2 GROWTH HORMONES IN PLANTS 



The significance of chemical correlation in plant physiology 

 and morphogenesis has captured the imagination of botanists ever 

 since the discovery in plants of chemical substances which might 

 be called hormones. The word hormone, derived from the Greek 

 opMcico and meaning "I arouse to activity," was suggested by 

 Hardy and first applied in animal physiology by Starling (1906) in 

 discussing the substance secretin ; it was later defined by Starling 

 (1914) as "any substance normally produced in the cells of some 

 part of the body and carried to distant parts which it affects for 

 the good of the body as a whole." It has been shown (Huxley, 

 1935) that all gradations exist between hormones and local 

 activating substances and between the latter and ordinary 

 by-products of metabolism wliich are less specific in regard to the 

 nature of the structures acted upon. The word hormone was used 

 for the first time in connection with plants by Fitting (1910), who 

 found that a substance present in orchid pollen caused swelling of 

 the gynostemium in the orchid flower. 



In dealing with the phenomena of growth, careful distinction 

 has not always been maintained in the past between the sub- 

 stances which may be correctly termed hormones and certain 

 other types of materials. In comparatively recent years, a 

 number of terms, more or less useful but not particularly well- 

 defined, have been proposed to designate newly discovered 

 functional materials, such as hormone, enzyme, vitamin, bios, 

 Wuchsstoff, etc. Although these terms may overlap in meaning, 

 they are temporarily useful until more information is available. 

 The definition of a hormone is given above in biological terms 

 because hormones play a role only in hving organisms; they are 

 chemical substances which have a specific influence on correlation 

 and differentiation of the organism. They are effective when 

 present in minute amounts and control growth in plants in some 

 way other than by direct nutritive means. The substances 

 influencing growth through direct nutritive effects include 

 vitamins (accessory or protective food factors in animals), 

 hios (substances that apparently function much like vitamins 

 in the growth of certain plants — Miller, 1930; Kogl, 1935, 

 Mitt. XIV), etc. Enzymes are produced also by living organisms 

 and promote chemical reactions either within or outside the 

 organism and are not used up. In some instances a given sub- 

 stance may fall into more than one category. Plant-growth 



