122 STRUCTURE AND FUNCTIONS OF LEAVES 



oleander, and Smilax aspera, the leaves do not attain their maxi- 

 mum power of transpiration as soon as they are fully grown. 

 Such a leaf transpires more when fifteen months old than when 

 three months old. 



The fall of the leaf is preceded by important changes in the 

 contents of its cells. Much of the sugary and protoplasmic con- 

 tents of the leaf disappears before it falls. These valuable mate- 

 rials 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 leafstalk has been 

 removed, and no chance for fungi to attack the bark or wood 

 and cause it to decay. In compound leaves each leaflet may 

 become separated from the petiole, as is notably the case with 

 the horse-chestnut leaf (Fig. 99). In woody monocotyledons, 

 such as palms, the leafstalks do not commonly break squarely 

 off at the base, but wither and leave projecting stumps 011 

 the stem. 



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 indi- 

 vidual 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. 



