Structure and evolution of female gametophyteof gymnosperms

Structure and evolution of female gametophyteof gymnosperms

1. STRUCTURE AND EVOLUTION OF FEMALE GAMETOPHYTE OF GYMNOSPERMS PRESENTED BY:- GOSWAMI KHUSHBOOBEN.B. M.SC.BOTANY SEM-1 PAPER - CBO - 402 DEPARTMENT OF LIFE SCIENCE H.N.G.U. ,PATAN.

2. CONTENTS Introduction Structure of female gametophyte Evolution of female gametophyte References

3. INTRODUCTION Gymnosperms (Gymnos = nacked and Sperma = seed) are naked seeded plants. These group of plants have ovules which are not encosed and thus the seeds remain nacked.

4. Most of the gymnosperms are evergreen, woody perennials with shrubby or tree like habit. They show xerophytic characters. Many of them are very large in size.

5. Fig:- Female cone

6. STRUCTURE OF FEMALE GAMETOPHYTE Female gametophyte develop in female cone. The female cone also has a central axis on which bracts known as megasporephylls are present. In the female cone , megaspore mother cells are present in the megasporangium .

7. The megaspore mother cell divides by meiosis to produce foue haploid megaspores. One of the megaspores divides to from the multicellular female gametophyte, while the others divide to from the rest of the structure. The female gametophyte is contained within a stucture called the archegonium .

8. OVULE The ovules are naked and are borne on megasporophylls that are usually spirally arranged around a central axis. The ovules are sessile and those of cycas are the largest in the plant kingdom. The ovules consists of a parenchymatous mass of cells called the nucellus or the megasporangium .

9. The necellus encloses a single diploid megaspore mother cell (MMC) that undergoes meiosis to form a linear tetrad of 4 haploid megaspores, out of which only (usually the lower) remains functional, whereas the rest degenerate. The functional megaspore enlarges and undergoes free nuclear division to form large number of the nuclei.

10. This is the young female gametophyte . By now the nucellus becomes enveloped by a single massive integument that grows around leaving a small pore or the micropyle at one end. This is the micropylar end of the ovule. This integumented megasporangium is called the ovule.

11. Now centripetal wall formation starts around the free nuclei of the female gametophyte. It continues till the whole female gametophyte becomes cellular. The single integument in gymnospermous ovule consists of three layers : (1) Sarcotesta or the outer fleshy layer, (2) sclerotesta or the middle stony layer, and (3) the inner sarcotesta or the inner fleshy layer.

12. In pinus , Ephedra and gnetum the integument is partly fused and partly free from the nucellus. In cycas the integument is fused with the nucellus except in the apical region. In podocarpus gracilior the entire integument grows into a micropylar tube in mature ovule. The apical region of the nucellus degenerates to form a pollen chamber in which the semi –germineted pollen grains.

13. Towards the micropylar end the female prothallus developes two or more archegonia or the female sex organs. The archegonia have a short or long neck made of 2 (cycas, Zamia, Ginkgo), 4 (Taxus) , 8 (Biota), and 8 tiers of 4 cell each (Ephedra). The ventral canal cell is present in Ginkgo and Pinus where as it is absent in cycas and Ephedra, which have only ventral canal nucleus. The venter has a large egg or the oosphere.

14. In Biota orientalis, Cryptomeria and some other conifers the archegonia occur in groups and form and archegonial complex. Such groups may also develope at the chalazal end of the female gametophyte, e.g., Biota orientalis and Cryptomeria japonica .

15. EVOLUTION OF FEMALE GAMETOPHYTES The fundamental characteristic of all the spermetophytes “sensulato” to produce seeds and that the seed or more exactly the ovule that will develop into a seed on maturation , is a tegumented megasporangium that the plants disperse togather with their contents .

16. The nucellus of an ovule contains a megaspore that develops into the female gametophyte (prothallus, archegonia) and is hence comparable to megasporangium. It is surrounded by one or two protective structure called the integuments . In oder to elucidate the origin and evolution of ovule the botanists have to analyse ontogenetically and phylogenetically the various elements of this body .

17. The research in this direction has almost been exclusively resrticted to the integument , and it is still a subject of dispute. Regarding ovules morphological nature, three main theories have been advanced:- 1). The axial theory , which held that nucellus is of the nature of a bud bearing integuments that are regared as lateral foliar appendages.

18. 2). The sui-generis theory that regarded the ovule to be an independent structure borne on either foliar or axial organs. 3). The foliar theory regarded the ovule to be hpmplogous to, with three lobed leaflet of the female sporophyll, the two lateral lobes of the leaflet are fused to form the outer integument, the nucellus is an emergence borne on the cup shaped terminal lobe.

19. Benson (1940) produced that a pteridosperm ovule is a synangium in which the central sporangium has retained a single megaspore, where as the surrounding ring of sporangia became sterile and formed the integument. Oliver and Scott (1940) regarded the integument to be a multiple structure as evidenced by its free lobing. They regarded that the present integument was once a comparatively un specialized cupul like indusium.

20. De Haan (1920) concluded:- The integuments of pteridospermous, gymnospermous and angiospermous ovules are homologus. The integuments are not homologus with the indusial of ferns and lycopods. The integuments are originally composed of units that are now regarded as teloms .

21. Halle (1933, 1937) was of the view that the cupules and the synangi of pteridosperm ovules were borne on the leaves and were formed by the lateral concrescence of groups of cyclically arrenged teloms. Nucellar modification concept :- this theory was put forth and later elaborated by Andrews in 1961. According to this theory the lagenostomalean integument is derivable from the nucellus or sporangiun wall by its vascularisation and growth and lobing of the apex. it dose not account for the origine of cupule.

22. The telomes concept:- it is the synthesis of of the views of smith (1961), Long (1960) and Andrews (1961). They all consider the derivation of pteridosperm ovule from a terminal fern megasporangium surrounded by sterile and fertile telomes as those of hedesia corymbosa.

23. CONCLUSION The preridospermous ovule has arisen independently a number of times , but the process of evolution has followed a similar pettern , which is explained by the telome concept. This group has been regarded by Beck (1960, 1962, 1970) as ancestral to both pteridosperms and cordaitales. This is suggestive of a monophyletic origin for the gymnosperms .

24. REFERENCES Botany for a degree students – Gymnosperms - Vasistha P C , S. Chand & company www. Google.com .

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