CHAPTER V 



THE TRANSPORT OF GROWTH SUBSTANCES 



The growth substances that are formed in different parts of a 

 plant are transported to other regions where they exert specific 

 physiological effects on the tissues. For example, the substance 

 dispersed from the coleoptile tip influences the enlargement of 

 cells in tissues at some distance away. Following the definition 

 of Starling (1914), such a substance is a hormone, and the problem 

 that confronts us now is to explain the mechanism by which the 

 plant-growth hormones are translocated from the place of their 

 formation (or center of distribution) to the region of their func- 

 tional activity. 



The Avena Coleoptile. — The great majority of investigators 

 interested in the transport of plant hormones have used the oat 

 coleoptile as an experimental object. It has been shown that the 

 substance controlling growth in young Avena seedlings is dis- 

 tributed from the extreme apical portion of the first leaf above the 

 cotyledon (coleoptile). 



The Conducting Tissues. — From the facts presented in the ear- 

 lier chapters, it is clear that there is abundant movement of 

 growth substance from the tip toward the base of the coleoptile. 

 Under normal conditions, migration of the substance takes place 

 almost exclusively in a longitudinal direction, and very little is 

 transported transversely. In naturally or artificially twisted 

 coleoptiles, conduction of a stimulus (i.e., the transport of growth 

 substance) ordinarily follows the course of the vascular bundles 

 (Tammes, 1931). 



Rothert (1894) investigated the conduction of the phototropic 

 stimulus and thus the transport of growth substance in the Avena 

 coleoptile. Since it was found that conduction was not hindered 

 when both vascular bundles were severed, it was concluded that 

 the stimulus is transmitted in the parenchyma. Although there 

 may have been sources of error in Rothert's experimental method, 

 his conclusion that growth substance can move in the paren- 



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