XXX DISTRIBUTION OF FOOD-MATERIALS 409 



converted into an organ of support : the thickness of its 

 external cells prevents absorption and it contains no 

 chlorophyll. Hence the function of decomposing carbon 

 dioxide is confined to the leaves. 



We have thus as an important fact in the nutrition of an 

 ordinary terrestrial plant that its carbon is taken in at one 

 place, its water, nitrogen, sulphur, potassium, &:c., at another. 

 But as all parts of the plant require all these substances it is 

 evident that there must be some means by which the root 

 can obtain a supply of carbon, and the leaves a supply of 

 elements other than carbon. In other words, we find for 

 the first time in the ascending series of plants, just as we 

 did in ascending from the simple Hydra to the complex 

 Polygordius (p. 278) the need for some contrivance for the 

 distribution of food-materials. 



The way in which this distributing process is performed 

 has been studied chiefly in the higher plants, but its essential 

 features are probably the same for mosses. 



Water is continually evaporating from the surface of the 

 leaves, its place being as constantly supplied by water — with 

 salts in solution — taken in by the rhizoids. This trans- 

 piration, or giving off of water from the leaves, is one 

 important factor in the process under consideration, since 

 it ensures a constant upward current of water, or, more 

 accurately, of an aqueous solution of mineral salts. The 

 withering of a plucked moss-plant is of course due to the 

 fact that when the roots are not embedded in moist soil or 

 in water, transpiration is no longer balanced by absorption.^ 

 In the higher plants it has been found that the root-hairs 

 have an absorbent action independent of transpiration, so 

 that water may be absorbed in the absence of leaves. 



' Mosses, however, unlike most higher plants, can absorb water by 

 their leaves. 



