TREE PLANTING ON STREETS AND HIGHWAYS. 45 



leaf that is thickly shaded by others is apt to remain green until it withers and dies. 

 Where a twig or branch presses constantly on the surface of a leaf the part thus 

 covered remains green after the rest has turned yellow or red. If you cut your 

 initials from tin foil or thick paper and paste them on a large leaf the letters will in 

 time be sharply defined in green on a background of yellow or red. 



Why leaves should change color is as hard to explain or understand as why the 

 hair turns gray. The scientists who have written on the subject admit that there is 

 much to learn about the process and its cause. The leaf cells contain rounded 

 granules of green matter known as chlorophyll, a substance — or mixture of sub- 

 stances — to which the pure green color of ordinary healthy leaves is due. The 

 appearance of any other color, such as red, yellow or purple, would indicate the 

 presence of some substance accompanying the chlorophyll and disguising its color, 

 or even replacing it entirely.* 



The leaf cells contain, also, xanthophyll, a peculiar yellow coloring matter, which 

 remains after the decomposition or absorption of chlorophyll ; and erythrophyll, 

 which supplies the red or crimson shades found in matured leaves. 



Most of the scientific explanations of the change of color are so technical that 

 they are of little use to the general reader. The following extract from an article in 

 the Botanical Gazette for April, 1887, entitled, "The Autumnal Changes in Maple 

 Leaves," by W. K. Martin and S. B. Thomas, is instructive and interesting: 



"Chlorophyll, manufactured constantly under the influence of light, is constantly 

 undergoing decomposition by the metabolism of the cell. Under ordinary condi- 

 tions, the manufacture of chlorophyll is sufficient to cover up its decomposition, and 

 the leaf retains its green color. Under certain changed conditions, however, such as 

 intense light or diminished vitalitj-, the decomposition of chlorophyll exceeds its 

 manufacture, and xanthophyll (probably one of the products of decomposition) 

 appears. In other words, xanthophyll is being formed all the time, but only 

 becomes apparent when the manufacture of chlorophyll is checked. The condition 

 of intense sunlight gives us the occasional summer yellowness, while to lowered 

 vitality must be attributed the failure of chlorophyll manufacture in the autumn. 

 This lower vitality is brought about by diminution of light, lowering of temperature, 

 and probably causes in the plant itself. Xanthophyll then stains the chlorophyll 

 masses yellow, which were before stained green by chlorophyll. The red coloration 

 is brought about in a very different way, as erythrophyll is manufactured in the leaf, 

 and stains the cell sap, leaving the chlorophyll masses untouched. This red color- 

 ing matter cannot be discovered in any of the crude materials brought into the 

 plant, or in any other part of the leaves, except sometimes in the phlceum regions 



* Webb's Dictionary of Chemistry. 



