280 BIOLOGY. [Book hi. 



whicli there is most light, because on that side the anatomical 

 elements increase and multiply least actively. It is only the 

 very refrangible rays, blue, violet, and ultra-violet, which exercise 

 this retarding action. Nevertheless, the mean elongation is 

 ordinarily greater during the twelve hours of the day than 

 during the twelve hours of the night j but this result is due 

 to the mean elevation of the diurnal temperature, and it is 

 always in the morning at sunrise that growth reaches its 

 maximum.^ 



A. de Candolle, De Tries, Kbppen, Sachs, &c., have occupied 

 themselves with the influence of the temperature upon the ger- 

 mination and elongation of different parts of plants. Below 

 and above 50 degrees, vegetal life is generally impossible, and 

 it is ordinarily at about 30 degrees that it attains its maximum 

 of activity. This proposition has, however, only a general appli- 

 cation, as, for each species, there are special maximal and 

 minimal temperatures, and also a certain temperature particularly 

 favourable. 



At this stiitable temperature, development is as rapid as 

 possible; nevertheless it still takes place above and below it, 

 within Sufficiently wide limits, but much more slowly. 



From observations made by M. Boussingault upon grains of 

 barley, it has been concluded that to develop itself, a vegetal 

 requires a given and nearly uniform degree of heat.^ This given 

 quantity of calorific vibrations transforms itself into nutritive 

 and evolutive equivalents. A lower temperature produces the 

 same effect as the more favourable temperature, but naturally 

 in a longer time. According to this hypothesis, very high 

 temperatures must* be considered as agents of perturbation, as 

 forces too great, hurtful by their very excess, and of a nature to 

 shackle the phenomena of nutritive chemistry. 



In treating of nutrition and respiration, we have stated what 



1 Sachs, TraiU de Botanigue, pp. 886-890. 



2 Boussingault, E^onomie Rurale, Oonsidirie dams ses Eapports awec la Ohimie, 

 la PhyHque, et la MUem-ologie, t. II. 



