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 alge, 
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 Coleochete, for example, and that of 
Riccia, and the change from the motile zodspores 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 alge, 
e.g. Coleochete, 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 Characez, which otherwise 
differ profoundly from the Mosses, and so far as our 
