1908.] PUBLIC DOCUMENT — No. 33. 71 



The Physiological Constant for the Germi- 

 nating Stage of Cress. 



F. A. WAUCII; C. S. POMEROY. 



The subject of pli3%siological constants was studied several 

 years ago by the senior writer, and a report of certain investi- 

 gations made, to which the reader is referred for summaries of 

 the theories advanced by various investigators. ^ A brief state- 

 ment of the present accepted behef is here given, that the sulj- 

 ject may Ije properly understood by all. 



A physiological constant may be defined as the amount of 

 heat required to carry a plant through some certain stage of its 

 growth. Thus each species of plant and each phase of develop- 

 ment for each species would have its own physiological con- 

 stant. 



De Candolle, 2 writing over fifty years ago, set forth two 

 fundamental principles which are accepted as sound to-day : 

 "1. The active heat is the product of the degree of tempera- 

 ture and its duration. A more intense heat in a short time 

 produces the same efix3ct as a less intense heat in a longer time. 

 This is true, provided the range of temperature and the space 

 of tinje are limited. 2. Every plant requires a certain mini- 

 mum of heat for each of its physiological functions, as germi- 

 nating, leafing, flowering, etc. The temperatures below freez- 

 ing point have no eflect on plants, or at a certain low degree a 

 destroying one ; but there are many species on which the 

 lower degrees above the freezing point have no effect. There 

 is a starting point of vegetation for every species at a certain 

 degree of temperature ; every species requires a certain sum 

 of heat above a certain degree of temperature, distributed over 

 a certain space of time between a minimum and a maximum of 



1 F. A. Waugh, Vermont Agricultural Station report, II. (1898), pp. 263-272. 



2 Alphonse De CaudoUe, "Ueographie Botanique" (1855). 



