282 BOTANICAL GAZETTE [OCTOBER 
The two influences of tension and compression are thus both 
favorable to increase in the size of the organ subjected to them. 
Then if we accept the view of HEGLER and BorDNER, that tension 
may cause the walls to thicken, we have in an alternation of tension 
and compression all the necessary stimuli for increased size, growth, 
and strength in the part subjected to those stresses. It does not 
seem unreasonable to assume that intermittent or alternating 
compression and tension should have a greater stimulatory effect 
than a constant stress. It is very possible that under constant 
stress the internal equilibrium of stresses may come in time to be 
readjusted so that the external stress would be no longer felt. In 
experimental work this objection may be removed in part by increas- 
ing the stress from time to time. Yet it is easy to see that this 
could not have so much influence as the complete reversing of the 
stress from tension to compression and vice versa. 
Summary : 
In the foregoing work the four woody stems show no increase 
in mechanical strength or in the amount or kind of mechanical 
tissue under the influence of longitudinal compression. 
In young herbaceous stems the development of mechanical 
strength in the tissues is somewhat retarded by a longitudinal ~ 
compression caused by comparatively heavy weights. 
Neither light weights nor heavy weights have any appreciable 
effect upon the growth and strength of herbaceous stems whi 
have already formed a cylinder of mechanical or woody tissue. 
Continuous longitudinal compression causes no marked differ- 
ences in the size or form of any part of the stem which is subjected 
_ to the compression, excepting of course mechanical changes whic 
might be caused by excessive compression. 
The investigation for which these results were obtained was 
carried out under the direction of Professor F. C. NEWCOMBF, 
to whom I wish to acknowledge my sincere thanks for his kindly 
interest and timely suggestions. 
UNIVERSITY OF MICHIGAN 
ANN ARBOR, Mica. 
