SALT RELATIONS OF VASCULAR PLANTS 131 



climates where the air temperature rises in spring much more 

 rapidly than that of the soil. 



Amongst factors affecting foliar absorption, the surface area, 

 shape and arrangement of leaves are of foremost importance. In 

 general, leaves of typical monocotyledonous plants retain solutes 

 less readily and hence absorb them less efficiently than do leaves of 

 many dicotyledons. Before salts can enter a plant, the leaf surface 

 must be wetted and the ease with which this can be achieved depends 

 on, among other things, the fat content of the cuticle, the number of 

 appendages and the surface tension of the applied solution. Fogg 

 (1947) found great differences in the wettability of leaves depending 

 on the species examined, the age of the leaf and its water content. 

 Roberts et al. (1948) demonstrated the presence of layers of pectin- 

 aceous material in the cuticle of apple leaves which are continuous 

 with similar layers in the walls of the epidermal cells below. It is 

 possible that solutes can move along these pathways into the leaf. 

 Although hairs tend to decrease the wettability of a leaf because 

 they trap air bubbles, they increase the amount of solution held at 

 the leaf surface once wetting is achieved. Addition of surface- 

 active substances or wetting agents to the solution promotes the 

 absorption of salts under some conditions. Uptake is clearly 

 related to the length of time an aqueous solution remains in contact 

 with the leaf surface, and therefore it is affected by factors such as air 

 movements, temperature and humidity which influence the rate of 

 evaporation. 



There is some controversy about the importance of stomata as 

 ports of entry of solutes into leaves. Crafts (1933) claimed that the 

 pores of fully-open stomata are generally small enough for surface 

 tension to prevent penetration of aqueous solutions. In support of 

 this, Rodney (1952) found no correlation between the number and 

 condition of stomata and the rate of entry of calcium nitrate and 

 ammonium sulphate into leaves of apple trees. Tiny drops of 

 solution do sometimes penetrate stomata directly when a heavy 

 spray is applied, leading to infiltration of the sub-stomatal spaces, 

 and increased absorption. According to F. M. Scott (1950) a layer 

 of cuticle commonly covers the inner walls of leaf epidermis and the 

 walls of mesophyll cell, adjoining stomatal cavities, so that entry 

 through the stomata does not necessarily absolve the nutrients from 

 traversing cuticularized cell walls. 



