LEAVES 



613 



which consists of a rapid ascent of water between the stems or of a rapid 

 filling of air spaces (or of both in Sphagnum and Leucobryum) ; the second 

 phase is represented by a slow osmotic movement, as in root hairs. 

 The water which is absorbed so rapidly by lichens and by cushion mosses, 

 is lost more slowly by transpiration, Sphagnum being able to absorb as 

 much in a minute as is lost by ordinary transpiration in a week. While 

 some cushion mosses are mesophytic (as in Bar- 

 tramia and some species of Dicranum), others 

 tend toward xerophytism (as in Leucobryum), and 

 even Sphagnum may be called a bog xerophyte. 

 Lichens usually are pronounced xerophytes, and 

 (as with some mosses) absorption must be a com- 

 paratively infrequent phenomenon (particularly 

 since only liquid water can be absorbed in quan- 

 tity), while exposure to transpiration is frequent. 1 

 For aerial absorption to be advantageous in such 

 plants, it must be accompanied by an ability to 

 endure prolonged desiccation, an ability possessed 

 by lichens and by some mosses in superlative 

 degree. 



FIGS. 901, 902. 

 Aspects of Polytrichum 

 commune, a xerophytic 

 moss: 901, a leafy 

 shoot, as seen when 

 the water supply is 

 adequate; 902, a simi- 

 lar shoot that has been 

 exposed to desiccation, 

 the loss of water hav- 

 ing caused the leaves 

 to become appressed 

 to the stem ; when the 

 base of such a shoot is 

 placed in water, the 

 leaves soon assume the 

 position seen in Fig. 

 901. 



Vascular plants. The aerial leaves of ferns and seed 

 plants are cutinized, and water absorption commonly is so 

 slight as to be without significance. 2 Wilted leaves, when 

 placed in contact with water, absorb enough to enable 

 them to recover their usual turgescence, but this phenom- 

 enon probably is of little significance in nature. Water 

 absorption has been predicated as a function of many 

 living leaf hairs, especially in xerophytes, largely, perhaps, 

 because their water supply is scant, and because no other 

 use is known for these hairs. The felted hairs of Centaurea 

 have been thought to be useful in this way, since the leaf can take up thirteen percent 

 of its weight, when placed in water for a day. Salt-secreting hairs, as in Reaumuria, 

 have been shown to absorb water. Recent experiments show that xerophytic leaves as 

 a class possess less capacity for absorption than do leaves in general, any water taken 

 up being lost with rapidity, since it does not penetrate to the living cells. Nor 

 should leaf absorption be expected a priori, above all in xerophytes, since a cuti- 



1 Some recent experiments appear to show that certain mosses in a few seconds can ab- 

 sorb sufficient water vapor to increase their weight 25 to 75 per cent, and that fruticose 

 lichens may absorb enough water vapor to be of appreciable advantage. 



2 In the filmy ferns, which are largely delicate plants of tropical forests, cutinization is 

 slight and absorption by leaves often surpasses in amount absorption by subterranean 

 organs. 



