610 ECOLOGY 



plants, however, have soil roots, so that experimentation is necessary 

 to determine the absorptive region. The presence of cellulose epider- 

 mal walls and of a slightly developed conductive system has led to a 

 general belief in the dominance of leaf absorption. 



That in attached submersed plants root absorption is not inconsequential has 

 been shown in some instances, lithium salts having been supplied to the medium 

 around the roots and later detected in the plant, and the ascent of water in the 

 conductive tract has been observed; the abundant development of root hairs in 

 many instances and the greater luxuriance of plants in loam than in gravel also ap- 

 pear to argue for root absorption. However, there is scarcely any doubt that in 

 water plants leaf absorption generally is much greater in amount than is root ab- 

 sorption, though it is likely that plants differ in respect to the relative impor- 

 tance of the two; for example, Potamogeton may depend more upon root absorp- 

 tion than does Elodea, since it grows less readily in an aquarium without a soil 

 substratum. Leaf absorption would seem the more advantageous, since it does not 

 involve the development of a conductive tract and the consequent translocation of 

 water. 



Absorption by chlorophyll-bearing organs in land plants. Algae and 

 lichens. Some green algae (as Vaucheria) and various blue-green 

 algae form felts on moist soil, the entire surface being absorptive, as in 

 water forms; here the wet soil may provide constant water contact, 

 though transpiration at times may be excessive. No group illustrates 

 absorption by the chlorophyll-bearing organs better than do the lichens. 

 For example, the reindeer lichen (Cladonia rangiferina, fig. 898), which 

 is stiff, brittle, and gray when dry, becomes soft, flexible, highly elastic, 

 and green when wet, by reason of the large amount of water it is able to 

 absorb. Upon entering the lichen, the water passes rapidly by capillar- 

 ity between the hyphae and more slowly through the cell membranes 

 and lumina. Lichens as a class are exposed to strong transpiration, and 

 soon lose most of the water that is so quickly absorbed. 



Bryophytes. Probably all mosses and liverworts are able to absorb 

 water through the chlorophyll-bearing organs, whenever they are 

 moistened by rain or dew. In some leafy liverworts water catchment 

 and absorption are facilitated by a cuplike arrangement of the leaves, 

 but most liverworts and many trailing mosses (as species of Hypnum) 

 grow so near the substratum that water absorption generally is possi- 

 ble without special structural arrangements. The situation is different 

 in erect mosses with more or less solitary stems (as Polytrichum, Catha- 

 rinea, Climacium), in which rhizoid absorption probably exceeds leaf ab- 

 sorption in amount, the latter being confined chiefly to periods when 



