Plant Qrowth'Suhstances 



and Allison and Hoover's (1933) "co-enzyme R", which can 

 be obtained from cane sugar; also reported to be widely 

 distributed. 



One more class of substances needs to be mentioned as 

 having growth-regulating properties towards plants, and that, 

 strangely enough, is formed by the vitamins, especially 

 vitamins C (ascorbic acid) and Bj (thiamin). 



This outline classification has been made partly for the 

 sake of completeness, and partly to render intelligible the 

 paragraph on nomenclature which follows: it is uneasy to 

 discuss the action of any group of substances on plants unless 

 we are sure which group we are discussing. No doubt the 

 borderlines between the above-mentioned groups will shift 

 as well as become clearer with increasing knowledge. As 

 things are, it would be a pity to lump together in one com- 

 prehensive group such substances as : auxenolonic and ascor- 

 bic acids; the phenyl-, indolyl-, and naphthyl-acetic acids 

 and esters and nitriles thereof; together with adrenalin and 

 impure theelin: under some such name as "phytohormone", 

 merely because they can all affect plant growth in some way. 

 Cf. Eyster and Ellis (1924, C). 



What a Hormone is^ 



The term "hormone" is inapplicable to designate a class of 

 substances which affect the plant's growth only after having 

 been applied from without. The first hormone was discovered 

 by Bayliss in 1902. It was a substance (not yet properly 

 identified) secreted by a part of the intestine, and acting as a 

 sort of release for digestive processes lower down. Pre- 

 viously, it was thought that body processes were controlled 

 by telegraphic impulses along nerves, but Bayliss showed that 

 some processes were controlled, not by intangible messages, 

 but by actual substances. The word "hormone" was adopted 

 to signify a chemical messenger. Whereas a painful sensation, 



* Part of this section, as well as the introductory paragraphs of Chapter I, 

 have been reproduced from the article "What is a plant-hormone?" (Fertil- 

 iser, Feeding Stuffs, and Far?n Supplies Journal of 28th July, 1937, 23, 407) 

 by kind permission of the Editor of the Fertiliser. 



92 



