240 PLANT LIFE 
somes. It invariably happens, so far as at 
present is known, that when the eggs are 
fertile at all they produce new ferns directly, 
that is, without fertilisation. Moreover, even 
the tissue cells of such a prothallus may 
change their mode of growth, and develop 
into fern plants without the definite produc- 
tion of sexual organs at all. 
Such a departure from the normal course 
of life history strongly emphasises the relation 
of meiosis to fertilisation, but it does more 
than this. It indicates that the striking 
difference between the fern plant and the 
prothallus is not itself essentially bound up 
with those nuclear changes which are inti- 
mately associated with the sexual phases. 
It points rather to the conclusion that in these 
plants the life history, with its two different 
stages, may have developed in coincidence, 
though not in causal connection with the 
separation of the sexual process into two 
stages. It would clearly be futile, in the face 
of the evidence, to attempt to maintain the 
existence of a causal relation between the 
nuclear changes and the characteristic differ- 
ences between the two stages of the life 
history of the fern. In this way we may 
understand the continuance of the alternate 
appearance of fern and prothallus, even when 
the cellular rhythm no longer obtains. 
Considerations of space preclude the follow- 
ing up of this matter in any detail; it may, 
however, be said quite generally that wherever 
