176 FOUNDATIONS OF BOTANY 
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 com- 
pound leaves each leaflet may become separated from the 
petiole, as is notably the case with the horse-chestnut leaf 
(Fig. 102). In woody monocotyledons, such as palms, the 
leaf-stalks do not commonly break squarely off at the base, 
but wither and leave projecting stumps on the stem 
(Plate VI). 
The brilliant coloration, yellow, scarlet, deep red, and 
purple, of autumn leaves is popularly but wrongly sup- 
posed 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 per- 
haps 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 foli- 
age may come about before there has been any frost. 
188. Tabular Review of Experiments. 
[Continue the table from Sect. 128.] 
