DISTRIBUTION OF INHIBITORS IN PLANTS 407 



immediately and directly affect its own absorption, and absorption in gener- 

 al, as soon as it gains access to the root cells. If inhibitor distribution 

 throughout the plant is desired, the concentration of the inhibitor sur- 

 rounding the roots must be below that level where the roots are damaged . 

 To gain entrance to the conducting system, the inhibitor must pass through 

 at least two plasma membranes and it is likely that penetration of ionic 

 inhibitors will be relatively slow because of the specialization for uptake 

 of certain physiologically important ions. 



An inhibitor applied to the surface of a leaf meets with four possible 

 barriers before entering an epidermal cell. From the cell outwards there 

 is the plasma membrane, the epidermal cell wall, the cuticular layer, and 

 the extruded waxy deposit on the surface. The cell wall probably seldom 

 presents any barrier to diffusion and the plasma membrane behaves typi- 

 cally and shows selective permeabilities just as in animal cells. It is the outer 

 layers which present some unique problems. In the first place, penetration 

 is generally not through the stomata because of capillary forces preventing 

 access of the solution; if surfactants are included with the inhibitor, stomatal 

 penetration is possible. This is quite similar to absorption of substances 

 through the hair follicles and sebaceous glands of the skin. The waxy material 

 is deposited in various ways characteristic of the plant and presents a bar- 

 rier to diffusion mainly by preventing adequate contact between the surface 

 and aqueous solutions. The wax is, however, often spongy and may swell 

 after contact with water, the penetration rate of a dissolved substance in- 

 creasing. The herbicide 2,4-0 is absorbed very slowly from ordinary aqueous 

 solutions when applied to leaves; a surfactant increases penetration five- 

 fifteenfold. The absorption and distribution of 2,4-D is discussed thor- 

 oughly by Loomis (1955). The cuticle is composed of pectin and the fatty 

 material, cutin, in various proportions, possibly in some cases associated 

 with cellulose. The permeability properties of the cuticle are poorly under- 

 stood and its role in inhibitor penetration can only be surmised. In any event, 

 penetration of large molecules or ionic substances into epidermal cells is 

 slow, although lipid-soluble substances may enter readily. The entrance of 

 2,4-D into phloem tissue occurs at a rate of 304 /^/hr in bean leaves but 

 once within the phloem, transport is much more rapid and may amount to 

 10-100 cm/hr (Day, 1952). 



Translocation of Substances in Plants 



Many substances necessary for plant growth must be transported be- 

 tween tissues. The salts and nitrogenous material absorbed by the roots 

 must be transported upwards into the growing regions of shoot and leaf, 

 while the organic substances synthesized in the leaves are transported to the 

 roots. It is probably along these normal pathways of movement that inhi- 

 bitors are transported. The excellent reviews of translocation processes by 



