LEAVES 



609 



spiration, are equally effective in checking absorption. Where the outer 

 walls are of cellulose, absorption and also transpiration may take place, 

 so that leaf absorption is advantageous chiefly where transpiration is low 

 or wanting. 



Absorption by chlorophyll-bearing organs in water plants. Algae 

 and bryophytes. In nearly all water plants the outer walls are of cel- 

 lulose, and, since there is constant water contact and freedom from tran- 

 spiration, no conditions are more suitable for water absorption by chlo- 

 rophyll-bearing organs. In many algae there is a homogeneous green 

 body which absorbs water through its entire surface. In other forms, 

 as in Chara and Bryopsis (figs. 1074, 1075) and in large marine algae (fig. 

 751), there are rhizoids, which are 

 regarded as anchorage organs. 

 Probably the rhizoids are permeable, 

 but the much more extensive chlo- 

 rophyll-bearing portion with its 

 permeable walls is vastly more 

 important from the standpoint of 

 absorption. The aquatic liverwort, 

 Riccia fluitans, has a homogeneous 

 green body without rhizoids, its 

 mode of absorption being compara- 

 ble to that of algae with similar 

 aspect. In aquatic mosses (as in 

 Fontinalis) water probably enters 

 chiefly through the leaf surfaces. 



Vascular plants. There are 

 some rootless aquatic ferns and seed 

 plants in which all water and salts 

 must enter through the leaf or stem 

 surface, as in Utricularia vulgaris 

 (fig. 909) and in Ceratophyllum. In 

 Salvinia there are synthetic floating 

 leaves and absorptive water leaves, 

 the latter being finely dissected and 

 quite unlike ordinary leaves (fig. 

 897). In Wolffia (fig. 997) there is a thalloid body, of which the 

 submersed lower part is a region of absorption, while the emersed upper 

 part is a region of synthesis and gas exchange. Most submersed seed 



FIG. 897. A plant of Salvinia, 

 natans, showing the broad floating leaves, 

 whose upper surfaces have aerial rela- 

 tions, and the dissected, descending sub- 

 mersed leaves, which absorb water and 

 salts from the medium and which bear 

 reproductive organs; note the abundant 

 leaf hairs. From COULTER (Part I). 



