410 CRAFTS 



the discovery of the activating function of acids, it became apparent that 

 penetration of plant cuticle demanded the proper chemical environment. A 

 satisfactory theory of absorption has been elaborated that explains the role 

 of acid in the activation of dinitrophenols and the means by which the 

 phenoxy compounds may be formulated to foster absorption (Crafts, 1945). 

 More recently Orgell (1954) has drawn attention to the role of adsorption in 

 the uptake of foliar sprays and has shown that there is a peak of uptake on 

 the alkaline side of neutrality as well as the plateau on the acid side of the 

 pK value. 



In essence, these workers suggest that for ready absorption through the 

 cuticle a molecule must be uncharged or positively charged and it must be 

 soluble in the cuticle. The uncharged molecule can approach the cuticle, dis- 

 solve in it, diffuse through, and, if sufficiently water-soluble, part from it into 

 the epidermal cells and so migrate into the mesophyll. The positively charged 

 particle is attracted to the negatively charged cuticle and is adsorbed to it; 

 if soluble in the cuticle it may readily pass into and through it. However, 

 in order to part from the cuticle its positive charge must be removed or 

 neutralized; this could be accomplished by exchange with the contents of 

 the epidermal cells or by enzymatic splitting of the molecule. Accumulating 

 evidence indicates, in the case of urea, that the molecule is split to CO2 and 

 some nitrogen compound (Tukey, 1954). 



The theory of the ready absorption of the associated parent molecules of 

 dinitro and phenoxy herbicides seems well established from over ten years' 

 experience. Apparently Dalapon follows the same rule (Wilkinson, 1956). 



Maleic hydrazide and amino triazole seem not to follow this rule. From 

 research in progress, these molecules seem to enter the leaf through an 

 aqueous pathway. This is extremely interesting in view of Lambertz' (1954) 

 evidence for the presence of protoplasmic strands in the cuticle. Also pertinent 

 are the observations of Scott (1955) and Loomis (1956) that the cuticle 

 under the electron microscope has a sponge-like appearance. If it can be con- 

 clusively shown that the cuticle combines lipoid and aqueous phases, making 

 possible the entry into the leaf of either fat- or water-soluble compounds, a 

 much wider array of compounds may find use as herbicides. Much research 

 is still needed here. 



An important point in the absorption of herbicides is that if they are to 

 be translocated they must be able to migrate into the vascular strands with- 

 out seriously impairing the photosynthetic mechanism of the leaf, for their 

 ultimate transport into the roots depends upon food movement. This de- 

 mands a sort of selectivity within the treated plant, for the leaf tissues must 

 be spared whereas the root and bud tissues must be killed. For the chloro- 

 phenoxy compounds, differences in maturity may account for this selectivity. 



Surfactants are used in most spray formulations, and much research sup- 

 ports the claims that they aid in penetration. Undoubtedly they aid in wetting, 



