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locality about the time of the ^Yintel■ awakening, and we should par- 

 ticularly consider the temperature that follows after the thermometer 

 has passed the freezing point, as well as the quantit}' of light radiated 

 by the sun. 



It must therefore be admitted that cold, as long as it does not 

 destroy the life of the phmt, may be more or less severe or more or less 

 prolonged, and thus lower the average j^early temperature, without 

 causing an}?^ marked ditference in the epochs of vegetation. This 

 reflection explains, independent of all hypothesis, that for any equable 

 mean annual temperature the acceleration in vegetation should be 

 in favor of localities where the annual variation is the greatest, par- 

 ticularly in northern countries, where the frost prevails during many 

 months of the year and where duing many of the following months 

 the sunlight never ceases to fill the sky. Admitting the hypothesis 

 that the action is proportional to the sum of the squares of the tem- 

 peratures, the results are still more positive; for, other things being 

 equal, the greater the annual variation the greater will be the sum of 

 the square of the jjositive ordinates in the curves of temperatures. 



I will now present some conclusions that one can deduce from all 

 that precedes. I must first of all warn my readers that this work 

 must be considered only as an attempt to solve a problem as difficult 

 as it is interesting, the principal elements for the solution of which 

 are still wanting. 



1. A great number of factors combine to produce variations in the 

 periodic phenomena of vegetation, the most important of which 

 in our climate is temperature. 



2. It may be estimated that the progress of vegetation is in pro- 

 portion to the sum of the temperatures, or, better, to the sum of the 

 squares of temperatures, calculated above the freezing point, starting 

 with the epoch of the awakening of vegetation after the winter sleep. 



3. The cold of winter, if it does not injure the vitality of the plant, 

 does not cause any perceptible retardation in its future development, 

 particularl}^ if the ground has been covered with snow. 



The effects that can be produced by the cold of winter must, how- 

 ever, be considered, and especially the condition of the plant when 

 it entered upon its winter sleep, a condition which should correspond 

 to a certain sum of acquired temperatures (or heat stored up). As 

 to the ripening of the harvest and because plants develoj) under the 

 infl-uence of the sun, we must consult a thermometer exposed to its 

 direct action, and not a thermometer exposed to its direct action, and 

 not a thermometer placed in the shade, as is connnonly done. 



4. The temperatures at night are not comparable with those of the 

 day as to their effects on vegetation. The (piantity of light received 

 by the plants must also be taken into consideration. 



5. An increase of 1° in latitude produces about the same retarda- 

 tion in vegetation as an increase in elevation of 100 meters; that is to 

 say, in our climate, a retardation of about four days. 



This result should be looked upon as only a kind of average of 

 quantities that vary during the year, the differences of latitude and 

 elevation having scarcely any real influence furtlier than as they 

 produce differences of temperature. 



6. The variations of temperature, other things being equal, are 



