GENERAL FEATURES OF THE ABSORBING SYSTEM 217 



general mode of nutrition of a plant, while it also throws some light 

 upon the nature of its habitat and upon its other ecological 

 peculiarities. 



In conclusion, the general features which are common to all the 

 different types of absorbing system may be briefly indicated. The 

 principle which, above all, dominates the construction of this system, is 

 that of the maximum exposure of surface ; for, clearly, the quantity of 

 nutritive material that can be absorbed by osmosis, is ceteris paribus 

 directly proportional to the area of the absorbing surface. With 

 regard to the disposition of absorbing tissues, it is obvious that the 

 latter must be situated near the periphery, and in fact must consist of 

 superficial cell-layers. A third feature of absorbing cells, and one 

 which greatly facilitates diosmosis, is the unthickened character of 

 their walls ; this condition is, however, not incompatible with the 

 development of local thickenings for mechanical purposes. Mention 

 must finally be made of a purely physiological but very characteristic 

 and important feature of living absorbing tissues, namely, their 

 capacity for producing and excreting acid substances and enzymes ; 

 were it not for this property the solution of solid food materials would 

 be very slow, and in many cases quite impracticable. 



//. THE ABSORPTION OF INORGANIC COMPOUNDS. 



A. THE ABSORBING SYSTEM OF SUBTERRANEAN ROOTS. 



The subterranean root-system of terrestrial green plants, the axial 

 and foliar organs which, in certain cases, perform the functions of 

 roots and the roots of water plants, carry out the absorption of water 

 and soluble salts with the aid of tissues which are essentially of the 

 same type in every case. In discussing this form of absorbing tissue, 

 we shall throughout regard the ordinary subterranean root as the typical 

 absorbing orran. 



Absorption is not carried on over the whole surface of a subter- 

 ranean root-system. It can, on the contrary, be easily shown that the 

 functional absorbing tissue only occurs on the youngest rootlets, and 

 that it is, moreover, confined to a definite zone, which begins close behind 

 the subapical growing region, and ends at a varying distance from the tip 

 owing to the degeneration and exfoliation of its cells on the older parts 

 of the root. In the case of the commonest type of absorbing system, 

 which is characterised by the presence of " root-hairs " (see below), the 

 restricted distribution of the actual absorbing tissue may be demon- 

 strated by means of a very simple experiment. All that is necessary 

 is to dig up a plant carefully with its roots intact, and to attempt to 



