often called perianth.
It gets thickened but becomes succulent at maturity. On its epidermis are stomata, and inside are abundant sclereids and laticiferous ducts. Numerous hairs are present at the base. The next to arise is the inner envelope, sometimes described as outer integument. Its apical part forms a tapering ring- shaped rim, the stomata differentiate on its epidermis and laticifers appear at the time of pollination.
The fibrous elements and sclereids of this layer contribute to the stony layer in a seed. The last to be formed is the integument which is fused with the nucellus at the lower part. It elongates considerably into a micropylar tube called ‘style’, which degenerates before or at the time of pollination. Neither stomata nor sclereids are present in the integument.
The nucellus is well-developed and quite massive, with a nucellar cap, and a pollen chamber. At the time of pollination a circular rim or umbrella-shaped structure (flange) develops from the integument (Fig. 18.4H). Initially, it is one- or two-cell thick but later becomes massive. The flange is rudimentary or missing in some species.
A female gametophyte is tetrasporic in origin this condition is similar to several angiosperms but unknown in gymnosperms (save for Welwitschiri).
Also a striking feature, the female gametophyte of Gnetum shares with that of Welwitschia is the absence of archegonia. On the entry of pollen tube on the female gametophytes can be seen differentiation of cells, with one or more nuclei, having a large nucleus and dense contents.
These are egg nuclei/cells surrounded by a couple of small and degenerated cells which are considered to be consumed for the nourishment of the egg during fertilization. An interesting aspect of differentiation of egg is that neither all eggs mature at one time nor do all the pollen tubes come in contact with the female gametophyte. Differentiation of eggs continues even after the fertilization of the first one.
In the process of fertilization both male nuclei are discharged into the egg cell, one of these nuclei enlarges and moves towards the egg nucleus while the other (non-functional) degenerates. The nucleoli of egg nucleus and of fusing male nucleus disappear. The zygote formed is identified by its spherical shape and dense cytoplasm.
In G. gnemon, zygotes usually occur in pairs. It is due to double fertilization, wherein each of the sperm nuclei fuses with a separate female nucleus. Double fertilization is regular in G. gnemon.
As for embryogeny of Gnetum there are variable accounts.
A zygote gives rise to a small protuberance into which moves its nucleus. It may divide into two cells. Either both or a single cell may give rise to the tube. Alternately, the zygote gives rise to a branched tube and its nucleus passes into one of the branches. As for tubes, these have been described as proembryonal tubes, suspensor tubes or primary suspensor tubes.
In this account, these tubes are referred as primary suspensor tubes to differentiate these from suspensors which develop during later stage of embryogeny. These tubes elongate, become septate, get coiled and move towards the chalazal end to penetrate into the female gametophyte.
Some of these tubes are seen growing on the gametophyte and may even come out of it and enter the nucellus. Exceptionally, in G. africanum the zygote and its derivatives produce a row of cells, each of which forms a tube.
Initiation of embryo development occurs at the tips of tube/s by cutting off a small cell. This cell divides longitudinally to produce a quartet. Further divisions in this quartet result into a globular embryo.
As for initial cell of embryo, it is described to be peculiar cell because it is cut off by a wall elongating in the direction of tube. However, this cell becomes normal.
Polyembryony is possible in many ways, either each of primary suspensor tube develops an embryo at its tip, forming many embryos or an embryonal mass at the tip of secondary suspensor may proliferate to give rise to additional embryos.
Polyembryony is of high order in Gnetum. Not only there are several gametophytes in each seed and several zygotes in each, there are also multiplication of embryos from each zygote. However, only one embryo reaches maturity in a seed.
In contrast to the varying accounts given above, zygote is described to undergo a few free- nuclear divisions. After the 4-nucleate stage, two pouch-like processes arise on one pole of the embryo. These processes grow towards chalaza and branch out. At 8- or 16-nucleate stage, the nuclei migrate into these branches eventually resulting into many uni-nucleate tubes.
Seed in most species is elongated but it is oval in G. gnemon. The nucellus is represented by a thin strip, the endosperm is massive and surrounded by three layers of seed coat supplied with vascular traces. The outer (sarcotesta) is green, of parenchymatous cells interspersed with sclereids, on the epidermis of which are haplocheilic stomata.
The next layer is complex, sclerotesta, divisible into three zones, outer one is of dark cells with indurated walls, middle one is palisade and the inner one is fibrous and lignified. The last layer, integument-endotesta-is fused with the nucellus.
The seeds are shed at the immature stage of embryo. G. gnemon seeds are shed in April and germinate in September, whereas dormant seeds of G. ula take one year to germinate. However, the embryo on isolation can be made to germinate. The germination is epigeal.