Chap. 3,] Senfilility of Plants to Light, 27 



fun. The air from filk was much fuperior to that from, 

 vegetables. 



Plants have a remarkable fenfibility to light; they 

 unfold their flowers to the fun, they follow his courfe 

 by turning on their ftems, and are clofed as fo. n as he 

 difappears. Vegetables placed in rooms where they 

 receive light only in one direction always extend them- 

 felves that way. If they receive light in two di regions, 

 they direct their c.ourf" iowards the ftrongeft. Trees 

 growing in thick forefts, where chey only receive light 

 from above, direct their (hoots almoft invariably up- 

 wards, and therefore become much taller and lefs 

 fpreading than fuch as (land fingle. This arTectioii'for 

 light feems to explain the upright growth of vege- 

 tables, a curious phenomenon, too common to be much 

 attended to. It has been afcertained by repeated ex- 

 periments, that the green colour of plants is entirely 

 owing to light ; for plants reared in the dark are well 

 known to be perfectly white. 



If we take a fucculent plant, and exprefs its juice, 

 the liquor appears at firft uniformly green ; but allow 

 it to ftand, and the green colour feparates from the 

 watery fluid, and falls to the bottom in a fediment. If 

 we collect this fediment it will be found to be of an 

 oily nature, for it dues not diflblve in water, but it will 

 in fpirits of wine, or oil, to which it imparts a green 

 colour. As the fun produces the green colour in plants, 

 and as this refides in an oily matter, it was formerly 

 concluded that light furnimes the oily matter of ve-. 

 getables, and that it effects this by furmfhing the prin- 

 ciple of inflammability. The new chemical do Brines, 

 however, afford a much more fatisfactory explanation 

 of the effect of the fun's rays in producing the oily 

 matter in vegetables. Vegetable matter confifts in 

 general of carbon, hydrogen and oxygen - 3 the lun's rays 



produce 



