76 HOW TO KNOW THE MOSSES 



Andreaea is separated from the true mosses on ac- 

 count of the structure of the capsule, which is incon- 

 spicuous because it is about the same dark color as the 

 rest of the plants and is raised only a little above the 

 end of the stem or branch. It can best be seen, when 

 dry, by looking across the plants to the light. When 

 dry, the four parts of the capsule are spread apart; 

 when moist, they are closed and resemble the end of a 

 branch. Andreaea forms a close, dark growth on rocks 

 somewhat similar to that of Grimmia (p. in), Ortho- 

 trichum (p. 117), and Hedwigia (p. 172). It is much 

 less frequent than any of these, as it occurs only in 

 mountainous regions, and the stems and branches as a 

 rule are shorter and much more slender. The tufts are 

 less compact than in Orthotrichum, and the stems usu- 

 ally much shorter than in Grimmia and Hedwigia, 

 The presence of the fruit in any case will distinguish 

 the genus. 



