136 THE STUDY OF PLANT COMMUNITIES * Chapter VI 



of chlorophyll with decreasing light intensity and an ability of 

 most plants to produce chlorophyll at light intensities considerably 

 below those necessary for effective photosynthesis. 



The opening and closing of stomata can usually be correlated 

 with light, but there are enough exceptions to give warning against 

 generalizations or interpretations based on the principle of alter- 

 nate opening and closing with light and darkness. In some plants, 

 stomata may open at night; in others, light seems not to be a con- 

 trolling factor. Where stomatal movement seems directly respon- 

 sive to light, other factors may at any time become more impor- 

 tant and modify or counteract the effects of light, as when stomata 

 close during the day if the water supply is insufficient. However, 

 stomatal movement is usually correlated with light changes and, 

 when other conditions are favorable, is apparently caused by tur- 

 gidity changes in the guard cells resulting from metabolic activity, 

 which varies with light. The opening and closing in turn may 

 modify effects of light by varying gas exchange related to photo- 

 synthesis and rate of loss of water by transpiration. 



The production of certain auxins or growth-controlling sub- 

 stances in plants is inhibited by light. As a result, through them, 

 size, shape, movements, and orientation of parts may be influenced 

 by light. A plant grown in complete darkness, since it produces a 

 maximum of auxins, elongates excessively, with poorly differenti- 

 ated tissues throughout and with almost no supporting structure. 

 These characteristics in an intermediate condition are often rec- 

 ognizable in plants grown in heavy shade, as under a forest canopy 

 or in close stands where plants shade each other. Such plants tend 

 to be tall and spindly with widely spaced nodes and relatively few 

 leaves. The better the light, the stouter and more compact the in- 

 dividual will be. 



Should illumination be one-sided, the increased production of 

 auxins on the shaded side usually stimulates sufficient extra elonga- 

 tion on that side to turn the growing portion of the stem toward 

 the light. Some species— sunflower, for instance— are so sensitive 

 to such differences of light that the floral portions shift from east 

 to west with the sun daily as differential elongation in the stem 

 progresses from one shaded side to the other. 



The orientation of vegetative parts is such that every leaf re- 



