give an illustrated account of male and female gametophytes of ginkgo?
Answers
Answer:
Gametophytes in Ginkgo:
(i) Male Gametophyte:
Major aspects of the development of male gametophyte of Ginkgo biloba (Fig 10.22) are similar to that of Pinus. Male gametophyte starts developing in situ, i.e. within the sporangium.
Friedman (1987) studied the growth and development of the male gametophyte of Ginkgo biloba using three-dimensional computer reconstruction (Fig. 10.22). Pollen germination starts with a diffuse swelling of tube cell. Tube cell becomes very elaborate with a proximal un-branched tube and a much branched haustorial system.
Ginkgo Biloba
Microspore is the first cell of the male gametophyte. Each microspore is a rounded structure having thin intine and thick exine layers. A centrally located nucleus contains one or two nucleoli and remains surrounded by dense cytoplasm. An un-thickened portion, called pore, is also present in each microspore. It is a region where exine is not covering the intine.
Two unequal cells are formed by the first mitotic division in the microspore. These are called 1st prothallial cell and inner cell (Fig. 10.23 A). The 1st prothallial cell is smaller than that of inner cell. The inner cell again divides forming a 2nd prothallial cell and an antheridial initial (Fig. 10.23B).
By this time the 1st prothallial cell starts to degenerate. The antheridial initial then divides forming a generative cell near the 2nd prothallial cell and a tube cell (Fig. 10.23D). The tube cell does not divide any further. This is the 4-celled stagesGinkgo Biloba. Structure and Development of Male Gametophyte
Pollination in Ginkgo biloba takes place sometime in April. At the time of pollination the ovule secretes a mucilaginous substance. Further development of the male gametophyte takes place in the pollen chamber. In pollen chamber the generative cell of the 4-celled young male gametophyte divides into a stalk cell, which is close to the 2nd prothallial cell, and a body cell.
Now the intine protrudes out near the pore and functions as a pollen tube (Fig. 10.23E). The pollen rube swells up and advances towards the archegonia by protruding into the nucellus. Just before fertilization the body cells divides into two, and the wall formation is at right angle to the base of pollen grains.
The blepharoplasts appear on the body of these so-formed two cells which function as sperms or male gametes (Fig. 10.23F). The structure of these multi-flagellated sperms, as well as their behaviour in the tube at the time of fertilization has been investigated by Shimamura(1937), Lee (1955) and Favre-Duchartre (1956).
Motile sperms in gymnosperms were observed for the first time on September 9, 1896 by Hirase, and his this discovery is considered as one of the most remarkable events in plant morphology. The ultrastructure of male gamete of Ginkgo has been studied by Gifford and Lin (1975)ii) Female Gametophyte:
The development of the ovule and female gametophyte in Ginkgo biloba has been studied by Carothers (1907). Out of the four megaspores formed from the megaspore mother cell, only the lowermost remains functional and the remaining three degenerate (Fig. 10.24) A).
The nucleus of the functional megaspore migrates towards the micropylar end and divides into two followed by a number of free-nuclear divisions forming hundreds of free nuclei.
A large vacuole is present in the megaspore at this stage (Fig 10.24B). Now the wall formation starts (Fig. 10.24B,C) and progresses from the periphery towards the centre. Dunng wall formation first the anticlinal walls are formed followed by the vertical walls.
Each cell generally contains one nucleus but in some cells 2-3 nuclei are also seen. Due to more rapid cell divisions on the micropylar end a pole-like structure develops. This is called tent-pole (Fig. 10.24D). The female gametophyte possesses abundant chlorophyll.
Ginkgo Biloba. Development of Female Gametophyte
The development of archegonium starts from the cells towards the micropylar end of the female gametophyte. According to Favre-Duchartre (1958) the archegonial initials start appearing sometime in June.
Each archegonium possesses a short neck made up of only four cells and a small venter having a central cell. The central cell later on forms a ventral canal cell and an egg cell (Fig. 10.25). According to Bierhorst (1971) only two cells are presentEach archegonium possesses a short neck made up of only four cells and a small venter having a central cell. The central cell later on forms a ventral canal cell and an egg cell (Fig. 10.25). According to Bierhorst (1971) only two cells are present in the archegonial neck.
Ginkgo Biloba. A Mature Archegonium
Ultra structural study of the female gametophyte of Ginkgo biloba has been made by Dexheimer (1973). He observed four zones based on food reserves.
These are:
(1) Lipid zone,
(2) Starch proteolipid zone with large vacuolated cells,
(3) Starch zone made of large vacuolated cells with peripheral cytoplasm, and
(4) Deep or central zone with very little reserve contents.