CHAPTER VI 



THE INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



PART I 



GENERAL VIEW 



SECTION 20. The Formal Conditions for Growth. 



A SUFFICIENT supply of food and water and a certain temperature 

 form three conditions essential for all life and growth. Oxygen and 

 calcium are necessary for most plants, but not for all, while particular 

 organic compounds serve as food for some metatrophic plants, but not 

 for others. Light is necessary to the aerial organs of green plants, but 

 not to non-chlorophyllous plants and organs. With regard to all these 

 factors, moreover, the quantitative requirements of different plants vary 

 within wide limits. 



Growth is also influenced by various non-essential agencies, some of 

 which, such as poisons and the like, are not present under natural 

 conditions. Poisons, when presented in sufficient dilution, may allow 

 slow and feeble growth, but when an appropriate dilution is reached 

 the plant commonly responds by a physiological counter-action, involving 

 an increase of metabolism, and frequently also of growth and of move- 

 ment. A similar reaction, coupled with a special formation of healing 

 tissue, is shown as the result of injury. These reactions, as well as 

 the heliotropic, geotropic, and nyctitropic ones, are of great importance 

 to the plant under normal conditions, but nevertheless they are not 

 essential factors for growth, which may take place equally well when 

 none of them are exercised. As a matter of fact, every external agency, 

 when sufficiently intense, must exert some influence or other upon the 

 organism, and even magnetic forces must affect plants, directly or in- 

 directly, in some way or other 1 . 



On the other hand, the absence or insufficiency of any one of the 

 essential factors renders growth impossible, and a retardation and ultimate 

 cessation of the latter infallibly occurs when either the temperature or 



1 Cf. Ewart, On Protoplasmic Streaming, 1903, Clar. Tress, pp. -15-52. 



