276 BOTANICAL GAZETTE [ocTOBER 
within plaster casts; as they increased in diameter, the pressure 
became greater. It was found that under these conditions the 
cells remained alive and active, yet the walls remained thin. Even 
the walls which had already begun to thicken ceased to become 
thicker as pressure increased. 
My experiments in which very young sunflower stems were 
subjected to heavy weights showed fairly well the retarding effect 
of a longitudinal compression, for the amount and the strength 
of the mechanical tissue was found to be less in the experimental 
than in the control plants. 
THE STIMULATORY EFFECTS OF COMPRESSION 
Very little is to be found in botanical literature upon longr 
tudinal pressure, or compression, as a stimulus, and still less as to 
its effect or lack of effect upon mechanical tissues. In nearly 
every instance in which the stimulatory effect of pressure 1s mel- 
tioned, it is loosely applied or very evidently connected with other 
causes of stimulation. Prerrer states (op. cit., p- 124): 
Various stimulatory influences are exercised by tension, pressure, 
mechanical vibrations or disturbances. 
and 
What he says concerning the influence of compression has already 
been discussed under mechanical effects. : 
In nearly all researches, compression has been associated with 
other stimulatory influences. Those observations in which the 
author attributed the observed effects to pressure, or compression, 
are discussed under this head. 
Har Ic (10) found that trees which were subjected to the action 
of a west wind produced thicker annual rings upon the east side 
than upon the west side. His explanation is best given in his 
own words: : 
Der Druck des Windes iibt einen Reiz auf das Plasma der Cambiumschich, 
aus, welche in zweckentsprechender Weise durch gesteigertes Wachsthum up 
durch Dickwandigkeit der Organe auf diesen Reiz reagirt. 
These observations have not been confirmed by experiment, 
and UrsPRUNG (32) has shown by his observations and experl- 
ments that longitudinal compression cannot account for these 
phenomena. 
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