474 BOTANICAL GAZETTE [JUNE 
SUMMARY AND CONCLUSIONS 
These changes in structure under changing conditions which 
were observed upon a number of tendrils in each condition and 
were found constant in each case, almost without exception, have 
but one meaning to the writer. In experiments where tension 
was introduced, the marked increase in thickness of pith walls, 
which was found only when the .factor of tension was present, 
can be explained only by the theory that this thickening is due to 
the longitudinal pull on the tendril, by which the tensile strength 
of the tendril is increased. 
That pith may serve as mechanical tissue is a thing for which 
no evidence has heretofore been offered. DEBARY says (Comp. 
anat., p. 533): 
The only demonstrable change in the pith during the phenomena of 
secondary growth is that it sooner or later, rapidly or slowly, dies off and dries 
up. The possibility of a change in the pith caused directly by the growth in 
thickness is not, indeed, excluded a priori. For ... . the increasing pressure 
. ... exercised on the pith [by the xylem] may lead to anatomical changes 
in the latter. In what cases and in what form such changes may possibly take 
place are questions which have not been investigated, and to the solution of 
which there is scarcely any safe clue; the possibilities will not be discussed here. 
Worcitzky noted a thickening of the walls of the pith in the 
tendrils of Passiflora caerulea, P. triloba, and P. quadrangularis, 
“‘after a support had been securely grasped.”’ He also adds “the 
purpose or cause of this was not found”’; and MacDouGat noted 
that lignification extended to the pith in the basal part of tendrils 
of P. caerulea. 
In view of the results of observations and experiments presented 
in this paper, I maintain that this thickening of the walls of the 
pith cells in Passiflora caerulea is an adaptation, where tension 
acts as an irritation-stimulus, for producing greater tensile strength 
to the tendril where needed. 
In the series of experiments on the contact portion, the great 
increase in xylem below the part in contact, accompanying the 
increased pressure, leads to the conclusion that contact-pressure 
has a marked effect upon the structure of the tendril. That a 
pressure of 20 grams does not cause a decided increase in xylem 
