PLANT METABOLISM 407 



methyl ester of naphthalene acetic acid and tnaleic hydrazide will keep 

 stored potatoes and carrots from sprouting. 



H H 



C C 



HC^ ^C^ "^C-CH.-COOCHs CH-CONH 



HC^ ^C\ ^CH CH-CONH 



C C 



H H 



Naphthalene acetic acid, Maleic hydrazide 



methyl ester 



Growth substances will hasten the rooting of cuttings (Fig. 15-8) and 

 will inhibit the development of buds. By application of growth substances 

 to flowers, seedless fruits can be produced. 



The most important use of plant growth substances is as herbicides, or 

 weed killers, and this application has greatly modified current agricultural 

 practice. General herbicides such as oils or acids have been employed 

 for many years, but the plant growth substances have the great advan- 

 tage of acting as selective herbicides. For example, much lower con- 

 centrations of 2, 4:-dichlorophenoxy acetic acid ("2,4-D") are required to 



0-CHo-COOH 0-CH,COOH 



I " I 



c c 



HQ^'''"^,CC1 HC^ ^CCl 



I II I II 



HCW4)/CH CIC^ XH 



C X 



CI CI 



"2,4-D'' "2,4,0-T" 



2,4-Dichlorophenoxyacetic acid 2,4,5-Trichlorophenoxyacetic acid 



kill dicotyledonous plants (most broad-leafed plants such as beans and 

 clover) than to kill monocotyledonous ones (cereals, other grasses, lilies, 

 etc.). Thus broad-leafed weeds can be destroyed in a field of grain 

 without damaging the grain. The closely related substance "2,4,5-T," 

 especially in the form of an ester dissolved in an oily carrier such as 

 kerosene, is very effective for killing weedy brush. Both compounds are 

 very useful for the eradication of such weeds as poison ivy. The way 

 in which selective herbicides act is still obscure. 



MINERAL NUTRITION 



The essential elements for the nutrition of plants include the macro- 

 nutrient elements: C, H, 0, N, P, K, S, Mg, Ca, and Fe; and the micro- 

 nutrients: Cu, Zn, B, Mn, and Mo. As was pointed out in the discussion 



