74 PENHALLOW ON MECHANISM 
As the tendril advances in age, several important structural changes occur. The 
whole hypodermal tissue increases slightly in thickness, and simultaneously the tissue in 
the region of each vibrogen band, and for the full depth of the zone, cl, becomes so modified . 
that the component cells enlarge strongly—chiefly in a tangential direction—while they 
also become much more thin walled, and all traces of collenchymatous thickening dis- 
appear. This causes a strong localization of the collenchyma to the regions between the 
vibrogen bands, where it retains its original character (Fig. 3) without much change 
beyond an increase in the size of the cells. Within the region, pr (Fig. 1), there arises a 
layer of cambium which forms a continuous zone. From this arise bast bundles, one for 
each of the vascular bundles already noted. The former remain quite distinct to the end 
of their growth, and are usually widely separated. From the inner face of the cambium 
tissue, there arise new wood cells, which now become develcped so generally as to render 
the original bundles conjunctive, thus giving rise to a continuous zone of wood which 
continually increases in thickness. 
At a very early period in its growth, each vascular bundle develops from two to three 
vessels and ducts. Ultimately, all the fibrous elements became highly lignified (Fig. 4 8). 
That this condition may be hastened in time, and possibly increased by contact, can 
hardly be doubted; but it is equally true that such changes occur normally where there 
is no contact, e.g., the sections here exhibited were taken from a freely coiled tendril. 
In comparing the tendrils of Vitis with those of Cucurbita, several important struc- 
tural differences become apparent. The much greater number of vibrogen bands in the 
former, and their somewhat regular distribution, at once suggest greater regularity in the 
figure described, as well as a general equality of motion in all directions. Also in Vitis, 
the inferior development of the collenchyma is consistent with, and may serve as a proper 
explanation of, the much lower degree of sensitiveness there manifested. While the 
general changes incident to maturity of parts are the same in any case, it is noteworthy 
that in Vitis there is no distinct zone of bast which fulfills the function of that tissue in 
Cucurbita, and upon the xylem portion of the vascular bundles must depend that resist- 
ance to general elongation which is so essential a factor in circumnutations ; though 
undoubtedly, in this case, unequal growth of opposite sides is of far greater importance 
than unequal tension of component tissues, so that torsion would here be of less value as a 
factor, than in Cucurbita where it is generally more marked. 
During the circumnutations, distinct torsions occur. These are readily determined by 
tracing the course of the vibrogen bands, from which it becomes apparent that the tendril 
is frequently twisted to the extent of one-half revolution upon its own axis. If no object 
is grasped during the active period, the tendril ultimately coils upon itself; but having 
grasped an object, it perfects a double spiral similar to that in Cucurbita. It is also noted 
that tendrils which have coiled freely, do not become so hard and dry as those which have 
secured attachment, from which it would appear probable that contact produces a more or 
less marked effect in accelerating, or, at least, in increasing the maturity and strength of 
parts, a view which gains strong confirmation also from the very marked differences in 
these respects to be found in Ampelopsis.' 
It would thus appear that the general features of cireumnutation in Vitis and Cucur- 

! Darwin, Climbing Plants, 148, 
