188 Plant Tissue Culture 



and Koepfli, 1939, 343; White, unpublished). 

 But it is also possible to observe, at concen- 

 trations just exceeding the threshold level for 

 growth retardation, a characteristic shorten- 

 ing and thickening of the root axis, resulting in a 

 crowding of the branch primordia into a relatively 

 small space (Fig. 48) (Leonian and Lilly, 1937, 

 219). This gives an apparent but not real increase 

 in the number of branches and simulates a ' ' root- 

 ing" response, although in point of fact a rapidly 

 growing culture of the same age, grown in a nutri- 

 ent lacking indole acetic acid, would normally 

 form just as many branches but scattered over 

 from 10 to 20 times as great a length of root. Root 

 cultures deprived of potassium continue to grow 

 in length with unimpaired vigor but become pro- 

 gressively more and more slender and fail to 

 branch, as if all the potassium present in the orig- 

 inal explant was being retained in the apical meri- 

 stem so that none was available for the formation 

 of branch primordia. Omission of iodine from the 

 nutrient likewise leaves the culture quantitatively 

 unimpaired but results in a spasmodic growth, the 

 tips dying after 3 or 4 days and a series of new 

 branches developing, giving a picture quite differ- 

 ent from that produced by potassium deficiency 

 (White, in press). 

 Changes in activity. As has already been 



