204? THYTOTOMY. 



321. 



Evajx)ration has an essential influence on the economy of the 

 plants themselves, and on the whole economy of nature. The 

 activity by which tlie plant empties itself of its superfluous 

 matters, operates as an incitement to the other functions, and 

 a plant is, in truth, the more healthy, the more freely it eva- 

 porates. Yet there may be an excess in this also, especially 

 when not only unformed juice, but the prepared and proper 

 sap, is given off". The sudden and powerful operation of 

 the sun-beams after a passing drizzling rain, favours not un- 

 frequently the perspiration of oxidized slime and of sweet 

 drops, which are known by the name of " honey-dew.^ 

 Swarms of insects are thus invited, whose young brood over- 

 spread the surface of the leaves as a fine powder, and ren- 

 der them incapable of performing their functions. Tliis is 

 the simple explanation of the blight, or of the mildew, as 

 every person may convince himself by observation, (426.) 



322. 



The evaporotion of leaves has a great influence on the ge- 

 neral economy of nature. As in the transition from the form 

 of drops to that of vapour, a greater portion of heat is con- 

 sumed, the quicker this transition takes place ; we find in 

 this fact a principal cause of the low temperature which the 

 juices of living plants exhibit even during the greatest sum- 

 mer heat. Nay, the shade of a leafy tree will alwaj^s afford 

 a greater coolness to sentient animals, than the shade of life- 

 less objects. 



The influence which the evaporation of leaves has upon the 

 whole atmosphere, as well as upon the earth and its waters, 

 produces very extensive effects. Forest regions are not only 

 cooler, but also more productive of rain, than steppes and 

 sandy deserts, where vegetation is entirely wanting. All the 

 streams of the world have their sources in mountain chains 

 covered with woods ; and although the melted snow is their 

 immediate cause, they would neither continue to be poured 

 along, nor grow to a river, unless forests and woods, by their 

 evaporation, incessantly afforded tlie necessary stores of wa- 



