118 EVOLUTION OF PLANTS 



becomes detached as a little lid (operculum), and the 

 detachment of this is aided by the formation of an 

 elaborate system of tooth-like structures (peristome) 

 about the mouth of the capsule. These teeth are ex- 

 tremely hygroscopic, and by their movements they not 

 only help to throw off the operculum, but also to empty 

 the capsule and disperse the spores, which, when ripe, 

 lie loosely in the capsule, owing to the drying up and 

 withering of the delicate interior tissues. 



SUMMARY 



While there is little question that the Bryophytes 

 have arisen from forms similar to certain green algae, 

 it must be admitted that so far as existing forms are 

 concerned the relationship is at best a remote one. It 

 is true a direct comparison can be made between the 

 sporophyte in Coleochaete, for example, and that of 

 Riccia, and the change from the motile zoospores of the 

 one to the spores of the other can be explained by the 

 abandonment of the aquatic habit by the Bryophytes. 

 The gametophyte, itself, offers no serious difficulties, 

 retaining in Anthoceros, for instance, apparently the 

 single chloroplast in each cell found in so many algae, 

 e.g. Coleochsete, and the structure of the thallus is 

 hardly more complex than in these ; but when an 

 attempt is made to compare the sexual reproductive 

 organs it must be admitted, especially as regards the 

 archegonium, that the difference between the two 

 groups is a very great one. The nearest approach in 

 this respect is found in the Characese, which otherwise 

 differ profoundly from the Mosses, and so far as our 



