80 The Ottawa Naturalist [October 



WHY THE LEAVES CHANGE THEIR COLOR. 



The change in the color of leaves in autumn is not, as many 

 people suppose, due to the action of frost, but is a preparation for 

 winter. All during the spring and summer the leaves have served as 

 factories, where the foods necessary for the trees' growth have been 

 manufactured. Ths food making rake- place in numberless tiny cells 

 of the leaf and is carried on by small green bodies which give the 

 leaf its color. These chlorophyll bodies, as they are known, make the 

 food of the tree b) combining carbon taken from the carbonic acid gas 

 of the air with hydrogen, oxygen, and various minerals supplied by 

 the water which the roots gather. In the fall when the cool weather 

 can- lowing down of the vital processes, the work of the leaves 



comes to an end. The machinery of the leaf factory is dismantled, so 

 to speak, the chlorophyll is broken up into the various substances of 

 which it is composed, and whatever food there is on hand is sent to 

 the body of the tree to be stored up for use in the spring. All that 

 remains in the cell cavities of the leaf is a watery substance in which 

 a few oil globules and crystals, and a small number of yellow, strongly 

 tive bodies can be seen. These give the leaves the yellow color- 

 ing so familiar in autumnal foliage. 



It often happens, however, that there is more sugar in the leaf 

 that cm be readily transferred back to the tree. When this is the i - 

 the chemical combination with the other substances produce- many- 

 colored tints varying from the brilliant red of the dogwood to the more 

 austere red-browns of the oak. In coniferous trees, which do not lose 

 their foliage in the fall, the green coloring matter takes on a slightly 

 brownish tinge, which, however, gives way to the lighter color in the 

 spring. 



While tin color of the leaf is changing, other preparations are 

 being made. At the point where the stem of the leaf is attached to 

 the tree, a specal layer of cells develops which gradually sever the 

 tissues which support the leaf. At the same time Nature heals the cut, 

 so that when the leaf is finally blown off by the wind or falls from its 

 own weight, tlie place where it grew on the twig is marked by a scar. 



Although the food which has been prepared in the cell cavities is 

 sent back to the tree, the mineral substances with which the- walls of 

 the cells have become impregnated during the summer months are 

 retained. Accordingly, when the leaves fall they contain relatively 

 large amounts of valuable elements, such as nitrogen and phosphorus 

 which were originally a part of the soil. The decomposition of the 

 lea* ults in enriching the top layers of the soil by returning these 



elements and by the accumulaton of humus. That is why the mellow 

 black earth from the forest floor is so fertile. (From the Forest 

 Service, U. S. Dep. Agric. ) 



