STUDIES IN CRYPTOGAMS 



199 



MOSSES (Bryophyta) 



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 liver- 

 worts. They grow on trees, stones, and on the 

 soil both in wet and dry places. One of the 

 common larger mosses, known as Polytrichum 

 commune, may serve as an 

 example, Fig. 295. 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 in- 

 stance, to the peculiar action 

 of the green living leaves 

 under the influence of chang- 

 ing moisture-conditions. The 

 inner or upper surface of the 

 leaf is covered with thin Ion- FlG _ P( $ LYTRICHUM COMMUNE . 

 gitudmal ridges of delicate 



Cells Which Contain chlorO- /,/. fertile plants .one on the left in fruit; 



, ., , .. m, anthendial plant. 



phyll. These cells are shown 



in cross-section in Fig. 296, as dots or granules. All the other 



tissue of the leaf consists of thick-walled, corky cells which do 



FIG. 296. SECTION OF LEAF OF POLYTRICHUM COMMUNE. 



not allow moisture to penetrate. When the air is moist the green 

 leaves spread out, exposing the chlorophyll cells to the air, but in 



