CHAPTER VII. 



STERILISATION. 



IT has been the practice from early times of Descriptive Botany to designate 

 the leaves which produce the sporangia in Ferns and other organisms as 

 the fertile leaves, those which carry out a vegetative function only the 

 sterile leaves. The similarity of form which these show one to another 

 readily established their close relationship : middle forms are frequently 

 found between them, partly sterile, partly fertile ; and any conversion of 

 the fertile into the sterile would, to preserve uniformity of terms, be 

 designated a process of sterilisation. The term thus applied to a leaf, or 

 a pinna as a whole, will be properly applied also to its constituent parts, 

 and so ultimately to the individual cells composing them; and thus, 

 wherever a cell that is normally sporogenous is diverted from that 

 function to any vegetative office, the process may be styled one of 

 sterilisation of that cell. It seems necessary thus to justify this use of the 

 word, since recent investigation has attached a definite structural meaning 

 to the change involved in those cells which are diverted from the office of 

 spore-production. Its cytological significance lies in the fact that chromo- 

 some-reduction, characteristic of fertile cells, does not take place in 

 them. Without the historical explanation it might appear strange to 

 describe this change of nuclear behaviour as sterilisation ; but on the 

 grounds of old custom this term will be retained throughout the present 

 discussion. 



In the Archegoniatae and in Seed-Plants the tetrad-division is the 

 criterion of the fertile or sporogenous cell. It is true that among the 

 highly specialised Seed-Plants this tetrad-division may sometimes be omitted ; 

 but putting these exceptions aside, it is the formation of a spore-tetrad 

 which is the final distinctive mark of a sporogenous cell as distinct from 

 a vegetative cell. But long prior to the appearance of this distinctive 

 condition the sporogenous cells may in most cases be recognised with a 

 high degree of certainty. They commonly form a clearly defined sporogenous 

 group, distinguished by the dense protoplasmic contents of its cells, and 



