480 HETEEOMOEPHISM AND ALTEENATION OF GENEEATIONS. 



those of Polytrichum, Bryum, Hylocomium, Andrecea, and Sphagnum. The spores 

 of the asexual generation germinate on a moist substratum, giving rise to a tubular 

 filament which becomes segmented, and gives rise to a considerable growth ot simple 

 character, known as the protonema (see fig. 350 'O- Certain rows of cells of the 

 protonema are colourless and penetrate the ground as rhizoids, the others are 

 extended on the soil and are bright green in colour. After a while bud-like 



structures arise here 

 and there upon the 

 protonema (see fig. 

 350^); these de- 

 velop into leafy- 

 Moss -shoots, upon 

 which the sexual 

 organs are borne — 

 usually in little 

 clusters. Thus, in 

 Mosses, the sexual 

 generation has two 

 stages; the proto- 

 nema and the leafy 

 Moss-plant. From 

 the latter arises the 

 asexual generation 

 or sporogonium. In 

 many of the Liver- 

 worts the sexual 

 generation is much 

 simpler, consisting 

 of a thallus, in 

 which the arche- 

 gonia and anther- 

 idia are sunk. How- 

 ever, a great variety 



is met with amongst them, but the main relations of the sexual and asexual genera- 

 tions are much as in Mosses. 



It will be noted that in Mosses the sexual generation is much more complex a 

 structure than the corresponding structure (the prothallium) in Ferns. The asexual 

 generation, on the other hand, in Mosses is never independent, whilst in Ferns it 

 becomes so quite soon and attains, in the latter group, to much greater structural 

 complexity than in the Mosses. 



Amongst the large assemblage of simple plants which together constitute the 

 class Thallophyta we find in some forms an incipient alternation of generations on 

 the lines already described for Mosses, Ferns, &c.; in others there is no suggestion of 



Pig. 362. —Asexual and sexual reproduction in SaprolegDiaceae. 



1 Formation of asexual zoospores in Achlya. " Oogonia with antheridia and fertilizing 

 tubes. 3 Fruit. All figures x 300. (After Sachs.) 



