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 hot always the case. The 
FIG. 94. Calla palustris. -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 
