SALT RELATIONS OF VASCULAR PLANTS 117 



transpiration rates clearly cause an enhancement of the passive 

 component whereas factors favouring rapid metabolism, e.g. 

 aeration and moderately high temperatures favour active absorption. 

 Mass flow is relatively less important at low external salt concentra- 

 tions, because it varies linearly with concentration, whereas active 

 absorption usually does not (see Chapter 4, pp. 55-7). Damaged roots 

 absorb salts passively to a greater extent than intact ones, and 

 mature roots may also be expected to present less resistance to 

 passive movements of salts, than do young actively growing root 

 systems. The predominating influence of metabolic mechanisms of 

 absorption in young plants under normal conditions is indicated by 

 the existence of root pressure, and by the preferential transference of 

 certain ions from roots into shoots (see below). 



Although the evidence shows that there are effects of trans- 

 piration rate on salt absorption, at least under some conditions, this 

 does not necessarily mean that transpiration served a useful function 

 in mineral nutrition. There is no evidence that when the rate of 

 transpiration is low, plants suffer from salt deficiency. Hoagland 

 (1944) showed that barley plants grown for several weeks with salt 

 supphed only during the night when transpiration was low, contained 

 as much salt at the end of the growing period as did other plants 

 which received salts during the hours of daylight when transpiration 

 was much higher. Species which naturally have a low transpiration 

 rate, tend to grow and absorb salts more slowly than do species 

 which are characterized by rapid transpiration, but it is not likely 

 that the different rates of growth are due to diff'erences in salt supply 

 induced by the level of transpiration. The complex interrelationships 

 of growth, transpiration, salt absorption, photosynthesis and other 

 metabolic processes cannot be disentangled at present. 



C. Transport across the Root Cortex 



Ions entering the root surface move predominantly in a trans- 

 verse direction across the cortex towards the stele (see Fig. 3, p. 15). 

 Those which are not retained by intervening cells are transferred 

 mainly into the xylem, and from there are carried into the shoot. A 

 small fraction moves into the phloem and travels to the root apex 

 (cf.p. 112). 



The passive component of salt probably travels principally in 



' M.S.A.P. 



