178 0N THE MODE OF ACTION OF HEAT ON PLANTS. 



XXIII. — On the Mode of Action of Heat on Plants, and 

 especially on the Effect of the Sun's direct Rays. By M. 

 Alphonse de Candolle. 



(From the Bibliotheque Unrverselle de Geneve, March, 1850.) 



Whenever it is endeavoured, to explain facts in vegetation by 

 means of temperature, therm ometrical data are made use of, 

 such as are supplied to us by the observations of meteorologists. 

 At first everything was referred to mean annual temperatures, 

 but as these could not be brought into harmony with the greater 

 number of facts, the means of seasons were afterwards taken 

 into consideration, and then monthly means. Finally, M. 

 Boussingault has introduced the most logical course : that which 

 consists in reckoning the time during which any phenomenon of 

 vegetation continues, and the mean temperature during that 

 time. Thus, supposing a plant has taken 20 days to ripen its 

 seeds from the period of flowering, and that the mean tempera- 

 ture during those 20 days has been 10°,* it will be said that the 

 heat received by the plant has been 200° ; the number of days 

 may have been 10, and the mean temperature 20°, to produce 

 the same heat of 200°, which figure will express the aggregate 

 heat necessary in the species to produce a certain effect in the 

 plant. 



But, on applying this calculation to different phenomena of 

 vegetable life and to different climates, it is soon evident that it 

 can only be approximative. In some instances, indeed, results 

 are obtained so discordant that one is led to doubt altogether the 

 value of the process. 



The causes of error are really numerous ; and if we do 

 not succeed in ascertaining them, if we cannot determine the 

 corrections required at least by the most important of them, it 

 is to be feared that our comparison of facts in vegetation with 

 facts in temperature will remain very vague and unsatisfactory. 

 I do not pretend to enumerate the whole of the causes of error 

 which may be imagined ; it will suffice to indicate the following. 



1. The time which should be taken into account is in many 

 cases very difficult to fix. Thus, the moment when germination 

 commences, when buds begin to swell, the period of maturity of 

 several seeds, are points much more difficult to ascertain than is 

 generally supposed. M. Boussingault (' Economie Rurale,' vol. 



* The degrees of temperature given in this paper are those of the Centi- 

 grade thermometer, which are retained in the translation on account of the 

 facility of calculation and the advantage of representing the freezing point 

 by 0°, more especially as the temperatures given are merely by way of 

 illustration. 



