150 PEINCIPLES OF GARDENING. [CH. IV. 



sap, and, indeed, stops the circulation to it of that 

 fluid, the absorbent powers of the organ are re- 

 versed, and instead of carbonic acid it inhales 

 oxygen. The effect is speedily perceptible. Gallic 

 acid forms, and this, modified by the differing saline 

 constituents of different leaves, changes the hue of 

 their green colouring matter, called chlorophyllite or 

 chromulite, into various tints of yellow, red, and 

 bro^\Ti. This is the general effect of acids acting 

 upon vegetable greens, and that it is the cause of 

 the autumnal change of colour in leaves, is proved 

 by the fact, that if a green leaf be dipped into an 

 acid it assumes the same hue, and if a red or yellow 

 leaf be dipped into an alkaline solution, it is ren- 

 dered green — the alkali, evidently, neutralizing the 

 acid that had wrought the unnatural change of 

 colour. 



The hints and warnings which these facts suggest 

 to the mind of eveiy reflecting practitioner are nu- 

 merous. They explain and enforce the necessity of 

 a regular, and by no means, as to quantity, indiscri- 

 minate, supply of water to plants ; the importance 

 of shading after their transplanting, and of a free 

 circulation of air, &c. ; and the necessity of keeping 

 the leaves as clean and as free from injuiy as 

 possible. The leaves of plants must often be re- 

 moved ; and in some instances this is done mth 

 essential benefit ; but the horticulturist should con- 



