SECT. II 



PHYSIOLOGY 



267 



to nearly vertical supports, although, on account of the manner in which 

 the tendrils coil, they can grasp only slender supports. A few tendril- 

 climbers are even able to attach themselves to smooth walls. Their 

 tendrils are then negatively heliotropic, and provided at their apices with 

 small cushion-like outgrowths, which may either develop independently 

 on the young tendrils {Ampelopsis Veitchii), or are first called forth by 

 contact irritation {Ampelopsis hederacea). Through their sticky ex- 

 cretions these cushions become fastened to the wall and then grow 

 into disc-like suckers, the cells of which 

 come into such close contact with the sup- 

 porting wall that it is easier to break the 

 lignified tendrils than to separate the hold- 

 fasts from the wall. Fig. 209 represents 

 the tendrils of Ampelopsis Veitchii (Vitis in- 

 rondans). The suckers occur on its young 

 tendrils in the form of knots. In Ampe- 

 lopsis hederacea the suckers are only pro- 

 duced as the result of contact, and the 

 tendrils of this plant require thin supports. 

 Sometimes, as in the case of Lopho- 

 spermum scandens (Fig. 210), the leaf-stalks, 

 although bearing normal leaf-blades, be- 

 come irritable to contact stimuli and 

 function as tendrils. Of leaf-stalks which 

 thus act as tendrils, good examples are 

 afforded by Tropaeolum, Maurandia, Solarium 

 jasminoides, Nepenthes, etc. The subsequent 

 modifications occurring in more perfectly 

 developed tendrils are not noticeable in 

 the case of petiolar tendrils, although the 

 coiling portion of the leaf-stalks of Solarium fig. 209.— Portion of a climbing 

 jasminoides do become strongly thickened S^^ff S™ 

 and lignified; while the leaf- blades of have fastened themselves to a 

 Clematis, by remaining small for a time, sm ° oth lra11 h y means of holtl - 

 enhance the tendril-like character of their 



leaf-stalks, and by bending backwards also assist in maintaining the 

 initial contact with a support. At other times the midribs of the leaf- 

 blades themselves become prolonged, and assume the function of 

 tendrils (Gloriosa, Littonia, Flagellaria). In many species of Fumaria 

 and Corydalis, in addition to the leaf-stalks, even the leaf-blades of 

 the leaflets twine around slender supports, while the parasitic shoots 

 of Cuscuta (Fig. 185) are adapted for both twining and climbing. 



F. Curvatures of growth due to Variations in Light and Temperature 

 The flowers and foliage leaves of many plants exhibit the peculiarity 

 that their different sides (the upper and under sides of foliage-leaves and 



