GROUP IV. GYMNOSPERALE. 429 



The male organ is a rudimentary antheridium consisting of two 

 cells, the stalk-cell and the generative cell. 



The male cell is derived from the generative cell of the anther- 

 idium which travels into the pollen-tube; this cell undergoes 

 division into two similar cells, near the apex of the pollen-tube, 

 both of which are, as a rule, functional male cells. In Cycas, 

 Zamia, and Ginkgo, each of these two cells actually developes into 

 a large multiciliate spermatozoid ; whilst in those forms in which 

 the male cell is not a spermatozoid, it is of somewhat spherical 

 or oval form. When, as in Juniperus, and other Cupressinese, 

 several archegonia are' fertilised by means of a single pollen-tube, 

 repeated cell-division takes place in the pollen-tube. 



The Female Individual is a prothallium (sometimes called 

 endosperm) developed within the macrospore. The germination 

 of the macrospore begins with the division of its nucleus ; nuclear 

 division is repeated until a large number of nuclei are formed, 

 lying in the parietal protoplasm of the spore ; free cell-formation 

 then takes place, walls being formed between the cells so that the 

 interior of the macrospore is lined by a layer of cells which grow 

 and divide until the cavity of the macrospore is entirely filled. 

 It is characteristic of Gymnosperms that the development of the 

 prothallium is uninterrupted, and that it is completed before the 

 female organs are developed and, consequently, before fertilisation 

 can have taken place. 



The female prothallium is a mass of parenchymatous tissue, 

 which does not, as a rule, protrude to any extent from the spore, 

 and which, in consequence of the exclusion of light, is destitute 

 of chlorophyll ; the only exception to this rule is offered by the 

 Cycadacese where, if the female organ is not fertilised, the pro- 

 thallium, resuming its growth, protrudes through the micropyle 

 and turns green in the light. 



The female organ is an archegonium, and is developed from a 

 single superficial cell of the female prothallium at its micropylar 

 end. The mother-cell generally divides transversely into two ; 

 an upper, the neck-cell ; a lower, the central cell : the neck-cell 

 usually divides, by two vertical walls, into four cells, which form 

 the neck ; the central cell grows, and divides transversely at its 

 upper end so as to cut off a small cell, the canal-cell, which lies 

 in the canal formed by the separation of the neck cells, and a 

 large cell which is the female cell or oosphere (Fig. 252). 



The number of archegonia developed on the female prothallium 



