S88 



ECOLOGY 



resting stages are able to endure the high temperatures of hot springs or the low- 

 temperature of liquid hydrogen, and are able to withstand the desiccation of the 

 desert. Even algae, though characteristic hydrophytes, may occur, apparently un- 

 protected, on dry rocks or in the snow and ice, the blue-green algae in particular 

 being about as resistant as are the bacteria. Lichens absorb and transpire water 

 quickly, enduring long droughts in a desiccated condition without injury ; indeed, 

 they may be regarded as among the most resistant 

 of plants, in spite of their lack of obvious protective 

 structures, a fact that is all the more remarkable 

 since they are complexes of algae and fungi, groups 

 which separately flourish best in water and in moist 

 woods respectively. Desiccation in these plants 

 induces merely a resting stage or stage of suspended 

 animation, in which the small amount of water 

 needed to preserve life is retained with great tenac- 

 ity. Very probably it is the ability of these plants 

 to retain this necessary modicum of water that 

 accounts for their great resistance to detrimental 

 factors. The ultimate cause of resistance here would 

 seem to be some "specific property of the proto- 

 plasm," whose nature is as yet unknown. 



Protection by coverings of snow and of dead 

 leaves. — The ma~ tie of fallen leaves which covers 

 the ground in forests, and the dead leaves remain- 

 ing on the grasses and other plants of meadows and 

 swamps, are of great value in protecting herbaceous 

 vegetation from the rigors of winter. On some trees, 

 as in Yucca (fig. 847) and in various palms (fig. 951), 

 the leaves or leaf bases remain on the stem after 

 death, forming a thick protective layer. Similarly, 

 in cold climates the snow cover is of great protective 

 value, although winter thaws and irregular drifting 

 often leave the ground bare and the vegetation un- 

 protected. In alpine meadows the deep and long- 

 enduring mantle of snow most effectively protects 

 the delicate alpine herbage from the severities of 

 winter. Layers of snow or leaves tend to conserve 

 the soil warmth, and thus are of value in protecting 

 the subjacent vegetation from the deleterious effects 

 of sudden temperature changes, but they are of 

 much greater value in reducing transpiration to a minimum at a time when the 

 low soil temperature prevents absorption. The winter killing of unprotected wheat 

 and of other vegetation is in most instances due to excessive transpiration rather 

 than to freezing. 



Summary on transpiration and carbohydrate S5mthesis. — A review 

 of the preceding pages indicates the existence of a reciprocal relation 



Fig. 847. — A tree yucca 

 (Yucca arborescens); note the 

 rigid, needle-like, many-ranked 

 leaves, which lop back against 

 the stem, serving long after 

 death as a protective covering; 

 among the shrubs is the creo- 

 sote bush (Larrea tridentata); 

 Victor, California. — Photo- 

 graph by E. W. COWLES. 



