[Chap. XLVIII 



MOSSES AND LIVERWORTS 



641 



Locally, the occurrence of some species is limited by the acidity or 

 alkalinity of the habitat. The pigeon-wheat moss (Polytrichum), the 

 wind-blown moss (Dicranum), and the white-cushion moss (Letico- 

 hrijum ) grow almost wholly in acid situations. The cord moss (Funaria) 

 and the silvery Bryiim are largely limited to alkaline or neutral areas. 

 Other mosses, such as Climachim and Mniwn, grow equally well in both 

 acid and alkaline habitats. Some species of bryophytes survive in very 

 acid, and others in extremely alkaline, substrates. 



Many mosses withstand freezing temperatures at any stage of develop- 

 ment, and in subarctic lands live under snow and ice for the longer part 

 of every year. Whenever the snow disappears, food-making, growth, and 

 reproduction proceed from the stage at which they stopped the previous 

 season. In rock crevices on the mountains of the Antarctic the Bvrd Ex- 

 pedition found living mosses and lichens, where temperatures are seldom 

 above freezing, and in winter often 75"^ F. below zero. Mosses also occur 

 in the water of hot springs. 



Their endurance of cold and drought is not dependent upon any spe- 

 cial anatomical structures. Owing to their habit of growth as compact 



Fig. 295. A tutt ot llcdwigia on a rock substrate where it may become dry and 

 brittle during droughts, but will grow again when it becomes moist. Photo by 

 E. S. Thomas, Ohio State Museum. 



