Cucurbita maxima and C. Pepo. 187 



While the vibrogen tissue constantly maintains its full tension, 

 the wood cells are becoming continually thicker, therefore 

 more permanent, less capable of extension and hence constantly 

 developing a greater resistance to the general elongation of the 

 organ as a whole. So long as this mutual tension remains con- 

 stant, i. e., so long as the tension of the woody structure is 

 released by continued growth in proportion to the development 

 of the vibrogen, for just such time will the motions of the ten- 

 dril be comparatively regular. With increasing positive 

 tension, however, its normal release will necessarily become 

 more intermittent on account of the modified character of the 

 cells in which it occurs, and the resultant motion of the whole 

 tendril arm must show corresponding irregularity. 



Coiling about a /Support. — The coiling about an object of sup- 

 port with which the tendril comes in contact, results from 

 a combination of causes already discussed. We have al- 

 ready seen that at the period of contact, the tip first of all 

 twines closely about the object, while the basal portion actu- 

 ally passes by, through continuation of the movement, until 

 excess of torsion draws it up into a double spiral. The coiling 

 of the tip is the direct result of irritation as already shown by 

 Sachs and Darwin, and as the latter explains,* it is developed 

 by a shortening of the side in contact with the object, the same 

 change, i. e. condensation of structure through cessation of 

 growth and loss of water, operating here as in the previous 

 cases, and with Darwin, we can hardly agree with Sachsf 

 that the coiling is in any way promoted by accelerated growth in 

 the unirritated side. 



When once the growth in length is arrested, as it appears to 

 be soon after coiling, the maturity of the tissue is the predom- 

 inating feature. Each tendril must be regarded as having a nor- 

 mal period within which all the changes of structure are to be 

 accomplished. This may be, and undoubtedly is, shortened by 

 mechanical influences which are sufficiently long continued, so 

 that the maturity of a young tendril is much more rapid after 

 having grasped a support than would otherwise have been the 

 case. This has already been pointed out by Darwin. On the 

 other hand, it is difficult to conceive that the normal period is 

 at all prolonged for the purpose of enabling the tendril to find 

 a support. Unless a tendril comes in contact with an object 

 before its tissues reach a certain stage of development, it can- 

 not grasp a support at all. This is well shown by the fact that 

 old tendrils which are near the end of their activity, often catch 

 hold but imperfectly, or if they secure a hold, fail to properly 

 perfect their double spiral. 



Free Coiling. — The changes incident to the free coiling of the 



* Climbing Plants, p. 181. fText Book, p. 859. 



