74 Selby : Studies in etiolation 



jected to etiolation by MacDougal and others shows that less 

 than half exhibit such elongations. 



An important correlation-reaction is shown by the basal buds 

 of etiolated stems. Numbers of such buds usually dormant are 

 awaked in darkness and develop stems. 



None of the facts derived from the study of the behavior of 

 etiolated plants may be construed to indicate that light exercises a 

 retarding effect on growth, however. If such a conclusion is to be 

 maintained it must be justified on other grounds beside the results 

 of studies of the action of plants in darkness. 



A consideration of the structure of the plants studied in the 

 work described above leads to the conclusion that when grown in 

 darkness the tissues do not attain full development and that the 

 differentiations by which the separate tissues are distinguished ap- 



* 



pear to be carried out only partially or not at all. The degree of 

 incompleteness depends to some extent upon the organographic 

 relations of the structures taken into consideration. This failure 

 in the perfection of the tissues, which may even include their 

 non-appearance, is very naturally coupled with a prolonged growth 

 of the meristematic cells which by repeated division may thus in- 

 crease either the thickness or the length of a stem. 



Directly and indirectly, light seems to exert a stimulative effect 

 upon the morphogenic processes leading to the differentiation of 

 the tissues. If an etiolated shoot is brought into illumination, the 

 portions in which the embryonic tissue has not gone beyond a cer- 

 tain age carry on development approximating the normal. Older 

 etiolated tissues may form chlorophyl or undergo additional thick- 

 ening of the walls, but no other differentiations in form are pos- 

 sible. 



The formation of prosenchymatous cells in the concave side of 

 a geotropically curving stem of Aschpias, in which they were 

 lacking from the opposite convex side, is an excellent example of 

 pure mechanical induction. In normal stems the development 

 must set up, through the tension of the various tissues of the 

 stem, stimuli which would induce the formation of prosenchyma 

 regardless of the action of light. Now in the etiolated stem the 

 bast-strands seem to be induced only by the bending strains ex- 

 erted on the stems by their tendency to fall over, and the stresses 



