268 The Living Plant 



(b), development of water-storing tissues and organs, (c), ar- 

 rangements that minimize loss by transpiration. 



The absorbing system of typical plants, as the reader now 

 knows very well, consists principally in the innumerable root 

 hairs, which draw water osmotically from a wide area all around 

 them. Plants that live in dry places usually exhibit, either as an 

 individual adjustment or a structural adaptation, a marked 

 intensification of one or more of the features involved in this 

 absorption; — that is, the number of young roots is larger, the 

 hairs are more profuse, the osmotic solutions are stronger, or the 

 total range of the root system through the soil is greater. The 

 increased profusion of hairs in drier situations is manifest when 

 young roots are grown in damp air, w^here they make a far 

 greater display than ever they do in the soil; w^hile the much 

 wider range and greater freedom of branching attained by root 

 systems in plants that grow in dry places, helps to explain why it 

 is that the plants of the deserts are spaced so widely apart, with 

 large open areas between them. The presence of stronger osmotic 

 solutions inside the absorbing root hairs is distinctive not only of 

 some desert plants, but also of others which grow in a different 

 situation where water is hard of absorption even though present 

 in plenty, — namely, in salt marshes, where the water itself is a 

 markedly osmotic solution of appreciable strength. As was 

 shown in the chapter on Absorption, osmotic absorption by roots 

 is dependent on a superiority in strength of the inner over the 

 outer solution, and is the slower and harder the more nearly the 

 two approach the same concentration. It is this difficulty of 

 osmotic absorption from salt water which explains why large 

 vegetation, while crowding as close as it can to fresh-water 

 streams and lakes, keeps awa}^ from the corresponding situations 

 along the margin of the sea. 



The storage of water is the second of the methods protective 

 in plants against dryness. All living cells of all plants, indeed, 

 possess plentiful stores of water in their sap-cavities, which 



