164 PLANT GROWTH SUBSTANCES 



and treated terminally. The pith showed practically no meristematic 

 response, but the cells of the inner cortex, endodermis, primary phloem, 

 cambium, and the rays flanking the phloem showed marked activity. 

 The roots which developed were related to the rays in the same manner 

 as those developed in apical tumors of decapitated plants. 



These responses indicate that almost any parenchymatous tissues may 

 become meristematic when growth-regulating substances are applied 

 to them. It is obvious also that the same general types of responses 

 are incited by a large number of these substances, although there are 

 marked differences in the degree or intensity of responses of the several 

 tissues in relation to the specific substances. The difference in response 

 with respect to the kind of substance employed is greater than it is to 

 the concentration of a single substance applied over a rather wide range 

 of the higher concentrations. It is, however, impossible to say just what 

 the concentration of the substance which reaches any cell or group of 

 cells may be when applications are made in the various ways listed. There 

 is no doubt that the substances are soluble and that they diffuse into and 

 among the cells of the plant, but the rate at which this may occur is 

 difficult to determine. 



Thus it would appear that: 



1. In the presence of growth substances and proper food and nutrient 

 supply, cells of recognizably differentiated tissue systems may dedif- 

 ferentiate, become embryonic, and proliferate. From these derivatives, 

 tissues quite distinct in type may be differentiated (for example, vascular 

 bundles from phloem, endodermis, pith, etc.). 



2. In general not wholly new responses are evoked, because many of 

 the tissue systems respond similarly to changes in nutrients and food 

 supply, and to wounding and other environmental changes (for example, 

 heahng of wounds, rooting of cuttings, delay of abscission). 



3. Although no genuine new tissues arise the nature of their distribu- 

 tion and proportions and pattern may be profoundly affected. 



4. To this extent morphogenesis is brought under control, or may be 

 manipulated, as is now done in so many instances in the agricultural 

 field by the use of these compounds. 



5. The course of development apparently is changed, but more than 

 a growth substance is required; foods and nutrients play important 

 roles in the quantity and quality of response. 



6. Age of tissues, or perhaps more basically the chemical nature of 



