Growth Substances 375 



velopmental problems (Zimmerman and Wilcoxon, 1935; Zimmerman, 

 1951b; and an extensive series of other papers ) . 



The literature in this active field is large, and in a brief space little 

 more than a very general introduction to it can be made. The student 

 may be referred to a number of books, symposium volumes, and reviews, 

 among which are the following: Boysen-Jensen (1936); Went and Thi- 

 mann (1937); Avery and Johnson (1947); Thimann (1948); Skoog 

 (1951); Soding (1952); Audus (1953); Leopold (1955); Wain and 

 Wightman (1956); and the Eleventh Symposium of the Society for Ex- 

 perimental Biology (1957). 



The term hormone was first used by Starling in 1904 with reference to 

 secretin, a substance important in animal physiology. Such hormones are 

 regarded as "chemical messengers" since they are produced in one part 

 of the body and carried to some other part, where they affect develop- 

 ment and various physiological processes. Their discovery marked a great 

 advance in an understanding of the chemical control of growth and 

 differentiation. 



With the demonstration that there are substances in plants which are 

 physiologically active in similarly small amounts, the term hormone was 

 carried over into plant physiology by Fitting (1909) in relation to a 

 substance in orchid pollen which produced swelling of the ovary. Various 

 phytohormones are now recognized. This word is not a particularly 

 happy one, however, for plants lack the efficient circulatory system of 

 animals. Indeed, many of these substances exert their effects in the 

 region where they are produced and thus are not "messengers" at all. 

 The most important natural plant hormones, the auxins, also differ 

 from typical animal ones in being relatively nonspecific and involved in 

 a great variety of growth processes rather than in particular ones. Huxley 

 ( 1935 ) discussed the relationship of these various substances, in animals 

 and plants, and suggested a classification and terminology for them. 

 Evidently there are many compounds and processes involved, and for 

 plants, at least, it seems preferable to use for all such morphogenetically 

 active materials the relatively noncommittal term growth substances. This 

 will be employed in the present discussion to refer to those substances 

 of whatever sort or activity that, in low concentration, are involved in 

 the control of growth, development, and differentiation. It should be 

 recognized that some of these substances are effective in retarding these 

 processes rather than in their stimulation. 



The history of the work on growth substances has been reviewed by a 

 number of authors (Leopold, 1955, Chap. 1; Went, 1951a; and Audus, 

 1953). Most of it dates from about the beginning of the second quarter 

 of the present century and was largely confined, at the start, to workers 

 in continental Europe. Among early students of the subject were Boysen- 



