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1910] PENNINGTON—LONGITUDINAL COMPRESSION 281 
Bicuer (4) also arrived at this conclusion as a result of trying 
to bring his results into harmony with BALt’s (1) conclusions. 
The application of the self-regulatory theory may seem to serve 
merely as a term to cover ourignorance. In reality it throws much 
light upon the whole question of stimulatory reactions, since it 
shows the great number of possibilities. In order to determine the 
influence of one factor, all the others must be taken into consider- 
ation, and their influence either eliminated or equalized by the use 
of both experimental and control plants under exact experimental 
conditions. | 
Considerable investigation has been carried on with respect to 
the various environmental stimuli. Further discussion of them, 
however, is not necessary in this place. In regard to the stimula- 
tory effect of stresses and strains, comparatively few exact data 
have been obtained. ; 
If we consider BorDNER’s results in regard to tension, it seems 
that a little more definite conclusion may be drawn from BUcHER’s, 
as well as from my own experiments. BtcHeErR found that when a 
stem is bent and then placed upon a klinostat the cell walls on the 
convex or tension side become thicker, and on the concave or 
compression side the walls remain thin, while the cells themselves 
become larger. In stems which were held in a horizontal position 
so that they could not bend upward under the stimulation of 
gravity, he observed the same results; the upper or stretched side 
had normal or slightly smaller cells with thickened walls, while 
the lower side had large thin-walled cells. Now BorDNER’s results 
show that, under proper experimental conditions, tension causes 
a strengthening of stems by producing thicker walls; and it is 
shown that compression, when not too great, causes large thin- 
walled cells. It seems reasonable to say that tension and compres- 
sion may be given as the two direct influences for the results 
observed by BiicHER. 
It is evident that the tension and compression do not have 
exactly opposite effects. We have seen that compression causes 
the cells to become larger and thus give greater surface for the 
osmotic pressure. Tension, as shown by the experiments of 
Nott and Hecterr, tends to accelerate growth and cell division. 
