Reproduction in Welwitschia

Reproduction in Welwitschia:-
> Welwitschia is strictly dioecious, i.e., two sexes are present in separate individuals. The inflore­scences develop from a series of several transverse ridges arising parallel to the leaf bases. The branching in inflorescences is dichasial, and each branch ends in an attractive cone.
> Several bracts or cone scales, arranged in opposite decussate manner, are present in each cone. Because of crimson or scarlet colour, the mature cones of Welwitschia provide a beautiful look. Martens (1961 1963, 1974, 1975, 1977) and his pupil Waterkeyn have done extensive work on the embryology of Welwitschia.
a. Male Strobilus and Male Flower:-
- A male or microsporangiate strobilus or male cone is a compound structure bearing a quadrangular cone axis. It contains several bracts or cone scales arranged in opposite decussate manner. In the axil of each subtending bract is present a male flower.
- Two lateral bracts and a perianth are also present in each male flower. The perianth is formed from two bract-like anterior-posteriorly placed structures.
- Inner to the perianth is present a whorl of six micro-sporangiophores which remain fused at the base to form a cup-like structure. A sterile ovule with a single integument is present in the centre of each male flower.
- At the top of each micro-sporangiophore is present a synangium. Each synangium is formed by the fusion of three microsporangia. Each microsporangium contains many pollen grains which are shed through a vertical slit. Pollination is effected either by wind or by insects.
Male Gametophyte:-
- Pollen grain matures into a three-celled male gametophyte (Sterling, 1963) but much is not known about the actual process of the male gametophyte development. The 3-celled stage includes a tube nucleus, a sterile cell and a spermatogenous cell. The sterile cell usually aborts even before the pollination.
- The spermatogenous cell gives rise to two male nuclei or sperm nuclei. Since the general process of the male gametophyte development resembles with that of Gnetum, it is assumed that the prothallial cells are absent in Welwitschia.
- According to Bomman (1972) Welwitschia is wind-pollinated. However, van Jaarsveld (1990) opined that it is insect-pollinated.
b. Female Strobilus and Female Flower:-
- The female strobilus, also called ovulate or megasporangiate strobilus or ovuliferous cone is also a compound structure like male cone. The axis of the female strobilus bears many broad decussate bracts or cone scales, in the axil of each of which is present a female flower.
- Inside the subtending bract of each female flower are present two small lateral bracts, two envelopes and a single nucellus. Out of the two envelopes the inner one functions as a true integument, and prolongs in the form of a long tubular micropyle.
- The outer envelope develops from two posterior-anterior primordia which fuse with each other in the early stages of the development. Some prefer to call this fusion product as perianth The perianth expands into a broad wing-like structure in the mature seed.
Female Gametophyte:-
- A single megaspore mother cell develops quite deep in the nucellus tissue. Its diploid nucleus divides meiotically but there is no cross-wall formation. Linear tetrad of spores is not formed, and a tetrasporic female pro-thallus develops directly. Haploid nuclei of young female gametophyte divide and redivide several times mitotically but there is no cross-wall formation. Thousands of free-nuclei are thus formed.
- At this stage the cell wall formation starts irregularly, and each cell contains varying number of nuclei. In several of these cells the nuclei fuse to form polyploids. Formation of archegonia has not been observed in Welwitschia.
c. Fertilization and Post-Fertilization Changes:-
- The fertilization process is quite unique in Welwitschia. At the time of fertilization, the pollen tubes elongate and grow downwards through the nucellus. Simultaneously the apical cells of the female pro-thallus elongate and form prothallial tubes which grow upwards.
- The pollen tubes and the prothallial tubes thus grow in opposite directions and come in contact with each other, somewhere in the nucellar cap.
- The contact walls between the two tubes dissolve and it is said that female nuclei pass into the pollen tubes and thus the male and female nuclei fuse with each other to form a fusion nucleus. The act of fertilization thus takes place in the pollen tube rather than in an archegonium or embryo-sac.
- The diploid zygotic nucleus divides and forms a 2-celled pro-embryo. The upper cell of this pro-embryo develops into a primary suspensor while the lower cell divides and re-divides to form a large number of secondary suspensor cells and a multicellular embryo. A cap of about eight cap cells protects the young multicellular embryo which is pushed down into the pro-thallus by the elongating suspensor.
- Welwitschia exhibits a high degree of polyembryony because many zygotes and young embryos are produced. However, only one embryo finally matures into a seed. A lateral finger-like process, called feeder, is present in the mature embryo.
- The winged outer envelope of the ovule matures into a papery structure. The seed germination is epigeal and both the permanent leaves appear soon after the germination. The cotyledons grow for about six months and finally die.