642 TEXTBOOK OF BOTANY 



cushions or as sponge-like mats, water accumulates during rains and fogs 

 by capillarity and remains available for much longer periods. When the 

 plants become dry, life processes are largely suspended; but in nature 

 recovery is possible even after prolonged droughts (Fig. 295). Moss 

 plants dried for several days in the laboratory fail to recover, but the 

 same species endure droughts of several weeks out-of-doors. Apparently 

 they secure enough water from the atmosphere at night to survive. The 

 spores of certain mosses, however, have germinated after being kept for 

 14 years in the dry air of a herbarium. 



Adequate light, a moist atmosphere, a rather constant water supply, 

 and suitable temperatures are all essential to the abundant growth of 

 most bryophvtes. Some grow in the entrances to caves and below rock 

 ledges, where the light is as low as 1/500 of full sunlight. Moss plants 

 have been found living in complete darkness. Obviously these plants 

 were living as saprophytes similar to the subterranean green algae. The 

 species of moss that lives at a depth of 180 feet in the water of Lake 

 Geneva, Switzerland, survives at an extremely low light intensity, and 

 must have a very low compensation point. The so-called "luminous 

 moss" (Schistostega) found under rock ledges and in dark recesses in 

 woods is probably the material basis of the fairy tales of "goblin gold." 

 The cobwebby threads of the green plant are easily overlooked among 

 the rock particles. From certain angles, however, the refracted and re- 

 flected light rays from the lens-like cells of the plant are seen as a golden- 

 green glow. 



None of the bryophytes is more than a few inches tall. They are 

 simple in structure, although many of them have organs that super- 

 ficially resemble leaves, stems, and rootlets. No conducting system is 

 present and the cell walls are not lignified. Mosses imbibe water rapidly, 

 even from moist air. The cells of the green parts of the plants have well- 

 developed round chloroplasts. No true parasites are known among bryo- 

 phytes, but many species are probably partial saprophytes. Moss com- 

 munities frequently become the substrates of lichens and die in their 

 shade. Their remains accumulate as humus on bare rock surfaces, and 

 become the substrate in which the roots of ferns and other plants may 

 grow. The remains of these plants in turn contribute further to the 

 humus, and the roots of the living plants bind the material together and 

 anchor it still more securelv. These are the pioneer stages of soil develop- 

 ment, and of vegetation on exposed rocks. 



Owing to their rapid and compact horizontal spreading, bryophytes 



