148 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



precisely similar, quantitative differences at least must exist in their ab- 

 sorptive and excretory powers, and even in the submerged non-septate 

 Caulerpa a differentiation of this character exists between the root and 

 shoot parts, while in young non-septate Mucor plants the differences existing 

 between the sub-aerial hyphae and the subterranean mycelium are still more 

 marked. 



Under normal conditions, the functional powers of which a plant is 

 capable are not all exercised, or at least not to the full possible extent. 

 Thus, aquatic organs are able to absorb gases directly from the air, while 

 sub-aerial organs can absorb water and dissolved substances to a greater 

 or less extent. Probably this power is not entirely absent from any leaf or 

 young stem, provided that, if necessary, wetting is rendered certain by the 

 removal of any waxy layer present. Nevertheless the cuticle and cork 

 commonly so diminish the power of absorbing fluids, that even under the 

 most favourable experimental conditions the loss of water by transpiration 

 cannot be made good by absorption through these layers, nor can a 

 sufficient supply of nutrient material be absorbed through cuticularized 

 epidermis. 



The members of a plant must necessarily be so constructed as to be able 

 under normal conditions to satisfy their own special needs and the general 

 requirements of the entire organism. Full attention must therefore be paid 

 to the importance and function of each organ, to the source and utilization 

 of water and nutrient material, as well as to the nature of the surrounding 

 medium, and the external influences which operate upon the organ in 

 question. All these factors together enable us to form a correct general 

 estimate of the demands made upon the functional activities of any given 

 organ. From considerations of this kind, it is readily explicable that in 

 sub-aerial organs the necessity of protection against excessive transpiration 

 causes the absorptive power for water and salts to decrease almost to 

 nil, while the parts growing in water or soil are incapable of a sub-aerial 

 existence either entirely or as far as young absorptive parts are concerned, 

 owing to the readiness with which they lose water by evaporation. In 

 general, when a particular purpose is in view, the other needs of the 

 organ, as well as the general conditions for existence, must be taken into 

 consideration, and hence the existence of a certain correlation of growth 

 between the root and shoot is essential. The former, as it develops more 

 fully, affords a firmer support and acquires an increased power of absorption. 

 Moreover, as has already been mentioned in more general terms (Sect. 6), 

 members of unequal morphological value may be entrusted with similar 

 physiological functions and vice versa. 



The different parts of an organ are not however of equal functional 

 importance. Those parts on which cork and cuticle are best developed are 

 the least permeable to water and dissolved salts, as well as to gases and 



