CH. IV.] THE LEAVES. 139 



though the water vras of the same temperature as the 

 greenhouse. 



The transpiration of plants decreases with that of 

 the temperature to wliich they are exposed, as well 

 as with the period of their growth. This explains 

 why the gardener finds that his plants do not require 

 so much water in cold weather, nor during the time 

 that elapses between the fall of their blossom and 

 the ripening of their seed. Duiing this period they 

 do not transpire more than one-half so much as 

 during the period preceding and attending upon their 

 blooming. The transpiration takes place from the 

 upper surfaces of the leaves ; and, if these surfaces 

 are coated with varnish, the leaves gradually decay 

 and fall, and the growth of the plant ceases until 

 fresh leaves are produced. Hence arises the benefit 

 which plants derive in rooms, gi'eenhouses, and 

 other confined inclosures, from keeping those sur- 

 faces cleansed with the sponge and syringe. Some 

 plants are particularly sensitive to injury from any 

 check to their transpiration, among wliich are the 

 tea-scented roses ; and it thence arises, that they 

 cannot now be cultivated in nurseiy-gardens near 

 London, where they once flourished when that me- 

 tropolis was less extensive. The advantage derived 

 by plants from having their leaves cleansed, was ex- 

 emplified by the following experiment : — 



Two orange trees, weighing respectively 18 and 



