PLANT PHTSIOLOOT. 85 



and this is replaced by absorption from cell to cell as in 

 the mosses. The fact that moisture escapes through the 

 open stomata has led to the assumption that they are for 

 the purpose of permitting moisture to escape, and that the 

 leaves of higher plants are "organs of evaporation." On 

 the contrary, the stomata are clearly for preventing as far 

 as possible the loss of water, while permitting the free 

 interchange of gases, and the leaf is rather a skilfully de- 

 vised structure in which a multitude of thin-walled cells 

 gorged with moisture are exposed freely to the air with a 

 minimum of loss of water by evaporation. The stomata 

 of the leaves and stem when open admit the external gases 

 to the intercellular spaces of the whole plant, and also 

 allow the internal gases to escape into the air. There is 

 thus a respiration in plants of the high organization of the 

 sunflower, but when examined closely this does not differ 

 in any essential from the simple absorption and excretion 

 of gases by a single-celled plant. 



155. Nutrition of Hysterophytes. — In the hysterophytes 

 (parasites and saprophytes) the solutions absorbed consist 

 partly or wholly of assimilated matter. When this in- 

 cludes the carbon products of assimilation the plant does 

 not develop chlorophyll, as in the dodders, Indian-pipes, 

 broom-rapes, and the vast assemblage of "fungi." "When, 

 however, there is little or no absorption of carbon com- 

 pounds, chlorophyll is present and the leaves are well 

 developed, as in the mistletoe. In the dodders the absorp- 

 tion is performed by suckers (outgrowths) on the stems, 

 and as a consequence the roots do not develop. In these 

 leafless, rootless, and eventually almost stemless plants 

 there is probably little assimilation of any kind ; they are 

 nourished much as the flower- and fruit-clusters of ordinary 



