LEAVES 



611 



the leaves are moist from rain or dew; in addition to lessened opportu- 

 nity for leaf absorption, such mosses also are exposed to greater tran- 

 spiration than are mosses in general. In other mosses and liverworts 

 rhizoid absorption probably supplements leaf absorption, but it is 

 probable that in most cases the latter exceeds the former (p. 517). 

 Remarkable cases of leaf absorption are afforded by various mosses 



FIG. 898. Cushion plants, which absorb water readily through the aerial organs; 

 the light-colored cushions are reindeer lichens (Cladonia rangiferina), and the dark-colored 

 cushion is a moss, Bartramia pomiformis ; Grand Marais, Minn. From MAcMiLLAN. 



that grow in dense cushions (as Dicranum, Leucobryum, Sphagnum, 

 fig. 898). Such cushions may be regarded as systems of capillaries con- 

 nected with the substratum ; thus a constant supply of water is available 

 for the leaves, except in very dry weather. Since each year the individual 

 stems of a cushion grow at the apex and die at the base, it is unlikely 

 that the soil rhizoids are of any significance in absorption, at least after 

 the cushion is a few years old; the aerial rhizoids that develop in some 

 species probably facilitate capillary activity. The most noteworthy of 



