144 PRINCIPLES OF GARDENING. [CH. IV. 



peculiar aroma, not only through the winter, but for 

 years, and be infinitely superior to any specimens 

 producible in the forcing department, for these are 

 unavoidably deficient in flavour. 



Leaves have the power of absorbing moisture as 

 well as of emitting it, which power of absorption 

 they principally enjoy during the night. 



During the day leaves also absorb carbonic acid 

 gas, which they decompose, retaining its carbon, 

 and emitting the greatest part of the oxygen that 

 enters into its composition. In the night this ope- 

 ration is in a certain measure reversed, a small 

 quantity of oxygen being absorbed from the atmo- 

 sphere, and a yet smaller proportion of carbonic acid 

 emitted. 



Carbonic acid gas in small proportions is essential 

 to the existence of leaves, yet it only benefits them 

 when present in quantities not exceeding one-twelfth 

 of the bulk of the atmosphere in which they are 

 vegetating ; though one twenty-fifth is a still more 

 favourable proportion; and as hot-beds, heated by 

 fermenting matters, rapidly have the air within their 

 frames contaminated to a much greater extent than 

 the proportions above-named, thence arises the injury 

 to the plants they contain, from a too long neglected 

 ventilation. The leaves turn yellow from the excess 

 of acid, which they are unable to digest, and which 

 consequently effects that change of colour which also 



