186 Plant Tissue Culture 



which they have no apparent connection. Thus, 

 cultures grown in a glycine-thiamin nutrient give 

 results which, at their best, are superior to the best 

 results obtained in a yeast nutrient, yet in a gly- 

 cine-thiamin nutrient they appear to be far more 

 sensitive to temperature changes than in a yeast 

 nutrient, so that at suboptimal or supraoptimal 

 temperatures the relations between growth rates 

 in these two nutrients may be just reversed 

 (White, unpublished). The presence of a large 

 number of substances of various character in the 

 yeast extract seems to "buffer" the cultures 

 against a too great sensitivity to other limiting 

 factors as compared to the few substances in 

 the synthetic (glycine-thiamin) solution. Perhaps 

 this is one reason for past failures to obtain satis- 

 factory growth of animal tissues in simple nutri- 

 ents. Studies of this sort bid fair to give us 

 considerably better insight into the functions and 

 interactions of the various nutrient molecules and 

 ions as well as other environmental variables in 

 the plant's economy. 



Qualitative changes. A tissue 's or organ 's reac- 

 tion to nutritional variables may, of course, be 

 studied qualitatively as well as quantitatively. 

 The reaction to indole acetic acid and its homo- 

 logues serves as a striking but by no means iso- 

 lated example. A graded series of concentrations 



