18 TREES 



with a delicate, transparent skin — a sort of etherealized 

 bark. What a vast quantity 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 pro- 

 portionately. 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 marvellous 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 at- 

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

 the ripening of fruit.) The waxy granules disintegrate, 

 and a j^ellow liquid shows its colors through the delicate 

 leaf walls. Now other pigments, some curtained from 

 view by the chlorophyll, others the products of decom- 

 position, show themselves. Iron and other minerals the 

 sap brought from the soil contribute reds and yellows and 

 purples to the color scheme. As drainage proceeds, with 

 the chemical changes that accompany it, the pageant of 

 autumn colors passes over the woodlands. No weed or 

 grass stem but joins in the carnival of the year. 



