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. 76), or in a few kinds of simple 

 leaves the blade may separate from its petiole, as it does in 

 the Boston ivy (Ampelopsis Veitchii). 



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. 



