59 
light, recovered their green colour when the light 
was restored. Thus French beans, which were much 
etiolated, acquired, in 24 hours, a sensible green tint 
by exposure to day-light in summer ; but, in the 
dark days of autumn, they still retained their whitish 
colour, even when thus exposed. He also noticed 
the great exertions which vegetables make to expose 
their leaves to the action of the sun ; thus, a tuberous 
root, confined in a vessel where the light was par- 
tially excluded, made, in five weeks, eleven turnings 
or movements to follow it *. 
286. The red and purple, as well as the green 
colours of vegetables, and all the varied hues of 
flowers, seem, likewise, to owe their perfection to the 
agency of light. DnGrew remarked, that those parts 
of the roots of plants, which remain under the soil, 
are generally white, while the parts exposed above 
ground are frequently coloured ; thus the tops of 
sorrel roots are red, and those of turnips and radi- 
shes are sometimes purple. These changes in the 
colours of the roots, as well as the green colour of 
the leaves, and the different colours of flowers, he 
attributed to the action of the air t ; but the experi- 
ments of Mr Davy (25.) prove that the colours of 
flowers, as well as of leaves, depend immediately 
on the operation of light. 
287. The changes produced in the other sensible 
properties of plants, by the agency of the solar rays, 
were first remarked by Scheele, who considered 
* Hecherches, &c. p. 340. 
t Anat. of Plants, p. 270. 2d edit. 
