CHAPTER IX 



GROWTH-SUBSTANCES FROM NATURAL 



SOURCES 



Agricultural Researches Having a Possible Bearing on 

 Growth- Substances in Soil and Manure 



I have experienced, that the black water taken from a 

 Dunghill, will make a Cabbage, or any of that Race, prosper 

 extremely. — R. Bradley, New Improvements of Planting and 

 Gardening, 171 8. 



The action of mixed micro-organisms on proteins and trypto- 

 phan (3-indole-a-amino-propionic acid; indole-alanine) pro- 

 duces indole-acetic acid and other related growth-sub- 

 stances, as well as substances capable of acting as nutrient 

 materials for plants. In nature and in agricultural practice, 

 such substances reach the plant directly from the excretions of 

 animals or via manure-heaps, in which they are mixed with 

 decomposition-products of straw and other vegetable mater- 

 ials. Natural decay of leaves and roots, and the operation of 

 composting, may also be supposed to yield growth-substances 

 and related bodies. Rhizopin (indole-acetic acid) has, for 

 example, been recognized as a product of a fungus, and 

 fungi play an important part in the decay of plant-residues. 

 Recent work by plant physiologists and by botanists has 

 tended to ignore the not inconsiderable mass of work done by 

 botanists on the assimilation of amino-acids such as tyrosine, 

 and by chemists on the existence of organic nitrogenous 

 compounds in soil. The early work on amino-acids was 

 almost all directed to proving their value as nutrients (sources 

 of nitrogen) when the amino-acid was used as sole source of 

 nitrogen. It therefore has only a comparative interest. 



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