CHAPTER XIV 



GROWING SEASON TEMPERATURES 



Horticulturists, particularly in the Old World, have recognized in 

 a manner the importance of growing season temperatures to fruit plants. 

 Most of the efforts at precise study of this nature, however, have been 

 made by those particularly interested in phenology. 



HEAT UNITS 



Various investigators have made efforts to show that, wherever 

 a given plant is grown, to complete its cycle that plant requires a certain 

 amount of heat. When it has received this amount of heat, whether 

 in n days or n + r or n -\- s days, it will have completed its cycle. The 

 outline of this idea was enunciated first, probably, in 1735 by Reaumur. ^ 

 Numerous writers since that time have attempted to refine the methods 

 used in studies of this sort. Adanson, for example, recognizing that 

 averages which included readings below freezing were misleading, inas- 

 much as such temperatures do not reverse plant activity but merely 

 suspend it, discarded all such readings. Others have assumed higher 

 temperatures as the zero points for their calculations. Gasparin con- 

 sidered that "effective temperatures" began at 5°C. He also considered 

 a thermometer in full sunshine on sod to show the true temperature of 

 the plant more nearly than one registering air temperature alone and 

 that "the warmth in the sunshine is to the warmth of the air in the shade 

 as though one has been transported in latitude from 3 to 6° farther 

 south. "^ DeCandolle^^ believed sunlight in itself to influence vital 

 processes independently of temperature, since several annuals which 

 he had under observation required a greater total of heat degrees for 

 flowering and for ripening in the shade than they received in full sunlight. 



The Relative Values of Different Efifective Temperatures.— Most 

 investigations in phenology until comparatively recent date have been 

 based on the assumption that, above the basic temperature which 

 initiates plant growth, each degree is of equal value with any other. 

 Lately, however, the principle of Van't Hoff and Arrhenius, namely, 

 "that within limits, the velocity of most chemical reactions doubles or 

 somewhat more than doubles for each rise in temperature of 10°C.," 

 has been shown to have considerable bearing on certain processes in 

 plants. As the Livingstons^*'^ point out, certain of the purely physical 

 processes involved in growth do not follow this principle and its applica- 



236 



