ss 
which are exposed to the sun, exhibit a green co- 
lour *. 
283. The celebrated Ray possessed still clearer 
notions on this subject. He remarked that plants 
continued green while they vegetated under a glass 
bell exposed to the light ; but when the light was 
excluded, by covering them with an opaque vessel, 
they lost their green hue, and acquired a whitish yel- 
low colour. Their stalks, at the same time, became 
long, slender and feeble, and their leaves small. The 
cause of the green colour he, therefore, rightly ascri- 
bed to the direct agency of light, rather than to that 
of heat or air f . 
284. These experiments were carried much far- 
ther by M. Bonnet, who caused peas and beans to 
grow in glass tubes, when their colour was not af- 
fected ; but in boxes, when the light was completely 
excluded, the plants became white. Even in wooden 
cases, however, the plants exhibited a green colour 
in particular parts, placed opposite to small perfora- 
tions which were made to admit the light. The ex- 
clusion of light, he adds, obstructs the development 
of the leaves, and promotes the elongation of the 
stem ; but neither want of air, nor a greater or less 
degree of heat, seemed to affect the degree of etiola- 
tion J. 
285. This philosopher farther discovered, that 
plants, which had become white by the exclusion of 
* Physiol. Veg, vol. iv. p. 265. 
f Hist. Plaatar. vol. i. p. 15. 
Recherches sur les Feuilles, p, 210. 331. 
