LEAVES 



615 



Probably the most xerophytic of all plants are the crustose lichens 

 (as £«(e«ia), which are either epiphytic or epilithic {i.e. growing on rocks), 

 appearing embedded within the substratum. Most crustose lichens ab- 

 sorb water chiefly through the upper surface, though some species have 

 upper surfaces which are not readily wetted or which are covered with 

 an impermeable crust. On dry rocks, at least, such plants absorb water 

 chiefly during or immediately 

 following precipitation, soon 

 drying out again, and enter- 

 ing a period of inactivity; 

 the active period may be 

 longer in the case of bark 

 lichens, owing to the greater 

 retentiveness of the substra- 

 tum. Foliose lichens (for ex- 

 ample, Parmelia; fig. iiii) 

 are more leaflike and are at- 

 tached to rock or tree surfaces 

 by evident rhizoids ; perhaps 

 also they are less xerophytic, 

 though capable of withstand- 

 ing prolonged desiccation. 

 Foliose lichens commonly ab- 

 sorb through both surfaces, 

 but mainly through the lower 

 surface, except in a few 

 instances (as in species of 

 Parmelia with black under 

 surfaces) . The gelatinous 

 lichens (as Collema) absorb 

 great quantities of water and remain for some time as mucilaginous 

 masses. Possibly the rhizoids supplement the chlorophyll-bearing 

 organs in absorption, but to what extent if any is unknown. Some 

 mosses (as Andreaea and Grimmia) are epilithic xerophytes and with- 

 stand prolonged desiccation without harm, reviving rapidly during rain 

 as a result of leaf absorption. 



Bromeliaceae. — The most remarkable instance among seed plants of absorption 

 by aerial leaves is in the Bromeliaceae, a family of tropical epiphytes, repre- 

 sented in the southern United States by Tillandsia, especially T. usneoides, the 



Figs. 904, 905.- — Absorptive scale hairs of 

 Tillandsia: 904, a scale hair, as seen in surface 

 view; 905, a scale hair, as seen in cross section; 

 the outer cells (c) are dead, and absorb water 

 readily when moistened, thus elevating the scale 

 and opening the channel, aa, along which water 

 passes by capillarity; subsequently the water 

 enters the living stalk cells (j); c, epidermis; 

 highly magnified. 



