CHAPTER VI 



THE SIGNIFICANCE OF GROWTH SUBSTANCES FOR THE 

 NORMAL GROWTH OF PLANTS 



GENERAL SURVEY OF THE EFFECT OF GROWTH SUBSTANCES 



UPON GROWTH 



Up to the present time, plant hormones have been studied 

 more extensively in relation to cell enlargement than in relation 

 to cell di\'ision. Consequently, the following discussion will be 

 confined almost entirely to those problems which deal with the 

 enlargement of cells. In general, it can be said that growth 

 takes place only when growth substances are present. Their 

 distribution and activity have been determined for diverse organs 

 in many kinds of plants; of these the Avena coleoptile is probably 

 the best known. 



The Avena Coleoptile. Structure. — The roots appear first in 

 the germination of the Avena seed ; shortly thereafter the coleop- 

 tile appears. The latter is the first leaf above the cotyledon and 

 differs from later leaves in having no lamina. It is a cylindrical 

 sheathing leaf which encloses the embryonic foliage leaves, and its 

 vascular relationship with the cotyledon is somewhat more 

 intimate than that of the succeeding fohage leaves (Avery, 1930). 

 Its diameter is about 1.5 mm.; in etiolated plants it may attain a 

 length of 6 cm. or more ; but in plants grown under normal condi- 

 tions, it seldom attains a length of more than 1 to 2 cm. The 

 coleoptile is devoid of green pigment if grown in darkness and 

 develops chlorophyll only sparingly under the influence of light. 

 Some varieties of oats seedlings when grown in the dark develop 

 an elongated first internode (a stem portion termed "mesocotyl" 

 in the older literature). 



In transverse section, the lower part of the coleoptile appears as 

 in Fig. 21; a longitudinal median section through the embryo 

 divides the coleoptile into two symmetrical halves, as indicated 

 by the broken hne in this figure. Embedded in the parenchyma 

 tissue in either half are the two vascular bundles; these 



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