EFFECT OF RADIANT ENERGY ON GROWTH 597 



tions in the quality of light. Even under experimental conditions it is difficult 

 to provide differences in light intensity which are qualitatively identical, and 

 this fact must be considered in evaluating the results of all such experiments. 

 We shall first consider the development of plants as it occurs in the com- 

 plete absence of light but under such conditions that food is available to the 

 growing parts from a storage organ such as a seed, tuber, or bulb (AlacDou- 

 gal, 1903). Seedlings of dicots which have developed in total darkness 



Fig. 132. Seedlings of bean {Phaseolus vulgaris) about two weeks old grown in light 



(left) and in total darkness (right). 



have spindling, yellowish, succulent stems on which the leaves fail to expand 

 (Fig. 132). Their root systems are also relatively poorly developed. In- 

 dividuals of the same species developed in full daylight, on the contrary, are 

 of normal stature, bear fully expanded green leaves, have relatively larger root 

 systems, and develop a less succulent type of stem structure. 



When seedlings of monocot species develop in total darkness the chloro- 

 phyll-free leaves are relatively narrow and more attenuated than the leaves 

 of similar plants which have developed in the light. 



The distinctive morphogenic development of plants which have grown in 

 total absence of light is called etiolation. Exposure of plants to light of a 



