133 
leaves, seem also to explain, not only why they lose 
their verdure in darkness, but the cause of that degra- 
dation and change of colour which they experience 
in autumn, at the period, and during the time of their 
fall. There are many reasons which make it proba- 
ble, that alkaline matter is constantly conveyed into 
the plant, during every period of its growth, and is 
thus constantly supplied to act on the colourable mat- 
ter of the leaves. When, therefore, vegetation fails, 
this alkali will be less abundantly supplied, and the 
acid will proportionally prevail. But not only will 
the alkaline matter, at this period, be thus diminished, 
but the quantity of acid will be increased ; for, as 
vegetation declines or ceases, spontaneous decompo- 
sition will begin, and the acid matter thereby deve- 
loped will counteract or overpower the alkali ; so 
that, according to the different proportions in which 
these ingredients meet, the colourable juices of the 
plant will be differently affected, and will thus ex- 
hibit those various brown and yellow tints, which 
compose an autumnal scene. 
385. But the green colours of vegetables, not on- 
ly pass to a yellow or brownish hue, but sometimes 
change to red. The leaves of the oak, of the pear, 
the vine, and barberry, often exhibit this colour : and, 
among herbaceous plants, the leaves of buck-wheat, 
strawberry, and amaranth do the same. It is only 
when they have attained to maturity, or are about to 
fall, or when they have been injured by insects, says 
M. Senebier, that they present a red colour. In 
those leaves which turn red before they fall, the red- 
ness first appears in the stalk , in others, the redness 
