GROWTH SUBSTANCES FOR NORMAL GROWTH 95 



the concentration is greater in young leaves, tending to decrease 

 as the leaves mature. It has been shown, also, that there is a 

 definite concentration gradient from the tip to the base of a leaf, 

 the concentration being low at the distal end and increasing 

 toward the base (Fig. 30). The increase toward the base is due 

 to the accumulation of growth substance in the proximal end of 

 the midrib and is correlated with greater longitudinal growth 

 ("polarized growth") of the midrib in this region. Inasmuch 

 as the application of growth-substance paste (lanolin method, 

 Laibach, 19336) to large veins brings about a bending (differential 

 growth) response, it may be assumed that it is the agent responsi- 

 ble for promoting the normal growth in length of the midrib and 

 larger lateral veins in the leaf; hence, growth substance is 

 responsible, at least in part, for the normal growth pattern 

 exhibited by the leaf {cf. Figs. 30 and 31). 



Axial Parts : Hypocotyls, Internodes, and Flower Stalks. 

 Distribution of Growth. — Rothert (1894) investigated the distribu- 

 tion of growth in the hypocotyls and epicotyls of dicotyledonous 

 seedlings. In seedlings with epigeal cotyledons the hypocotyl 

 usually elongates first. Enlargement of the growing point above 

 the cotyledons begins only after the growth in length of the 

 hypocotyledonary axis is completed. As long as the hypocotyl 

 is very short, it grows throughout its entire length; later the basal 

 portion ceases growing, and a growth zone of a rather constant 

 length (1 to 4 cm.) is established in its upper portion. Following 

 cessation of growth in this region the epicotyl begins to develop. 

 The distribution of growth in the epicotyl of Phaseolus is shown 

 in Fig. 32. 



The distribution of growth in stems with several elongating 

 internodes is often quite comphcated. It would not be of value 

 to discuss this question at length here, since the significance of 

 growth substance for these growth processes has not been 

 investigated. 



Very obvious light-growth reactions are exhibited by many 

 seedling axes. According to Blaauw (1915), a decided retarda- 

 tion of growth appears in Helianthus after brief illumination; 

 this is followed later by an increase in growth enduring for a 

 short period. According to van Overbeek (1933), the rate of 

 growth in the hypocotyl of Raphanus is decreased to about one- 

 half l)y illumination. 



