DEVELOPMENT OF PLANTS 281 



The protonema absorbs the earth substances through delicate 

 rhizoids and spreads over the ground, appearing to the eye as a 

 green mould. Buds finally appear on these branches at numerous 

 places and develop the leafy stems of the moss plant (Fig. 212, 

 b). This in part accounts for the dense colonies so character- 

 istic of the mosses. As soon as the young plants are established 

 in the soil by means of rhizoids that spring from the basal parts, 

 the protonema withers away, although in some genera it is per- 

 ennial and continues to produce new plants from year to year. 



no. The Relationship of the Bryophyta. — It appears probable 

 that there have been three lines of variation or evolution among 

 the Bryophyta that have had their origin in some simple thalloid 

 form most nearly related to the lower Jungermaniales. One line 

 has resulted in the highly differentiated thallus of the Marchanti- 

 ales with its simple sporophyte. Another, the Jungermaniales, 

 retaining the simple structures of the primitive thallus has modi- 

 fied its form into a leafy plant and developed a more complex 

 sporophyte, while the variations of the third line, or Anthocero- 

 tales, have resulted in slight alterations of the primitive type of 

 the thallus but in profound modifications of the sporophyte. The 

 variations of the sporophyte in the latter group appear to have 

 been very advantageous to the present conditions upon the earth 

 and doubtless lead to such forms as the sphagnums and other 

 mosses. The important feature in this evolution, already noted 

 among the algae, is the increasing importance of the sporophyte. 

 Among the green algae it was dependent upon the amount of 

 food stored in the gametospore. In the Bryophyta, it is better 

 nourished, owing to its essential parasitic habit. This led to a 

 larger growth and more complex development and it is especially 

 important to note that the sporophyte became more profoundly 

 modified by the stimuli of external conditions than did the gameto- 

 phyte. For example, we see in the sporophyte of Anthoceros 

 and of the mosses, a plant that is practically dependent upon the 

 gametophyte only for water and the earth substances. So the 

 way is prepared for a study of the next division where the sporo- 

 phyte finally comes to absorb its crude materials directly from 

 the soil and thus becomes an independent self-supporting plant. 



