386 RELATION TO ENVIRONMENT. 



The red and purple colors are found suffused in the cell-sap of certain 

 cells in the leaf much as we have found it in the cells of the red beet. The 

 yellow color is chiefly due to the disappearance and degeneration of the 

 chlorophyll while the leaf is in a moribund state. A similar phenomenon 

 is seen in the yellowing of crops when the soil becomes too wet, or in the 

 blanching of grass when covered with a board, or of celery as the earth 

 is ridged up over the leaves in late summer and autumn. A number of 

 different theories have been advanced to explain autumn coloring, i.e., 

 the appearance of the red coloring-matter. It has been attributed to the 

 approach of cold weather, and this has likely led to the erroneous belief 

 on the part of some that it is caused by frost. It very often precedes frost. 

 Some have attributed it to the action of the more oblique light rays during 

 autumn, and still others to the diminishing water-supply with the approach 

 of cool weather. The question is one which has not met as yet with a 

 satisfactory solution, and is certainly a very obscure one. It is likely 

 that the low temperature or the declining activities of the leaf affect certain 

 organic substances in the leaf and give rise to the red color, and it is quiU 

 certain that in some years the display is more brilliant than in others. 

 The color is more striking in some regions than in others and the different 

 soil, as well as climate, has been supposed to have some influence. The 

 North American forests are noted for the brilliant display of autumnal 

 color. This is perhaps due to some extent to the great variety or number 

 of species which display color. It would seem that there is some specific 

 as well as individual peculiarities in certain trees. Some individuals, 

 for example, exhibit brilliant colors every autumn, while others near of 

 the same species are more subdued. 



It has been shown by experiment that when sunlight passes through 

 red colors the temperature is slightly increased, and it has been suggested 

 that this may be of protection to the living substance which has ceased 

 working and is in danger of injury from cold. There does not seem to 

 be much ground for this suggestion, however. It certainly could not 

 protect the protoplasm of the leaf at night when the cold is more intense, 

 and during the day would only aggravate matters by supplying an in- 

 creased amount of heat, since extremes of heat and cold in alternation 

 are more harmful to plant life than uniform cold. Especially would this 

 be the case in alpine climates where the alternation of heat and cold be- 

 tween day and night is extreme, and brilliancy of the colors of alpine plants 

 is well known. It seems more reasonable to suppose that tlie red coloi 

 acts as a screen, as the chlorophyll is disappearing, to protect from the 

 injurious action of light, certain organic substances which are to be trans- 

 ferred back from the leaf to the stem for winter storage. So in the case 

 oi many stems in the spring or early summer when the young leaves often 

 have a reddish color, it is likely that it acts as a screen to protect the living 



