MINUTE STRUCTURE OF LEAVES. 125 
-Much of the sugary and protoplasmic contents of the leaf 
disappears before it falls. These valuable materials have 
been absorbed by the branches and roots, to be used again 
the following spring. 
The separation of the leaf from the twig is accomplished 
by the formation of a layer of cork cells across the base of the 
petiole in such a way that the latter finally breaks off across 
the surface of the layer. A waterproof scar is thus already 
formed before the removal of the leaf, and there is no waste 
of sap dripping from the wound where the leaf-stalk has been 
removed, and no chance for moulds to attack the bark or 
wood and cause it to decay. In compound leaves each leaflet 
may become separated from the petiole as the latter does 
from its attachment to the twig, as is notably the case with 
the horse-chestnut leaf (Fig. 75), or in a few kinds of simple 
leaves the blade may separate from its petiole, as it does in 
the Boston ivy (Ampelopsis Veitchit). 
The brilliant coloration, yellow, scarlet, deep red and purple, 
of autumn leaves is popularly but wrongly supposed to be 
due to the action of frost. It depends merely on the changes 
in the chlorophyll grains and the liquid cell contents that 
accompany the withdrawal of the proteid material from the 
tissues of the leaf. The chlorophyll turns into a yellow 
insoluble substance after the valuable materials which 
accompany it have been taken away, and the cell-sap at the 
same time may turn red. Frost perhaps hastens the break- 
up of the chlorophyll, but individual trees often show bright 
colors long before the first frost, and in very warm autumns 
most of the changes in the foliage may come about before 
there has been any frost. 
