40 



These remarks of De Candolle with reference to germination are 

 equally applicable to the whole period of growth of the piant. 



As to the method of calculating the sum total of temperatures 

 De Candolle found that it may be conducted in two ways, either by 

 adding together all the mean daily temperatures above 0° C. or by 

 omitting the useless degrees and adding all the others. This last 

 method would seem to be the most logical, but can rarely be 

 employed, owing to our ignorance of that minimum temperature 

 below which all must be omitted. On the other hand, if we consider 

 that a plant which vegetates between 10° C. and 30° C. has a maxi- 

 mum at 20° C, and if we seek the coefficients of growth correspond- 

 ing to each successive degree of temperature, we find, as Boussingault 

 has shown, that these coefficients vary for each degree as we depart 

 above or below the temperature most favorable to vegetation. 



Similarly De Candolle (1865) has shown that near the minimum 

 and near the maximum temperatures the rate of germination is more 

 difficult, and therefore slower, than at the intermediate or best tem- 

 peratures; consequently, both in germination and in subsequent veg- 

 etation, it is necessary to recognize the fact that calculations of the 

 •sums of heat in connection with the study of the geographical distri- 

 bution of plants are complicated with hypotheses and many sources 

 of error. 



Schuebeler (1862) shows that cultivated plants in northern coun- 

 tries have more highly colored flowers, larger and greener leaves, and 

 larger seeds, which are more highly colored and richer in essential 

 oils, than those of southern regions. Bonnier and Flahault (1878) 

 have shown the same facts for uncultivated plants. Both these 

 authors attribute this result to the prolonged action of sunlight, 

 and the latter shows that the A^ariations are exactly proportional to 

 the duration of sunlight. In Flahault's more recent observations he 

 shows that there must necessarily exist a relation between the quan- 

 tity of carbonic acid decomposed and the quantity of carbonaceous 

 matters formed by the plant, and that in general the sunlight has a 

 very remarkable inifluence on vegetation since it compensates in a 

 large measure for the deficiency of temperature. 



It is, furthermore, to this influence of light that Pauchon attributes 

 the singular fact that plants cultivated in high latitudes are endowed 

 with a vegetating power greater than that of southern countries, so 

 that when transported to the south their seeds ripen sooner than those 

 of the southern plants. This subject has been especially studied by 

 Tisserand in his memoir on vegetation in high latitudes, as cited by 

 Grandeau in his work on nutrition of plants. According to Tis- 

 serand a plant behaves in northern latitudes as a more highly per- 

 fected machine and one that performs better than southern jdants. 

 In regions where it has neither time nor heat it gains in activity and 



