310 Morpho genetic Factors 



illumination the two dimensions grow at about the same rate (Pearsall, 

 1927). 



As to the causes of etiolation there has been much discussion but no 

 final agreement. Auxin is undoubtedly involved, for it is well known 

 that sensitivity to it increases in darkness. Wave length of light is also 

 important here, for etiolation may be very different in red light and in 

 blue. The two processes of leaf growth and stem elongation may be 

 affected differently. 



Priestley (1926b) called attention to the fact that in etiolated stems 

 the endodermis tends to be well developed and to have thick-walled 

 cells. An etiolated stem is thus somewhat like a root in structure. He 

 suggests that for this reason water and nutrients, coming from the roots 



Fig. 13-1. Effect of etiolation on cell shape. Longitudinal section of cortical paren- 

 chyma of the stem of Vicia faba when grown in light ( left ) and in darkness ( right ) . 

 ( After Kolda. ) 



into the vascular cylinder, may be confined there and prevented from 

 passing outward. This would tend to accelerate growth at the tip of the 

 shoot and to check the development of leaves. 



The relation of light to normal and etiolated growth evidently involves 

 the problems of cell division and cell enlargement. In beans Brotherton 

 and Bartlett (1918) found that in the epidermis about a third of the 

 added growth in length of etiolated as compared with normal plants 

 was due to more cells and about two-thirds to longer cells, the rate of 

 both processes of division and enlargement being inversely proportional 

 to light intensity. Cell elongation has been shown in many other cases 

 to increase with diminished light intensity (Fig. 13-1). This is evident 

 not only in green plants but in fungi (Castle and Honeyman, 1935). 



