CHAPTER 6 



Plant Hormones 



by P. W. Zimmerman 



The mysterious forces which regulate the growth and movement of plants 

 have always been a subject of major interest to botanists. For many years 

 efforts were centered around essential mineral elements in fertilizers and 

 soil, with the thought that properly balanced nutrient solutions might lead 

 to an understanding of growth regulation. While plants grew at varying 

 rates according to the kinds and amounts of minerals supplied, there was 

 no evidence that tropisms, correlation of organs, flowering, and maturation 

 are regulated by fertilizers. The effect of the growing stem tip on growth of 

 other organs, the bending of stems toward light, the capacity of plants to 

 right themselves when placed m a horizontal position, the production of 

 adventitious roots, and the polarity of shoots and roots were phenomena 

 which were not controlled by mineral nutrients. There was, perhaps, some- 

 thing made by the growing plants, natural substances, which regulate 

 growth. 



The results of recent investigations take away some of this mystery and 

 indicate that, as in the animal kingdom, growth, movement, and matura- 

 tion of plants are regulated by chemical substances (hormones) produced 

 by the organism itself. In fact such substances, extracted from plants and 

 animals and re-introduced into normal tissue, cause hormone-like responses. 

 Physiologically active chemicals were prepared synthetically in the labora- 

 tory to take the place of natural hormones. As ^^^th animals, a single active 

 compound has several different effects on plants. For example, a single 

 treatment of a growing plant with a-naphthaleneacetic acid may cause 

 cell elongation, resulting in curvature of stems and epinasty of leaves, 

 proliferations involving cell division and induction of adventitious roots, 

 mhibition of buds, and regulation of rate of growth. These are all hormone- 

 like responses. The term "hormone" was borrowed from the animal field, 

 where it referred to a regulating substance produced in a particular ductless 

 gland but having its effects on organs or tissues some distance away. In 

 plants the terminal bud produces a substance which regulates growth of 

 axillary buds. Botanists have a number of terms used more or less synony- 

 mously with the word " hormone " — growth substance, growth regulator, 

 phytohormone, and auxin. The term "hormone" should be reserved for 

 natural substances, but it has a popular appeal and has been used loosely. 



Many controversial views were held before the modern growth substance 



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