FILICALES 



351 



generations which constitute it. Either can grow, nourish itself, and 

 even multiply independently of the other. It is true that the young 

 sporophyte is nursed temporarily in the parent prothallus. But this 

 is only a transient event and is soon over. A similar nursing period, 

 with much greater adaptive detail that lends added efficiency, is seen 

 in the Seed-Plants. The main difference between the Seed-Plant and 



FIG. 294. 



Nephrodium pseudo-mas, var. polydactylum. Tissue of prothallus where an apo- 

 gamous growth is to be found, showing on the left a cell with two nuclei, while an 

 adjoining cell has none. At the centre a nucleus is seen passing through a perforation 

 of the wall, and fusing immediately with that of the cell it enters. (After Farmer, 

 Moore, and Miss Digby.) 



the Fern lies in their spores. The former are heterosporous, the latter 

 are few exceptions) homosporous. The advantage of the large 

 female spore is that it contains already a supply of nourishment for 

 the young germ after fertilisation, so that a vegetative prothallus is 

 not necessary. Especially is this so in Seed-Plants where the spore 

 is retained in the tissue of the parent, and can draw nourishment 

 continuously from it. On the other hand, in the primitive homo- 

 sporous state the individual spore is small and is cast out to fend for 

 itself. It is then incumbent on the spore, when it germinates, at 



