CHAPTER XVII 

 HORMONES, AUXIMONES, VITAMINES, AND TOXINS 



REFERENCE has frequently been made, in preceding chapters, to 

 the effect of various stimulating or inhibiting agencies upon the 

 physiological activities of plant protoplasm. In the main, these 

 agencies are external to the plant and are either physical, such 

 as changes of temperature, amount of light received, etc.; or 

 chemical, such as variations in the salts received from the soil, 

 or common anaesthetics applied to the plants by man. A plant 

 grows normally under certain conditions to which it has become 

 adjusted by hereditary acquirements. When these conditions 

 are altered, the effect upon the functioning of the plant proto- 

 plasm may be either stimulating or depressing. Extreme changes 

 in environmental conditions generally result in the death of the 

 plant; but changes which do not result in the lethal condition 

 affect the plant by either stimulating it to more rapid physiological 

 activity or by depressing its normal growth or functions. As 

 has been pointed out, the same external influence, either chemical 

 or physical, which acts as a stimulant if it differs only slightly 

 from normal conditions, may become depressing, or positively 

 toxic, if present to a larger extent. 



There is also the possibility of the elaboration by the plant 

 itself of internal agents, or substances, which may have a definite 

 stimulating or inhibitory effect upon its metabolism and growth. 

 The study which has been given to these matters has practically 

 all been carried on within very recent years and is still in prog- 

 ress. Most of it is still in the experimental stage, in which no 

 definite conclusions are as yet possible. Hence, the most that can 

 be done at present is to give a brief review of the suggestions which 

 have been made thus far, as indicative of the uncertainty of 

 our present knowledge of these matters and of the general trend of 

 the investigations which are now in progress. 



Substances which are elaborated by plants and which are sup- 

 posed to have a definite stimulating or beneficial effect upon 



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