CHAPTER VII 



THE SIGNIFICANCE OF GROWTH SUBSTANCES FOR 



OTHER PHENOMENA 



The significance of hormones for the growth and development 

 of plants appears to be exercised mainly through some effect 

 upon the enlargement of cells. By regulating the increase in cell 

 size, growth substances control the growth of tissues and organs. 

 Recent investigations have extended the role of these substances 

 to include the initiation of roots, the production of tumors, the 

 stimulation of cell divisions in the cambium, and many other 

 important physiological and morphogenetic processes. 



Bud Development. — The phenomenon of apical dominance and 

 the inhibition of buds lower down on the shoot axis has been 

 interpreted variously as being due to differential distribution of 

 the food supply, the electrical pattern, or chemical regulators. 

 In recent years, the evidence in favor of some sort of chemical 

 regulation has come to the front. 



Since growth substance is a necessary factor for stem elonga- 

 tion, one might consider that the dormancy of resting plant 

 organs, for example, of buds, is caused by a lack of the growth 

 substance. If this hypothesis is correct, then the substance 

 might function to promote growth in dormant tissues. The 

 presence of growth substance in the periderm of dormant potatoes 

 would suggest that other factors must be involved also. 



In order to throw more light on this question, the results of 

 some unpublished experiments (by Boysen Jensen) dealing with 

 the influence of growth substance upon resting buds will be 

 discussed. A difficulty in the experimental set-up was encoun- 

 tered in bringing the growth-substance solution into the vicinity 

 of the buds. It was found that Forsythia is suitable for the pur- 

 pose, since there are diaphragms across the stem at the nodes. 

 If one bores into the stem, the interior can be filled with growth- 

 substance solution. Another method involved removal of the tips 

 from twigs of Salix, Syringa, and Aesculus. The basal end of 



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