IX Leaves 187 



as Prof. Vines has of late conclusively confirmed, a starch- 

 changing (diastatic) ferment in all green leaves. It is 

 indeed in the ordinary processes of everyday life that the 

 vitality of the leaf is most marvellous, especially in that 

 process by which the kinetic energy of the sun's rays, 

 entering the plant as a series of ethereal waves, passing 

 through the screen of chlorophyll, aids the living matter 

 to build up out of crude materials those complex organic 

 products whose potential energy may be again transformed 

 in heat and light or motion when we use them as fuel or 

 food. Nor in our appreciation of the life of the leaf 

 should we forget that it often contains enough of the 

 plant's essential material and vigour to enable it in 

 suitable conditions to grow into an entire plant, for 

 every one knows how a piece of Begonia leaf, for instance, 

 may be planted so as to produce a new plant with stem 

 and roots. 



Fall of the Leaf. — Throughout the summer the leaf 

 lives this intense Hfe, thriving in the sunshine, and produc- 

 ing a store of food-stuffs laid up in reserve in different parts 

 of the plant. But in autumn the vitality is checked, the 

 supplies of water which the leaves demand is no longer 

 afforded, transpiration and the movements of the sap 

 become very slight, and the leaves begin to die. But 

 before they die they surrender all that remains of their 

 life to the plant which bore them ; before the breath of 

 approaching winter all that is worth having of sugar and 

 more complex stuffs ebbs in gentle current from the leaves 

 to the stem. Then the leaf, useful in dying as well as in 

 living, begins to be cut off, for while the retreat of the 

 residues of the leaf's life is being accomplished, there has 

 also been preparation for^the leaf's fall. Across the base 

 of the leaf-stalk, in a region which is normally firm and 

 tough, there grows inward a partition of soft juicy cells, 



