X. CLIMATE OF PLANTS. 479 



True it is, however, that the gardener who raises early 

 potatoes by help of warm litter and night coverings, — who 

 brings forward his lettuce and celery on a gentle hotbed, 

 with the purpose of getting this produce earlier into 

 market than it would otherwise come in, — who forces his 

 cucumbers and melons by the application of higher heat, 

 • — or who gets his grapes and peaches ripened under glass 

 earlier or better than they could be ripened in our open 

 climate, — in these, and other such proceedings, works 

 under a practical conviction that greater heat during a 

 shorter time will produce results very similar to those 

 brought about by less heat continued through a longer 

 time, or applied less continuously. The fact is clear and 

 certain enough within gardening limits ; and it no doubt 

 must hold true to some considerable extent in reference 

 to the natural growth of plants. A shorter and warmer 

 summer may thus be equalised with a longer and cooler 

 summer, in its effects on the growth of some species ; 

 and in this manner may be determined the possibility of 

 their existence in two countries of climates very dis- 

 similar. 



But there must certainly be an early limit to tliis ten- 

 dency of time and temperature to balance each other, or 

 compensate one for the other. The idea is too easily 

 pushed to a reductio ad absurditm, to allow of that wide 

 extension and felicitously precise application to phyto-geo- 

 graphy, which M. De Candolle apparently expects from it. 

 The sum of the daily temperatures experienced by a plant 

 might be the same with very different results, destructive 

 in the first, — unuseful in the second, — successful in the 

 third, — injurious in the fourth, — destructive again in the 

 fifth, &c. For instance, 



