GROUP IV. — PHANEROGAMLA: GYMNOSPERM^. 



477 



The in<de organ is a rudimentary aritheridium consisting of two 

 cells, the stalk-cell and the generative cell. 



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

 theridium which travels into the pollen-tube (see Fig. 289) ; 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 



equivalent to spermatozoids ; in 

 Taxus, however, the two cells 

 differ in size, and it is only the 

 larger one that is a functional 

 male cell. The male cell 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 germina- 

 tion 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 macro- 

 spore is entirely filled. It is 

 characteristic of Gymnosperms 

 that the development of the 

 prothallium is uninterrupted, 

 and that it is completed, except in Gnetum, before the female 

 organs are developed and, consequently, before fertilisation can 

 have taken place. 



Fig. 302.— Fertilisation of two archegonia 

 by means of a single pollen-tube in Junipe- 

 ru» virginiana (x230: after Strasburger) : e 

 prothallial tissue ; r neck of archegonium ; 

 tt nticleus in the pollen-tube ; mp male pro- 

 nucleus, fp female pronucleus, in the 

 oosphere; v vacuole in the oosphere ; o« 

 lower part of oosphere. 



