ORGANIC EVOLUTION 



129 



remarkably in the size and predominance to which the 

 sporophyte has attained. 



The fern (Pteris) . The gametophyte of the fern will serve 

 well to connect with preceding studies. It is a little heart- 

 shaped thallus (called the pro- 

 thallium), hardly exceeding a 

 quarter of an inch in diameter 

 when grown. Its structure is 

 even more simple than that of 

 the thallus of a liverwort. 

 There is the same copious 

 development of rhizoids, con- 

 necting it with the soil, but 

 there is less differentiation of 

 the tissues of the thallus itself, 

 there being no sharp distinction 

 of special assimilatory paren- 

 chyma and no pores. The 

 growing point is in the notched 

 tip, protected as before by the 

 lobes extended at either side of 

 it. The archegonia and anthe- 



under side of the thallus, and 

 former are arranged in a cluster, 

 The sperms are motile 



FIG. 77. The fern, a, the gameto- 

 phyte phase, inverted and seen 

 from lower surface; s, the spore 

 from which it grew; r, rhizoids; 

 p, the growing point in the bottom 

 of the apical notch ; o, archegonia 

 and t, antheridia; 6,a single arche- 

 gonium in vertical section ; e, egg 

 cell; c, a single antheridium, with 

 developing sperm cells; d, a single 

 [mature sperm cell. 



open downward; the 



nearer the growing point (fig. 77). 



and swim about when mature, if favoring rain or dew give 



them opportunity. Many of them mature in advance of 



the maturing of the eggs of the same thallus, thus favoring 



cross-fertilization. 



Thus, it will be seen there is quite a general similarity 

 between the prothallium of the fern and the thallus of a 

 simple liverwort. But this phase of the fern is least 



