PLANT COMMUNITIES: SEASONAL CHANGES. 417 



of a Christmas tree lighted with numerous candles. The numer- 

 ous clusters of staminate flowers suggest the bundles of toys and 

 gifts, and one inquires if this beautiful aspect of some pines 

 when putting on their new growth did not suggest the idea of 

 the Christmas tree at yule time. 



728. The summer tints are more subdued. — As summer- 

 time draws on the new needles of the pine are unsheathed, the 

 light green tints of the forest are succeeded by darker and sub- 

 dued colors, which better protect the living substance from the 

 intense light and heat of midsummer. The physiological pro- 

 cesses for which the leaf is fitted go on, and formative materials 

 are evolved in the countless chlorophyll bodies and transported 

 to growing regions, or stored for future use. In transpiration 

 the leaf is the terminus of the great water current started by the 

 roots. Here the nutrient materials, for which the water serves 

 as a vehicle, are held back, while the surplus water evaporates 

 into the air in volumes which surprise us when we know that it 

 is unseen. 



729. Autumn colors. — As summer is succeeded by autumn, a 

 series of automatic processes goes on in the plant which fits it 

 for its long winter rest again. Long before the frosts appear, 

 here and there the older leaves of certain shrubs lose more or 

 less of the green color and take on livelier tints. AVith the dis- 

 integration of the chlorophyll bodies, other colors, which in 

 some cases were masked by the green, are uncovered. In other 

 cases decomposition products result in the formation of new 

 colors. These coloring substances to some extent absorb the 

 sun's rays, so that much of the nitrogenous substances in the leaf 

 may not be destroyed, but may pass slowly back into the stem 

 and be stored for future use. 



730. Fall of the leaf. — The gorgeous display of color, then, 

 which the leaves of many trees and shrubs put on is one of the 

 many useful adaptations of plants. While this is going on in 

 deciduous trees, the petiole of the leaf near its point of attachment 

 to the stem is preparing to cut loose from the latter by forming 

 what is called a separative layer of tissue. At this point the cells 



