THE PLANT AND ITS ENVIRONMENT 



159 



structures generally. The advantageous character of the 

 ordinary phototropic responses is obvious. 



Not only does light affect the position of plant organs but it has 

 a profound influence upon their structure. /The stems of green 

 plants grown in the dark are usually slender, much elongated and 

 provided with but little woody tissue; and their leaves are 



Fig. 77. — Phototropism. A mustard seedling growing with its root in water. 

 This plant was at first illuminated from all sides, but later from only one (shown 

 by direction of arrows). Note that the stem has bent toward the light and the 

 root away from it, and that the leaves have taken up a position at right angles to 

 the light. (After Strash urger) . 



greatly reduced in size, long petioled and undifferentiated inter- 

 nally. Chlorophyll fails to develop and the plant assumes a 

 pale yellow color. This general effect of darkness is known as 

 etiolation (Fig. 78) and begins to show itself whenever the supply 

 of light falls below the optimum either in duration or intensity. 

 If sufficiently pronounced, etiolation ultimately results in death. 

 The stimulus of light upon protoplasm evidently prevents the 

 abnormal growth which we see in etiolation, but how this effect 

 is brought about, we do not understand. 



Too little illumination is thus harmful to the plant, but 

 too much may be e(iually so through its to.xic effect upon proto- 

 plasm. To the blue, violet, and ultra-violet raj^s living substance 

 is particularly sensitive, and in many plants the position or 



