162 PRINCIPLES OF GARDENING. [CH. V. 



to the species from which the bud or scion is 

 taken. This is justified bj my observing that in a 

 garden so low-lying as to be veiy subject to an 

 ovei'flow of water, the only pear-trees which were at 

 all productive were those grafted upon quince stocks 

 — and the quince is well knovra to endure water 

 much better than either the apple or pear. 



The ascent of the sap, like the circulation of the 

 blood, is increased in rapidity by an addition to the 

 temperature in which the plant is vegetating, and 

 when it is flowing from incisions made in a stem at 

 various heights from the ground, a sudden reduc- 

 tion of temperature will cause a cessation of the 

 flow from the upper wounds whilst it continues from 

 those below. 



These facts indicate most satisfactorily why the 

 gardener finds his vines, peaches, and other plants 

 in the forcing-houses injured by keeping them in a 

 high temperature during the night. It is then, as 

 in the animal economy, that the individual functions 

 are renovated by a temporary repose, and if left to 

 the dictates of healthy nature, the sap, like the blood, 

 flows at night with a much diminished velocity. 



That plants do become exhausted by too unre- 

 mitting excitement is proved to eveiy gardener who 

 has a peach-house under his nile, for if the greatest 

 care be not taken to ripen the wood by exposure to 

 the air and light during the summer, no peach-tree 



