70 PENHALLOW ON MECHANISM 
(p. 869) and it certainly appears justified. We must, therefore, regard the collenchyma not 
only as influencing all the movements dependent upon growth, but also as that particular 
tissue which chiefly determines all movement caused by mechanical irritation, a view 
which is well supported by its presence in the tendrils of Vitis, Ampelopsis, Cucurbita, 
Sicyos and other vines, and the relations which it there bears to the movements of those 
tendrils. ; 
The unmodified fundamental tissue, consisting of large, rounded, thin-walled cells 
filled with protoplasm and chlorophyll, is that in which the most rapid, general and 
continuous increase occurs. As the central or pith region early loses its power of 
growth and shrinks away radially, it may be regarded as having no special value in the 
movements, and we must look in this respect entirely to that parenchyma which lies 
without the wood zone. In all of the parenchyma tissue (Plate IV. Fig. 1) at ¢ and a, there 
is found to be the greatest activity ; and this power of extension is so strongly developed, 
that even after the vascular elements have assumed their most lignified condition, and the 
tendril has permanently coiled up, the vibrogen tissue at a a’ will be found to retain its 
activity for some days. We must, therefore, infer, from this that the negative tension, as 
a whole, is developed most strongly in the parenchyma tissue, and particularly in the 
three bands of vibrogen which lie at aa’ a.” 
TorsIon.—Sachs' distinctly states that no torsions occur in Cucurbitaceæ. This is not 
confirmed by our own observations, however, since it has been observed to be a common 
feature of the circumnutations, that distinct torsions constantly occur. This is readily 
determined, not only by the vibrogen bands, but by the changes in the direction of the 
recurved tip. Similar torsion is also readily detected in the petioles of both tendril and 
leaf, and that it bears a most important relation to the circumnutation itself can hardly 
be doubted. So strongly are these torsions developed in the tendril arm, that the tip 
frequently rotates through 180° or 200°. The explanation of this torsion is not difficult, and 
has been given on many previous occasions by various observers. From what has already 
appeared with reference to the various tissues in their mutual relations of position and 
tension, it is clear that torsion must follow as a natural result of excessive elongation in 
the external layers, thereby exerting a positive tension upon those which are internal. 
IRRITATION.—Of the two sides of the tendril arm, that which is uppermost and 
slightly channeled is the least sensitive to contact. This bears a direct relation to the 
distribution of the collenchyma tissue, which we find to be more continuous and strongly 
developed on the lower and sensitive side. That the vibrogen tissue is not concerned in 
changes due to irritation, appears evident from the fact that the flexure never coincides 
with these bands, but is always toward the lower side of the tendril arm, conforming to 
the position of the collenchyma. The conclusion is justifiable, therefore, that the collen- 
chyma tissue is that which is directly concerned in such movements, through its capacity 
for strong variations in the contained water. ° 
A tendril subjected to local irritation for about thirty seconds, develops an abrupt 
curvature at that point within one or two minutes, and the bending continues so Jong as 


‘Text-book, 866. 
