PHYTOTOMY. 



dissolves completely in spirit of wine ; but, as it is not depo- 

 sited by water from this solution, but continuing mixed with 

 it, gives out a very nauseous hepatic smell, it is likely that 

 azote goes also to the composition of this green colouring- 

 matter. AVe shall come back again to this subject, and only 

 notice farther here, that a higher degree of vital activity in 

 leaves, awakened by the light of the sun, often produces a 

 blue colour from the green. In this case, there is probably 

 an excess of hydrogen above carbon, as the putrefactive fer- 

 mentation of woad and indigo, which is encouraged for the 

 production of the blue colour, seems to shew. The blue 

 colour of woad and indigo passes again, with mineral acids, 

 into green, and lastly into yellow. 



Decayed and falling leaves are yellow and red, because the 

 oxygen remains in them after the vital activity is gone. 



318. 



This peculiar breathing of plants through the leaves has 

 the most important influence upon their whole economy. By 

 this only the proper mixture of the juices, and the produc- 

 tion of fruit, is accomplished. It is hence that the leafing of 

 plants is so necessary to the setting and ripening of the fruit, 

 and that an attack of lightning, by which the leaves are de- 

 stroyed, is injurious to hops and to all other plants. The re- 

 ception of gaseous matters is equally important to tlie for- 

 mation of the proper juices, and to the perfection of every 

 other vital function. 



319. 



But we must also treat of the absorption of fluids in the 

 shape of drops or of vapours, and of their evaporation, as be- 

 ing important functions of the leaves. 



That the vapours and rain-drops are absorbed by leaves, 

 is evident to sight. This also is confirmed by the fact, that 

 a multitude of plants which have insignificant roots, yet grow 

 very freely by absorbing with their green surface the nourish- 

 ing fluidity of the atmosphere. In the parched deserts of 

 Africa, where the quantity of rain in a century rises scarcely 



