32 STRUCTURAL BOTANY 



When the microspore germinates it begins by cutting off 

 a little inactive cell (the prothallus-cell), and the pollen- 

 grain of a Gymnosperm does the same. The subsequent 

 divisions lead to the formation of the spermatozoids 

 in the Cryptogam, and of the generative cells in the 

 Gymnosperm. In both cases these are the bodies which 

 effect fertilisation. That the generative cells are homo- 

 logous with spermatozoids has long been recognised. Two 

 Japanese botanists have recently made the brilliant dis- 

 covery that in some of the Gymnosperms each generative 

 cell actually becomes converted into an active sperma- 

 tozoid resembling those of the Ferns (see p. 301). The 

 antheridium, i.e. the organ in which the male cells are 

 formed, is more complex in Selaginella than in the Gymno- 

 sperms, for in the latter it has almost become reduced to 

 its most essential part, the generative cells themselves. 

 The pollen-tube of the Gymnosperm is not represented in 

 Selaginella, for in the latter the whole contents of the 

 microspore are used up to form the prothallus-cell and 

 antheridium. The comparison of the development gives 

 us then the following chief results : 



Selagitiella. Gymnosperm. 



1. Spermatozoids = Generative cells. 



2. Prothallus and antheridium = Cell-group in pollen-grain. 



3. Microspore = Pollen-grain. 



4. Microsporangium = Pollen-sac. 



The student, however, must clearly understand that it is 

 quite useless to learn up the names of the equivalent 

 organs, unless he thoroughly grasps the developmental 

 facts on which their comparison is based. 



We will now compare the female organs in the two 

 Classes. It is best to start with the ovum, which is 

 beyond question the same thing in both. The ovum in 

 Selaginella is produced in an archegoniuni, which is 



