Transactions Texas Academy of Science. 
95 
On the other hand, it can not be denied that in certain cases, a me- 
chanical force may cause an increase in tensile strength. Worgitzky 12 
found that tendrils which had twined about a support were, in Passillora 
quadrangularis, twice, and in Cucurbita pepo, thirteen times as strong 
as those which had found no support. Nevertheless, he has not defi- 
nitely shown how much of this was due to mechanical tension and how 
much to contact irritation. The latter plays the chief role in the often 
very marked thickening of the petioles of leaf climbing plants. This ap- 
pears, according to von Derschau, to a certain extent even when contact 
irritation is not present and when the weights were fixed to the leaves 
instead of to the petiole direct. However, these results are not altogether 
free from objection, especially as the matter was not fully investigated. 
In those cases where tension and contact irritation were simultaneously 
increased, it is still a matter of doubt how much of the observed effect 
was due to tension and how much to contact. 
However, it can not be doubted that, in general, a similar compli- 
cated correlative interaction exists, as is everywhere the case when a 
gradual development of an organ which stands in the closest mutual 
relationship to the whole occurs. This is especially applicable to the 
development of the stems of fruits which stand in close correlative re- 
lation to the ripening fruit. It is indeed possible that the simple increase 
in weight of an apple or a gourd acts as a stimulant, and in a regulatory 
manner, causes a greater tensile strength in the fruit stem. 
That there are stimuli which can aid in the development of mechan- 
ical tissues will be seen from the results of Wortman 13 and Elfving , 14 
which we have sustained, according to which prevention of geotropic 
curving and forcible bending cause a tolerably well defined development 
of collenchyma, bast fibres, etc. Nevertheless no new tissues were 
formed, but only tissues normally present were strengthened and which 
would of themeslves, have been, in time, fully developed. Since this 
difference was a one-sided one, there results an actual difference between 
the convex and the concave sides which, in the case of Ricinus and 
Phaseolus, continued for from three to four weeks. If this difference 
decreased with increasing age or was finally equalized, it was not investi- 
gated. We have already seen that neither through forcible bending nor 
through prevention of geotropic curving could fibres of hard bast be 
developed in Helleborus niger. 
In those objects which, as the result of the prevention of geotropic 
curving, a one-sided thickening occurred, there was no increase in tensile 
12 Worgitzky. Vergleichende Anatomie der Ranken, Flora, 1887, p. 46. 
13 Wortman 1. c. 
14 Elfding, 1. c. 
