FERNS AND THEIR RELATIVES 81 
leaves and moistened by a friendly shower. At some 
point the cell-wall gives, and a second cell sprouts from 
its side. Steadily multiplying cells, it forms a flat plate 
upon the soil, usually in the outline of a kidney. In this 
generation we find no leaves nor trace of them, and, of 
course, no fibro-vascular bundles. Upon the top side it 
is merely a smooth, green plate, but upon the under side 
are the structures which will produce the fern plant, and 
also, driving their way downward, there are root-hairs 
sucking up moisture and mineral food. This green plate 
is called the prothalliwm, and, as we mentioned above, it 
is very rarely more than half an inch across, and usually 
very much smaller. 
You will remember that on the tips of the fertile moss 
branches we found antheridia and archegonia, the former 
being round bodies which finally split and allowed the 
particles of protoplasm to swim off on their fertilising 
business, the latter flask-shaped, and containing the cells, 
which, when fertilised, would stay in their place and grow 
up to produce spores. Exactly similar bodies are pro- 
duced on the “ prothallium,” but beneath it, projecting on 
to the surface of the ground. Very frequently you would 
only find one kind of body on the prothallium. In that 
case fertilisation only comes about by means of the 
swimming protoplasm voyaging along through the mois- 
ture of the ground until it finds on some other plate 
an archegonium ready to be fertilised. This readiness 
is shown in the same way as with the mosses. The cells 
_ that run down the neck of the flask (the swollen part 
is sunk in the prothallium) all turn to jelly, and the 
swimmer is attracted to the canal. It may have come 
off the same green disc, or, more probably, from a neigh- 
bour. Gliding down the canal, it combines with the cell 
G 
