42 



Growth 



25°. The greatest increase in cell size was with sugar and mineral salts. 

 Yeast extract tended to decrease extension. 



Brown and Wightman ( 1952 ) grew root tips of pea 3.0, 6.0, and 10.0 

 mm. long in sterile culture and found that the peak rate of division occurs 

 later and that its peak value is greater the shorter the initial tip. They 

 conclude that cell division in the meristem depends partly on synthesis of 

 metabolites there and partly on a supply of metabolites from more mature 

 regions of the root. 



I 3 5 7 9 II 



DISTANCE FROM APEX(MMS) 



Fig. 3-12. Changes in protein (circles) and water content (crosses) of bean-root cells 

 at increasing distances from the root apex. ( From Brown and Robinson. ) 



A basic problem in this field is that of protein synthesis. Brown and 

 Broadbent ( 1950) sliced a series of root tips into segments 0.2 to 0.8 mm. 

 thick and in each successive section determined the number of cells, the 

 protein content, the dry weight, and the amount of respiration. During 

 development from the meristematic to the fully extended state the average 

 cell volume increased thirtyfold and there was an increase in protein con- 

 tent and in respiration (Fig. 3-12). 



Genetic factors are concerned in cell activities in many ways. Beadle 

 ( 1931 ) , for example, found a gene for supernumerary divisions in maize; 



