Cucurbita maxima and C. Pejpo. 183 



collenchyma tissue is that in which the modification of tension 

 is first determined. 



Torsion. — We have seen in a previous paragraph, that when 

 the tendril arm becomes straight from the bud and begins its 

 circurnniUations, the tip is turned slightly downward, or toward 

 the rounded side of the arm. Soon after movement has com- 

 menced, it will be observed that the plane which the tip makes 

 with the basal portion of the arm, does not maintain the same 

 relative position, but that it continually changes. Thus the tip 

 may point up, or down, or even sideways. If we follow 'any 

 one of the collenchyma or vibrogen bands, throughout its en- 

 tire length, we shall also find it to assume a variably spiral form, 

 conforming to the direction of the tip. There is thus pro- 

 duced in the tendril, incident to its most rapid elongation, a dis- 

 tinct torsion which frequently turns the tip throngh 180°, and 

 sometimes through 270°. Moreover, it will be found that the 

 petiole of the tendrils develop a similar and strong torsion, as 

 is clearly defined by the longitudinal bands. 



The explanation of this torsion is simple when we remember 

 the character of the tissues composing the tendril, and the very 

 great difference of tension which is developed between them ; 

 while from our previous considerations, it must be clear that, 

 in both tendril arms and petiole, the torsion is but the natural 

 result of excessive growth in the vibrogen, exerting a strong 

 positive tension upon the collenchyma and wood tissues. 



Irritation. — Of the two sides of the tendril arm, that which 

 is uppermost and slightly channeled is the least sensitive, 

 while the power to respond to mechanical influences appears to 

 be centered in the lower portion of the arm where the collen- 

 chyma is most abundant, and this leads us to infer that it is 

 this tissue which is chiefly concerned in such changes. If the 

 vibrogen were the chief factor in such cases, then the change 

 of direction must follow one of three planes, to the right, the 

 left, or upward, and also the effect of irritation would be most 

 conspicuous when the vibrogen was touched. On the other 

 hand, however, since the bending is almost always toward the 

 lower side, at least never lateral, and since the collenchyma is 

 particularly sensitive, we have strong proof pointing to the 

 view that the bending is produced by a full maintenance of 

 tension in the vibrogen, while that of the collenchyma on the 

 side irritated is increased, and the growth of this tissue ceases 

 for the time owing to contraction of the protoplasm from the 

 cell walls, according to the well known influence of irritation 

 upon living protoplasm.* The power of the tendril to respond 

 to irritation, therefore, is correlated to the hygroscopic power 



* Weiss. Allgemeine Botanik, pp. 81 and 82. Darwin, Insectivorous Plants, p. 

 38, &c. Hackel, Quart. Journal Mic. Sci., April, 1869. 



