DEVELOPMENT OF PLANTS 309 



both surfaces of the peristome cells are thickened. These teeth 

 are so variously marked and fashioned that they furnish one of 

 the most characteristic features of the various genera of the 

 mosses. The peristome closes the mouth of the capsule in wet 

 weather, but on dry days it opens in a variety of ingenious ways 

 in the various species of mosses so as to effect a gradual scattering 

 of the spores. The seta also contributes in the distribution, 

 being very elastic and often hygroscopic (Fig. 205, D). 



The spores germinate and produce profusely branching chains 

 or filaments of green cells, called the protonema (Fig. 212). In 

 a few instances, thalloid structures are developed as in Sphagnum. 

 The protonenia absorbs the earth substances through delicate 

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

 green mould or fine algal cells. Goebel presents some evidence 

 tending to show that the mosses may have been derived from 

 branching forms of the green algae. 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 characteristic 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 perennial and continues to produce new plants 

 from year to year. 



1 10^ 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 perhaps led to the mosses. The important feature in this 



