136 PRINCIPLES OF GARDENING. [CH. IV. 



explained by the disoxygenizing power ^ntli which it 

 is gifted. The violet rays of the spectrum have this 

 power iu the greatest degree; and Sennebier has 

 ascertained by experiment, that those rays have the 

 greatest influence in producing the green colour of 

 plants. ^Yhen leaves are of any other hue than 

 green, they are said to he coloured. This variegation 

 is often considered to be a symptom either of tender- 

 ness or debility ; and it is ceitain, when the leaves of 

 a plant become generally white, that that mdividual 

 is seldom long lived. Mr. Knight, however, has 

 demonstrated that variegation is not a certain indica- 

 tion of a deficiency of hardiliood. 



The functions of the leaves appear to be a com- 

 bination of those of the lungs aiid stomach of animals ; 

 they not only modify the food brought to them from 

 the roots, so as to fit it for increasing the size of the 

 parent plant, but they also absorb nourishment from 

 the atmosphere. The sap, after elaboration in these 

 organs, difi'ers in every plant, though as far as experi- 

 ments have been tried, it appears to be nearly the 

 same in all vegetables when it first arrives to them. 

 The power of a leaf to generate sap is in proportion 

 to its area of surface, exposure to the light, and con- 

 genial situation. 



Leaves throw off a veiy considerable quantity of 

 water. Dr. Hales found that a cabbage emitted 

 daily nearly half its weight of moisture, a sunflower, 



