The Living Materials 57 



liminary work in developing a tissue and organ 

 culture technique, and practically all of the fun- 

 damental steps in that development have been 

 made first with root tips and only later applied 

 to other tissues. Such roots are, in fact, very 

 sensitive to differences in cultural conditions such 

 as temperature, pH (White, 1932, 63), osmotic 

 value (Bonner and Addicott, 1937, 42), concentra- 

 tion and proportion of nutrient salts (White, 1933, 

 141, 1937, 143; Bonner and Addicott, 1937, 42), 

 carbohydrate supply (White, 1932, 63, 1940, 172, 

 173), etc.; this sensitivity manifests itself either 

 in change of growth habit (qualitative) or change 

 in growth rate (quantitative). Growth rate can 

 be measured quite accurately by any of a number 

 of methods (see later), so that it is possible to 

 follow the effects of relatively small differences 

 in environmental conditions and to establish opti- 

 mal ranges for these conditions. 



There remains a variety of meristematic re- 

 gions which are more difficult to classify. The 

 entire young flower, and particularly the ovary, 

 is at first meristematic. This function is retained 

 in some tissues for a long time. Wehnelt (1927, 

 230) and subsequently Bonner and English (1938, 

 342) have taken advantage of this fact in using 

 the inner surface of bean pods as test-pieces for 

 a study of the growth stimulating properties of 



