1912] BRUSH—MECHANICAL TISSUE 475 
may be explained if we assume that the pressure exerted on the 
support by the coils of the part in contact which contract after 
grasping the support is equal to 20 grams for the whole area in 
contact. No constant change could be noted in the amount of 
bast present under these varying conditions, except that very 
little could be found in the free tendrils. As the bast plays only 
a comparatively small part in the Passiflora tendril, this tissue 
was not taken into consideration. 
The conclusion from these anatomical studies on Passiflora 
can only bé that contact-pressure causes a greater formation of 
xylem in the tendril, while longitudinal tension causes a thickening 
of the walls of the pith whereby greater tensile strength is secured. 
General conclusions 
To return to the problem of the present paper (as given in the 
introduction), my conclusion in regard to Passiflora caerulea is 
that those tendrils which function to support the plant, that is, 
that are under the influence of contact and tension, possess a greater 
breaking strength than those which have grasped no support 
(see table I). 
The cause of this greatly increased strength, as shown by the 
experiments on the middle third, basal third, and contact portion, 
and a study of sections of the same, is clearly due to a combina- 
tion of the two factors contact and tension, the cells of the xylem 
being increased both in number and in thickness of walls by the 
former stimulus, and the walls of the pith much thickened by 
the latter. Comparing the values of these two factors in the forma- 
tion of mechanical tissue in the Passiflora tendril, I conclude that 
contact plays by far the most important part, though the strength 
of the tendril may be still more increased (even 50 per cent) by 
the additional factor of tension. — 
As to the influence of contact upon the formation of tissues, 
we have had a large number of observations, not only in regard 
to tendrils, but also in regard to plant tissues in general. 
That tension also may act as a stimulus, and that thereby 
stronger tissues are built up in the plant, has been shown by the 
experiments of BoRDNER (3); this is substantiated by the observa- 
