THE IMPORTANCE OF THE ROOT-SYSTEM 149 



water-vapour. Hence as a general rule, the older parts of the root are of 

 only trifling importance for the absorption of water and salts, while the 

 root hairs, which die off further behind, markedly increase the absorptive 

 power of the younger parts. Wherever an exchange of water and salts is 

 necessary (Sect. 21), the cuticle remains permeable, but in other cases the 

 permeability for water and salts, and also for gases and water-vapour, is 

 more or less markedly diminished by the impregnation of the cuticle with 

 waxy and other substances. This is generally the case in the sub-aerial 

 parts, in which, however, at least in higher plants, the stomata and 

 lenticels afford open passages for gaseous exchange and for the exit of 

 water-vapour. 



Absorption and excretion are directly or indirectly influenced by all 

 agencies which awake special activities in the plant, or divert pre-existent 

 ones into new paths. The progress of development brings about modi- 

 fications in the quality of the absorptive membranes, or in the vital 

 activities of the plant or its members, or even causes certain organs to be 

 exposed to changed conditions of environment. In regard to the latter, 

 it is sufficient to recall the fact that the cotyledons of Ricinus and Pinus at 

 first absorb nourishment from the endosperm and later on function as green 

 foliage leaves, and that the young leaves of Butomus and Nymphaea are at 

 first immersed in water and subsequently float on the surface or rise above it. 

 It is not necessary to depict the various other changes which the progress of 

 development may bring about, or to enumerate the plants which are able 

 to live on land as well as in water, or those which after a parasitic stage 

 continue their life cycle as saprophytes. 



It does not belong to general physiology, but to special biology, to 

 describe the varied adaptations by which individual plants ensure for 

 themselves an adequate supply of nutriment and water, or the means by 

 which the necessary exchanges with the external world are maintained. 

 Hence only a few features of general importance can be dealt with 

 here, and since parasites and saprophytes may be more appropriately 

 discussed in connexion with the absorption of organic nutriment, no special 

 mention need now be made of them. 



SECTION 26. The Importance of the Root-system. 



The differentiation into root and shoot is accompanied by a marked 

 division of labour. Thus in terrestrial plants the roots usually fix the plant 

 to the soil and absorb water and nutritive material, while in order to satisfy 

 the greater demands made upon it as the developing shoot system above 

 ground increases in size, the root system must grow and become stronger, 

 so as to provide a firmer attachment and to render possible an increased 



