Plant Qrowth'Suhstances 



substances represent a real gain of immediate practical 

 importance. Moreover, the possibilities have not been fully- 

 explored, and it is confidently expected that the new growth- 

 substances will be useful in grafting and in further types of 

 propagative work in the nursery garden. 



The substances are sometimes called growth-promoting 

 substances, but that name is not sound as a general rule. It 

 often happens that a plant that has been treated with one of 

 these substances produces a whiskery out-growth of roots 

 over most of its stem and a great part of its leaves, but becomes 

 dwarfed in comparison with an untreated plant. Dr. H. L., 

 Pearse, writing in the January, 1937, issue of the Journal of 

 Pomology and Horticulture, has pointed out that the amount 

 of plant-capital is not altered by the application of an artificial 

 growth-substance such as phenyl-acetic acid, but that the 

 distribution of the plant's capital is thereby altered. In other 

 words, the more aerial root, the less stem and leaf, the sum 

 of matter belonging to the plant being apparently unchanged. 

 It is, therefore, better to call these substances growth-regulat- 

 ing substances, or, more simply, growth-substances. This 

 book is devoted to an exposition of the chemistry of plant 

 growth-substances; that is, substances, other than fertilizers 

 and nutrients, that can regulate the growth of plants. 



The non-chemical reader who glances at the later portions 

 of this book and is affrighted by the array of formulae, and 

 the words, often of more than Johnsonian ponderosity and 

 length, with their uncouth-looking jumble of syllables 

 hyphenated with figures and Greek letters, may think that it 

 is something like English, but what does it mean ? 



It means, in brief, the story of an attempt to adapt the 

 forces of nature whereby plants grow, not only in size, but in 

 the increase of their several parts — roots and leaves, stems and 

 fruit. It is the summary of an attempt to set our accumulated 

 knowledge to a new field of endeavour : the better control of 

 plant growth. Firstly, however, it has been necessary to under- 



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