210 



GROWTH OF PLANTS 



Figure 75. Tomato plants showing induced cell elongation and cell division. A, left 

 to right: control plant; three plants treated on the upper side of a leaf petiole with lanolin 

 preparations containing three different concentrations of a-naphthaleneacetic acid 

 (0.0005 per cent, 0.00025 per cent, and 0.0001 per cent, respectively) which causes cell 

 elongation, the degree of response increasing with increasing concentration. Photograph 

 taken after 24 hours. B, left: control plant; right: lanolin preparation containing 1.5 per 

 cent a-naphthaleneacetic acid applied around the upper end of the stem causing increased 

 cell division and swelling of leaves and entire stem. Photograph taken after 48 hours. 



stimulate cell division. The actual initiation of root primordia involves 

 some form of growth regulation which is not well understood. Root pri- 

 mordia usually are associated with proliferations involving stimulated cell 

 division. Several types of induced rooting are shown in Fig. 77.^'' 



The many groAvth substances do not all have the same degree of root- 

 inducing power, and there are many quahtative differences in the induced 

 responses. From a concentration standpoint alone the requirements vary 

 from 0.1 mg/1 or less for a- (2,4-dichlorophenoxy) -propionic acid to 40 mg/1 

 for /3-indolebutyric acid. Again the species vary in their capacity to respond. 

 For example, Ligustrum (privet) responds to a-naphthaleneacetic acid 

 but not to i3-indolebutyric acid; but Evonymus (strawberry bush) is just 

 the opposite, being sensitive to /S-indolebutyric acid and not to a-naphtha- 

 leneacetic acid. The usefulness of a root-inducing preparation can be ex- 

 tended and made to cover a wider range of species by including two or more 



