1910] PENNINGTON—LONGITUDINAL COMPRESSION 271 
tissue. It may be that at some time in the history of a plant it 
was susceptible to mechanical stresses, and that as a result of these 
stresses the plant gradually came to develop normally a certain 
amount of mechanical tissue without regard to the stresses to 
which it might be subjected. Whether this is the case or not isa 
mere matter of speculation, so far as this work is concerned, since 
of course experiments can throw no light upon that phase of the. 
problem. Yet some plants may still be able to respond in a slight 
degree to such stimuli. It was with this possibility in mind that 
such comparatively large numbers of plants were used and careful 
measurements made in order that individual variations might be 
_ obliterated in the average of a large number. 
It is possible and even altogether probable that the importance 
of such differences and variations may not have been given the 
proper amount of attention, or that they may have been overlooked 
entirely. In this work, however, the possibility of an error from 
both specific and individual variation has been eliminated by 
getting the averages of a large number of the same species for 
each experiment. Further discussion of this phase of the question, 
therefore, may be omitted. Heredity, however, does have a very 
important bearing upon the problem in another way. 
The ability to produce certain tissues may be hereditary but 
latent in a plant, and require a certain definite stimulus to make 
it appear. Many plants do not produce leaves unless they feel 
the stimulus of light; others, cacti for example, produce leaves 
only when deprived of light. , The experiments of Von DerscHau 
(35) and Worerrzky (37) show that mere contact may cause the 
production of a certain amount of mechanical tissue, and contact 
and strain together are necessary to cause a tendril to produce its 
normal amount of mechanical tissue. The results of my experi- 
ments show that, at least in the plants which were used in this 
work, no latent hereditary activity is brought forth by compression. 
CORRELATION 
Under this head may be considered all those observations upon 
the mechanical tissue in fruit-bearing stems and branches. DEN- 
NERT (6), REICHE (29), and Preters (28) found in the great num- 
