Plant Qrowth'Suhstances 



a plant, nor can it be defined in terms of its position in the 

 plant. Even thorns may grow underground. 



It is now fairly well known that true roots can be induced to 

 form in unusual positions: the property of being able to 

 induce the formation of aerial roots, and to cause cuttings to 

 form roots, is possessed by artificial growth-substances. This 

 has given rise to no small amount of confusion, regarding the 

 effects of the substances on existing roots. 



Many people have mistakenly assumed that an application 

 of a synthetic growth-substance to the normal underground 

 roots of common plants must be beneficial. To explain why 

 this assumption is wrong would take me beyond the province 

 of this book into the sphere of plant physiology, but it may be 

 hinted that the induction of new roots on rootless parts of 

 plants — cuttings or stems — is one thing, and the elongation or 

 growth of normally formed roots is another. While a root 

 cannot be defined by an appeal to its position, the effects of a 

 chemical stimulant may and do differ considerably, according 

 to the position and original function of the part of the plant to 

 which the substance is applied.^ 



Went (1938 D) has shown that it is possible to separate two 

 phases in the action of indole-acetic acid on the formation of 

 roots on cuttings. The first phase is tentatively identified 

 with a redistribution of the hormone rhizocaline within the 

 stem. This phase can be induced by a number of substances 

 not active in root-formation proper. The second phase can be 

 induced only by indole-acetic acid and similar substances; 

 this phase may be an activation of the accumulated rhizocaline. 



There is at present no warrant for supposing that the 

 addition of a synthetic growth-substance to a manure, or its 

 presence in a manure, must inevitably improve the growth of 

 ordinary underground roots. It is tempting to suggest that 

 part of the special value of organic manures lies in their 

 content of, or ability to form, substances like indole-acetic 

 acid. But, until more is known about the presence in organic 



^The formation of shoots on or near the tips of roots of Pogonia ophioglos- 

 sides has been recorded by Margery C. Carlson (Bot. Gaz., 1938, lOO, 215), 

 but the effect of synthetic growth-substances on this process does not seem 

 to have been examined. 



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