240 CAUSES WHICH LIMIT 



ture, at least in Europe and in all countries analogously situated. 

 According as the climate of any locality is more or less excessive, that 

 is, more or less different from one season to another, the vegetation of 

 a plant commences and finishes at different epochs of the year. The 

 lines of like temperature relate to fixed periods, and the vegetation of 

 any one species in Europe lasts during periods which are variable. 

 There can then be no agreement between these two classes of fact, 

 unless by some special accident. 



In arriving at the law which governs the limitation of species, I 

 have been led to dwell upon two principles, the truth of which is 

 admitted by all agriculturists and botanists, but whose combined 

 effect had until now received no adequate attention. 



It is plain that a great heat during a short period must produce the 

 same effect on plants with a less degree of heat during a longer term. 

 The cultivators who would force or retard plants do nothing but com- 

 bine the duration and the degree of heat. They thus succeed in pro- 

 ducing the flower or the matured fruit at a given day. M. Boussin- 

 gault has given these facts in a precise form by showing that, for 

 the greater part of our cultivated annual plants, when we count the 

 number of days that the culture has continued, and multiply that 

 number of days by the mean temperature maintained, we arrive at the 

 same product for each culture in all countries and for all years. The 

 heat acts then proportionally as regards its duration and its force. 

 But M. Boussingault has not presented the result of his calculations 

 under so general a form^ and in this he was right. There is, in effect^ 

 a second principle, which modifies the one in question, and which is of 

 equal importance, at least in botanical geography. 



This second principle is, that each species requires, for each one of 

 its physiological functions, a certain minimum of temperature. Not 

 only is any temperature below zero useless to plants, en account of the 

 congelation of its juices, but even those of 1°, 2°^ and 3°,* are useless 

 to a great nun.iber of species, and ought not to be computed among 

 the temperatures available for the plant. Cultivate wheat, for in- 

 stance, under a temperature constantly below 4°, though the plant 

 will live long, and the product of the number of days by the tempera- 

 ture reach a high number, yet the stalk will not grow tall nor will the 

 flower be formed. M. Ch. Martins has said, with truth, that each 

 species of the vegetable kingdom is a kind of thermometer which has 

 its own zero.f We should be wrong, then, to infer that 10° during ten 

 days would have the same effect on all plants as 5° during twenty 

 days. In both cases the sum of atmospheric heat is expressed by 

 100°; but for species which do not vegetate below 6°, for example, 

 the amount of 100 must be diminished by all the values between 5° 

 and 6°, which occur in one of the supposed cases, while for those 

 species which do not vegetate below 10°, if any such exist, the avail- 

 able heat would be reduced to 0. If we would estimate the heat really 

 useful to a species we must consider, in our calculation, only the values 

 above a certain degree of temperature, which varies according to the 

 species. Direct observation seldom permits of our verifying the min- 



* These degrees are centigrade. 

 ■\ Voyagt BoUmique in Norwegt. 



