SELBY: STUDIES IN ETIOLATION 75 
throughout the stem are otherwise equal, being simply those arising 
from turgidity. When such a stem is laid in a prostrate position 
and the embryonic cells of the apical portions of the stem produce 
a curvature in making an adjusting response, the stretching tension 
set up in the concave side of the stem acts as a stimulus inducing 
the formation of the mechanical tissue as described. 
The writer wishes to express his great obligations to Dr. D. T. 
MacDougal, Director of the Laboratories of the Garden, for ma- 
terial aid and constant encouragement in the course of this work, 
and to make further acknowledgment of special assistance in the 
interpretation of results. The illustrations are chiefly from draw- 
ings made by A. Mariolle, of the Garden, to whom his best thanks 
are due. 
OHIO AGRICULTURAL EXPERIMENT STATION. 
