Study Notes on Welwitschia| Gnetales

In this article we will discuss about Welwitschia. After reading this article you will learn about: 1. Introduction to Welwitschia 2. Vegetative Structure of Welwitschia 3. Anatomy 4. Reproductive Structures 5. Resemblances with Gnetum.

Contents:

1. Introduction to Welwitschia
2. Vegetative Structure of Welwitschia
3. Anatomy of Welwitschia
4. Reproductive Structures of Welwitschia
5. Resemblances between Welwitschia and Gnetum

1. Introduction to Welwitschia:

Dr. Friederich Welwitsch, an Austrian physician, explorer and botanist discovered Welwitschia in 1859 from Angola, an African country, and Dr. Joseph Dalton Hooker, a British botanist, named the genus in honour of Dr. Welwitsch. Welwitschia occurs in the deserts or such semi-desert regions of south-west Africa where the rainfall is extremely meagre.

Plants occur and survive in such adverse conditions for a very long period mainly because of the presence of an exceptionally long tap root system. The young plants, in which the tap root system is not yet fully developed, utilize the condensed moisture of dew and fog. Plants also occur widely in the Namib desert of Namibia.

The estimated life-span of Welwitschia is 400-1500 years. Although, the cause of the death is unknown, von Willert (1985) opined that it is due to lethal temperatures, caused by lack of transpiration due to shortage of water. As far as the photosynthetic pathways is concerned, Welwitschia was considered as a CAM (Crassulacean acid metabolism) plant, but studies of von Willert (1985) suggest it to be a C₃ plant.

2. Vegetative Structure of Welwitschia:

Welwitschia (Fig. 15.1) has been called the most bizarre or strange of all gymnosperms because the plant appears like a gigantic wooden radish or turnip, and the major part of its stem remains buried in the sandy soil.

The exposed part of the stem consists of a well-developed, woody concave disc bearing two very large (sometimes up to 2 metres long), strap-shaped leaves. Both the leaves are thick, leathery and opposite. They contain parallel veins and persist throughout life of the plant.

Along with parallel venation, countless longitudinal veins are interlaced by anastomosing veinlets which form irregular meshes or areoles. The leaves split into ribbon-like structures and extend in a twisted or contorted manner along the surface of the ground.

Throughout the life of the plant, both the leaves grow continuously from a basal meristem. The leaves bear syndetocheilic stomata. A long well-developed tap root system is the characteristic feature of this plant. Welwitschia plants have a very long life, sometimes as long as 1000 years or even more.

Apical Meristem:

Robin (1953) observed that in young seedlings, the apical meristem is very minute. Well-defined zones are not visible at this stage mainly because of irregular periclinal cell divisions in all directions. The apical meristem stops functioning very soon and ultimately dies. The growth is now restricted only in the peripheral region and this finally results in an increased girth.

Martens and Waterkeyn (1963) worked on the stem apex of Welwitschia and observed that instead of two, as was believed earlier, three pairs of foliar organs develop in an opposite decussate manner. First to develop are two short-lived cotyledons. Then develop the two main leaves.

The last to develop are two leaf-primordia which develop just before the apical meristem stops functioning and aborts. Both these leaf primordia give rise to scaly bodies. The latter have been variously interpreted as buds, or cones by earlier workers.

3. Anatomy of Welwitschia:

(i) Stem:

A thick and ridged corky layer covers the upper surface of the stem. A ring of conjoint, collateral, open and endarch vascular bundles is present in the young stem which also contains a centrally located distinct pith. Thick-walled spicular cells encrusted with calcium oxalate crystals are present in the cortex and pith both.

The older stems, however, contain a saucer-shaped mass. From this mass the vascular traces are given out to the leaves, tap root and also to the inflorescences. Successive zones of xylem and phloem, as observed in Cycas and Gnetum, are also observed in very old stems of Welwitschia.

Isolated vessels are also present in the stem. Vessels consists of about a dozen cells and are present in the form of coiled, worm-like masses. Abundant xylem parenchyma is also present. Sieve cells and phloem parenchyma constitute the phloem.

(ii) Leaf:

A layer of thickly cuticularized lower epidermis and an upper epidermis are present (Fig. 15.2). The stomata are syndetocheilic. Palisade tissue alternating with sclerenchyma patches are present below the epidermal layers on both the sides of leaf. Spongy tissue fills the space between the palisade layers of both the sides. Sclereids or spicular cells are also present in the spongy tissue.

A row of collateral vascular bundles with their xylem facing towards upper epidermis and phloem towards lower epidermis is present. Well-developed transfusion tissue surrounds each vascular bundle. A fibrous cap formed by a group of thick-walled cells is present on both the ends of vascular bundles.

4. Reproductive Structures of 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 (Fig. 15.3A), 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 (Fig. 15.3A,B). Martens (1961 1963, 1974, 1975, 1977) and his pupil Waterkeyn have done extensive work on the embryology of Welwitschia.

Male Strobilus and Male Flower:

A male or microsporangiate strobilus or male cone (Fig. 15.3 A) 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 (Fig. 15.3C).

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 (Fig. 15.3C).

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.

Female Strobilus and Female Flower:

The female strobilus, also called ovulate or megasporangiate strobilus or ovuliferous cone (Fig. 15.3B) 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 (Fig. 15.3D, F, G), 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 (Fig. 15 .3H).

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 (Fig. 15.31). Formation of archegonia has not been observed in Welwitschia.

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 (Fig. 15 .4 A). 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 (Fig. 15.4A). 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 (Fig. 15.4B).

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.

Cytology:

The somatic number of chromosomes in Welwitschia is 42. It has very asymmetrical karyotype with a clear difference between the shortest and the longest pair. Rod-shaped chromosomes with terminal centromeres are present. Its detailed cytology has been worked out by Khoshoo and Ahuja (1963).

5. Resemblances between Welwitschia and Gnetum:

Welwitschia resemble Gnetum because both show the following characteristics:

1. Vessels in their xylem;

2. Broad medullary rays in their xylem;

3. Dioecious nature of their plants;

4. Compound male and female strobili;

5. Almost similar structure of their microsporangia;

6. Almost similar mode of the development of their male gametophytes;

7. Ovules of which the inner integuments are produced into a long micropylar tube,

8. Either shallow or no pollen chamber;

9. Tetrasporic type of the development of their female gametophytes;

10. Multinucleate cells in their female gametophytes, the nuclei of which fuse and form polyploid nuclei;

11. Almost similar mode of pollination;

12. Dicotyledonous embryo;

13. A distinct feeder in their embryo;

14. Zygotes which do not form free nuclei.