Vegetable Physiology. 451 



is gifted. The violet rays of the spectrum have this power in 

 the greatest degree ; and Sennebier has ascertained by experi- 

 ment, that those rays have the greatest influence in producing 

 the green colour of plants. When leaves are of any other hue 

 than green, they are said to be coloured. This variegation is 

 often considered to be a symptom either of tenderness or debi- 

 lity ; and it is certain, when the leaves of a plant become gene- 

 rally white, that that individual is seldom long-lived. Mr. 

 Knight, however, has demonstrated that variegation is not a 

 certain indication of a deficiency of hardihood. 



The functions of the leaves appear to be a combination of 

 those of the lungs and 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, differs in evei'y plant; though, as far as expe- 

 riments have been tried, it appears to be nearly the same in 

 all vegetables Vv^hen 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 congenial situation. 



Leaves throw off a very considerable quantity of water. 

 Dr. Hales found that a cabbage emitted daily nearly half its 

 weight of moisture, and a sunflower three feet high perspired 

 in the same time 1 lb. 14ioz. But of all the plants of which 

 the diurnal perspiration has been ascertained, the Cornelian 

 Cherry (Cornus mascula) transpires the most, the exhalation 

 amounting to near twice the weight of the plant in twenty- 

 four hours. This aqueous expiration takes place chiefly during 

 the day ; is much promoted by heat, and checked by rain or 

 a reduction of temperature. 



On the free performance of this function of plants their 

 health is dependent in a very high degree ; and I believe that 

 half the epidemics to which they are subject arise from its 

 derangement. The clubbing of the roots of the jJ5rassica 

 tribe, I consider, arises entirely from it. In the drought of 

 summer, when the moisture supplied to a cabbage by its root 

 does not any thing near equal the exhalation of its foliage, to 

 supply the deficiency the plant endeavours, by forming a kind 

 of spurious bulbous root, to adapt itself to the contingency ; 

 in the same manner that, in dry situations, the fibrous roots 

 of Phleum pratense, ^lopecurus geniculatus, &c., acquire a 

 tuberous form, as bulbous or tuberous rooted plants, it is 

 well known, will exist in a soil so deficient in moisture as to 

 destroy all fibrous-rooted vegetables. 



Evergreens transpire less moisture than deciduous plants, 

 which would lead to the expectation that they are more 



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