CHAPTER VII. 
STERILISATION. 
Ir 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 s¢erilisation. 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 Gell. 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 ¢etrad-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 
