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 chlorophy] 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 Asclepias, 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- 
on the stems by their tendency to fall over, and the stresses 
