The Fall of the Leaves 



run from July until November. The tree anticipates the com- 

 ing of winter. Its buds are well formed by midsummer. Even 

 then signs of preparation for the leaf fall appear. A line around 

 the base of the leaf stem indicates where the break will be. Corky 

 cells form on each side of this joint, replacing tissues which in 

 the growing season can only be parted by breaking or tearing 

 them forcibly. A clean-cut zone of separation weakens the hold 

 of the leaf upon its twig, and when the moment arrives the light- 

 est breath of wind — even the weight of the withered leaf itself — 

 causes the natural separation. And the leaflets simultaneously 

 fall away from their common petiole. 



There are more important things happening in leaves in 

 late summer than the formation of corky cells. The plump green 

 blades are full of valuable substance that the tree can ill afford 

 to spare. in fact, a leaf is a layer of the precious cambium 

 spread out on a framework of veins and covered with a delicate, 

 transparent skin — a sort of etherealised bark. What a vast quan- 

 tity of leaf pulp is in the foliage of a large tree! 



As summer wanes, and the upward tide of sap begins to 

 fail, starch making in the leaf laboratories declines proportion- 

 ately. Usually before midsummer the fresh green is dimmed. 

 Dust and heat and insect injuries impair the leaf's capacity for 

 work. The thrifty tree undertakes to withdraw the leaf pulp 

 before winter comes. 



But how? 



It is not a simple process nor is it fully understood. The 

 tubes that carried the products of the laboratory away are bound 

 up with the fibres of the leaf's skeleton. Through the transparent 

 leaf wall the migration of the pulp may be watched. It leaves 

 the margins and the net veins, and settles around the ribs and 

 mid vein, exactly as we should expect. Dried and shrivelled 

 horse-chestnut leaves are still able to show various stages in this 

 *narvellous retreat of the cambium. If moisture fails, the leaf 

 bears some of its green substance with it to the earth. The 

 "breaking down of the chlorophyll" is a chemical change that 

 attends the ripening of a leaf. (Leaf ripening is as natural as 

 the ripening of fruit.) The waxy granules disintegrate, and a 

 yellow liquid shows its colours through the delicate leaf walls. 

 Now other pigments, some curtained from view by the chlorophyll, 

 •others the products of decomposition, show themselves. Iron and 



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