MOSSES 



189 



337. Spores and elaters of marchantia. 



setting free the spherical spores together with numerous filaments 

 having spirally thickened walls (Fig. 337). These filaments are 

 called elaters. When drying, they exhibit rapid movements by 

 means of which the spores are scattered. The spores germinate 

 and again produce the thallus of marchantia. 



MOSSES 



If we have followed carefully the development of marchantia, the 

 study of one of the mosses will be comparatively easy. The mosses 

 are more familiar plants than the liverworts. They grow on trees, 

 stones, and on the soil both in wet and dry places. One of the com- 

 mon larger mosses, known as Polytrichum commune, may serve as an 

 example. This plant grows on rather dry knolls, mostly in the borders 

 of open woods, where it forms large beds. In dry weather these beds 

 have a reddish brown appearance, but when moist they form beautiful 

 green cushions. This color is due, in the first instance, to the color 

 of the old stems and leaves and, in the second instance, to the peculiar 

 action of the green living leaves under the influence of changing mois- 



338. Section of leaf of Polytrichum commune. 



ture -conditions. The inner surface of the leaf is covered with thin, 

 longitudinal ridges of delicate cells which contain chlorophyll. These 

 are shown in cross -section in Fig. 338. All the other tissue of the 

 leaf consists of thick-walled, corky cells which do not allow moisture 

 to penetrate. When the air is moist the green leaves spread out, 

 exposing the chlorophyll cells to the air, but in dry weather the mar- 



