136 



THE VEGETATIVE FUNCTIONS OF PLANTS 



bleached to a pale yellow color, or even to white. The 

 chlorophyll has failed to develop. This condition is 

 called etiolation. Early studies of this phenomenon 

 seemed to indicate that light retarded growth in length, 

 but more thorough and more extended observations 

 clearly showed that such is not always the case. The 



I"iG. 94. — Calla paliistris. -A , Normal plant, grown in daylight; B etiolated 

 plant of the same age, grown in darkness. (After D. T. MacDougal.) 



stems of many kinds of plants {e.g., potato, pea, bean) 

 undoubtedly grow much longer in darkness than in light 

 (Fig. 93), but in other species the difference, if any, is 

 much less (Fig. 94). A disturbance of the normal illumi- 

 nation causes a general disturbance of the functions of the 

 organ or of the entire plant, so that not only growth, but 

 differentiation of tissues (development), nutrition, and 

 metabolism in general are more or less upset, and proceed 



