GROWTH SUBSTANCES FOR NORMAL GROWTH 



83 



place during the growth of the seecUing (Figs. 23, 24, and 25). 

 Their rate of elongation, therefore, is proportional to that of the 

 coleoptile as a whole (Avery and Burkholder, 1936). The under- 

 lying parenchyma and inner epidermal cells increase their number 

 2.9 times on the average (Fig. 25). Of all the cell layers, the 

 greatest number of cells from the tip to the base of the organ is 

 in the subepidermis, and each successive layer inward has fewer 

 cells. Most of the cell di^dsion takes place before the coleoptile 

 is 1 cm. long, or about one-fourth its final length (Fig. 25) ; it is 

 about evenly distributed throughout the whole length of the 



A B 



Fig. 22. — Diagrams of longitudinal sections through the tip of the Avena 

 coleoptile. A, section through the narrow dimension, which is indicated by 

 the dotted line in Fig. 21A. B, \n.ew through wide dimension with position of 

 vascular bundles shown by broken lines. Note in detailed strip the increasing 

 length of epidermal cells below the ends of the vascular bundles. 



organ, except for the zone between the pore and the tip where 

 there is no division w^hatsoever. Cell division and cell elongation 

 take place only in the direction of the long axis of the organ; i.e., 

 growth is polarized. From a length of 1 cm. to maturity, the 

 coleoptile grows in direct proportion to the elongation of its cells. 

 The course of growth is influenced, of course, by temperature, 

 humidity, and light. 



The distribution of growth in the Avena coleoptile has been 

 investigated by Rothert (1894), duBuy (1933), and more recently 

 Avery and Burkholder (1936). The lower half of the embryonic 

 coleoptile elongates to make up approximately three-fourths of 

 the total length of the mature coleoptile, while the upper half of 

 the embryonic coleoptile elongates to form the upper one-fourth 

 of the mature coleoptile. Although growth is taking place 



