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.] 



