THE INFLUENCE OF LIGHT. 69 
full brightness, and the colour of the leaves 
changed so fast, that by the middle of the same 
afternoon the whole forest, for many miles in 
extent, exhibited its usual Summer’s dress. Nor 
is this the only result of exposure to the influ- 
ence of light. As we shall presently have to see, 
the formation of the wood of plants is proceeded 
with under the influence of the light of day alone. 
All the complicated phenomena of digestion, and 
of the various chemical processes which take 
place in plants, are equally the results of the won- 
derful influence of the solar ray. Without light, 
no odoriferous flowers could adorn our earth; 
nor could any of the valuable and useful medi- 
cinal resins which we find in plants, be produced. 
Sometimes, indeed, human art avails itself of this 
fact; and when we wish to procure a vegetable 
full of sweet or limpid juice, and free from its 
natural acrid, or even poisonous properties, we 
cause it to grow in such a manner, as that light 
can scarcely exert the least influence on the por- 
tions weconsume. ‘Thus for instance—One among 
other reasons for tying up the tops of lettuces, so 
as to bleach all the heart-leaves, is to exclude the 
light, and thus obtain a mass of leaves free from 
