GROWTH SUBSTANCES FOR NORMAL GROWTH 115 



shown previously that auxin is ineffective at pH 7, more active at 

 pH 6, and about equally active at pH 4 and 5 (Dolk and Thimann, . 

 1932). From these facts it seems quite certain that Strugger's 

 acid-growth reaction is little more than a consequence of the 

 setting free of the auxin acid from its inactive salt. 



Inhibition of Growth in Roots. — Up to this point we have 

 observed only the promoting effect of growth substance. It 

 must not be forgotten that the rate of growth in roots is retarded 

 by growth substance. How the same compound can produce the 

 opposite effects in stems and roots has not been explained 

 satisfactorily. Cholodny (1931a, e) has proposed the hypothesis 

 that growth substance promotes the rate of development of 

 growing cells but shortens the length of their individual life 

 cycles. Elongation in the root lasts only a short time, and the 

 period of growth is shortened still more by growth substance; 

 i.e., if the latter is added, the cells mature quickly without 

 elongating. Growth in length of the root, therefore, is retarded 

 by growth substance. In the stem, on the other hand, the zone 

 of cell stretching is greater, and growth continues for a relatively 

 longer time; the period of growth in stems is shortened (i.e., the 

 rate is accelerated) by the addition of growth substance. There 

 is insufficient evidence to support this hypothesis at the present 

 time. 



SUMMARY 



Numerous experiments have shown that without growth 

 hormones, growth of the shoots of higher plants cannot take place. 

 Although hormones are not the only important factors concerned 

 in growth, they are essential for the normal enlargement of cells. 

 In the Avena coleoptile, Lupinus hypocotyl, and the foliage leaf 

 of Nicotiana, growth intensity has been shown to be correlated 

 with the differential distribution of hormones. Regions of stems, 

 leaves, etc., where such hormones are present, always appear to 

 undergo greatest growth in one dimension; i.e., growth is 

 polarized. 



The same substances that are essential for the growth of 

 coleoptiles, foliage leaves, hypocotyls, and stems inhibit the 

 elongation of roots over a wide range of concentration. 



The mechanism by which hormones promote growth is not well- 

 understood. There is some evidence that they are used up not as 



