Chemical Factors in General 



371 



Robbins and his colleagues (1929) have observed a shape difference re- 

 lated to potassium. Sweet potato plants grown with little potassium have 

 longer and more slender roots as compared with "chunkier" ones on 

 plants grown where this element is abundant. The difference seems to be 

 due to the greater cambial activity in the well-nourished plants as a 

 result of increased protein synthesis resulting from an abundance of 

 potassium. 



Zinc has indirect morphogenetic importance since it seems to be 

 necessary for the maintenance of auxin in an active state (Skoog, 

 1940; Fig. 17-2). 



Boron evidently is concerned with the development of the cell wall 

 and affects the process of carbohydrate condensation into wall material 

 (Spurr, 1957; Fig. 17-3). Reduction in amount of boron produces hyper- 



lAA+Co 



Fig. 17-4. Effect of cobalt on etiolated pea 

 stem segments supplied with auxin alone 

 (below) and auxin plus cobalt (above). 

 ( From Miller. ) 



O 



12 3 4 



PER CENT SUCROSE 



trophy and hyperplasia of tissues, especially the cambium, in tomato, 

 turnip, and cotton and affects the number and the maturation of the fibers 

 (Palser and Mcllrath, 1956). MacVicar and Struckmeyer (p. 320) ob- 

 served that day-length altered the boron requirement of soybeans. 



Cobalt has been found by several workers to increase the size of cells, 

 particularly in association with sucrose. Miller (1954) reports that in 

 etiolated pea stems cobalt salts plus auxin produced only slight elonga- 

 tion. Sucrose alone had the same effect, though it considerably increased 

 fresh weight ( Fig. 17-4 ) . When sucrose and cobalt were applied together 

 to the pea stem there was greatly increased elongation. He believes that 

 water uptake and wall growth are separate processes and suggests 

 that sucrose tends to increase cell volume but that cobalt promotes the 

 ability of the cell to enlarge its surface. 



