GROWTH HORMONES 

 IN PLANTS 



CHAPTER I 



INTRODUCTION AND HISTORICAL SKETCH 

 GROWTH AND GROWTH SUBSTANCES 



The growth of Hving organisms is of fundamental importance 

 to all students of biology. It implies a permanent increase in the 

 size of the whole organism or its parts as a result of the incorpora- 

 tion of materials from the environment. Growth is due chiefly 

 to the absorption of water, the synthesis of new protoplasm, 

 extension of cellular boundaries, and increase in weight. As an 

 organism grows, it becomes differentiated into parts that perform 

 specific functions. 



Growth in plants results from the integration of many internal 

 processes. In an attempt to analyze the discrete chemical 

 reactions and physical conditions that contribute to growth, it 

 may be useful to classify the substances concerned into two 

 groups: nutritional substances and regulating substances. In 

 the first group, considered in the broadest sense, belong water, 

 minerals, gases, and the organic foodstuffs which supply energy 

 for building the plant's structures. Following Huxley's sug- 

 gestion (1935), the second group of regulating substances may be 

 conveniently subdivided as follows: (1) localized chemical acti- 

 vators whose range of influence may be limited strictly to intra- 

 cellular activities {e.g., those concerned with various genie 

 effects) or to a comparatively small sphere; (2) hormones which 

 exercise specific effects upon cells or tissues other than those by 

 which they are produced. To the latter group of substances 

 belong those growth-regulating materials which are the subject 

 of this book. 



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