WEED control: applied botany 411 



and they dry down to a film that holds the chemical in intimate liquid con- 

 tact with the leaf surface. Do they do more than this? Conceivably some 

 surfactants may alter the cuticle, rendering it more permeable. Here again 

 research is needed to clarify this important point. 



Translocation. To kill roots following application to the foliage a herbicide 

 must pass down through the vascular system. Much research substantiates 

 the view that the phenoxy compounds are carried from leaves to roots along 

 with food materials via the phloem (for a review see Crafts, 1951). This 

 mechanism makes certain demands on the physiology of the plant, and many 

 field observations indicate a close correlation between the satisfying of these 

 demands and the success of translocated herbicides. Use of labeled 2,4-D has 

 greatly strengthened the evidence for the mechanism of transport (Crafts, 

 1956a, b; Leonard and Crafts, 1956). 



Evidently two problems confront the user of translocated herbicides. First, 

 he must find the proper time and condition for treatment ; second, he must use 

 a formulation that will penetrate the cuticle, migrate to the vascular tissue, 

 and move to the proper site of action without killing the intervening tissues. 

 The first requires that the plant has attained a degree of maturity so that 

 the leaves are hardy enough to take in the chemical; foods must be moving 

 to the roots; the roots in turn must be actively vegetative, having adequate 

 soil moisture for growth. Apparently, given the proper conditions, transloca- 

 tion seldom fails. Problems in this area involve penetration of the chemical, 

 partition from the cuticle, and freedom from adsorption by specific compounds 

 in the plant that hold the chemical away from its site of action. 



These problems are tied up with species susceptibility and can only be met 

 by careful studies in the greenhouse and in the field. Those who have worked 

 with modern herbicides realize that there remains at present a tremendous job 

 of fitting compounds into the hundreds of niches that exist in the field. And 

 this involves not only search for new compounds but also careful studies on 

 many of the old ones to make sure that the possibilities of formulation and 

 field practice have been exhausted. A very unfortunate aspect of the whole 

 program of synthesis and screening being carried on by industry today is 

 the fact that not enough weed and crop plants are being used, and the condi- 

 tions of screening are not varied enough to preclude the possibility of missing 

 valuable chemicals. 



Root applications. The application of herbicides to the soil for absorption 

 by roots presents a whole new set of problems. These involve such soil prop- 

 erties as chemical composition, particle-size distribution, colloid content, 

 reaction, organic matter, microflora, depth, etc. Also involved are precipita- 

 tion, temperature, vegetative cover, crop history, and the like. 



All soil treatments by herbicidal chemicals have been conveniently termed 

 soil sterilization, and they are arbitrarily classified as temporary and rela- 

 tively permanent (Robbins et al., 1952). When crops are involved, the treat- 



