4l8 CRAFTS 



if dosage is heavy, may be prolonged and may even result in death. Potato 

 tubers and onion bulbs are prevented from sprouting normally when the tops 

 are sprayed with MH before harvesttime. 



Amino triazole may kill some plants by a slow contact action; others 

 respond by producing chlorotic shoots, and death may eventually result from 

 starvation. At lower dosages a temporary chlorosis may be produced. Evi- 

 dently the enzymes responsible for chlorophyll formation are affected, but the 

 exact role of this response in plant death is not known. 



This is not the place to give a detailed review of herbicidal action. Such 

 reviews have been written by Norman et al. (1950), Blackman et al. (1951), 

 Crafts (1953), and Leopold (1955). The examples that have been given illus- 

 trate the great diversity of the mechanisms involved in plant killing with 

 herbicides. They indicate the scope of the field presented for physiological 

 and biochemical studies. And they round out the picture of the impact that 

 chemical plant control has had on agriculture. Use of herbicides undoubtedly 

 represents the greatest advance in agricultural technology since the intro- 

 duction of artificial fertilizers. 



Discussion. Those familiar with weeds and weed control recognize the 

 great contribution the science of botany has made to this field of knowledge. 

 Growth and development of weeds is typical of higher plants in general ; the 

 great competitive vigor of weeds is an example of "survival of the fittest" 

 under disturbed conditions; chemical weed control involves and exemplifies 

 many aspects of plant physiology; the methodology of weed control makes 

 many direct applications of the physiology and biochemistry of plants. 



It might be interesting, before closing, to examine the other side of the 

 coin: what has weed control contributed to botany? To illustrate this, one 

 might picture what happens in using a systemic herbicide. Following applica- 

 tion of a systemic herbicide to a plant, the chemical, if successful, penetrates 

 the cuticle, migrates across the mesophyll, enters the phloem where it mixes 

 with the newly synthesized foods in their movement to points of utilization, 

 and finally enters active cells, usually meristematic, where it brings about a 

 lethal reaction. In this same order I would like to point out some recently 

 acquired knowledge gained through research on herbicides. 



Absorption. Many chemicals can move through the cuticle of plants; this 

 so-called impervious coating is apparently more pervious than commonly 

 pictured. Some chemicals, applied in large amounts, alter the cuticle and enter 

 readily; others, used in smaller quantities, may move in as undissociated 

 molecules, soluble in the lipoid cuticle layers; others apparently penetrate 

 through an aqueous phase. Thus the cuticle seems subject to alteration by 

 strong or corrosive chemicals; surfactants in smaller amounts may increase 

 its permeability; and some organic herbicides enter in fat- or water-soluble 

 forms. It seems reasonable to assume that the cuticle is labile and composed 

 of at least two separate phases. 



