DIV. ii PHYSIOLOGY 355 



rigidity of their apices is maintained only by collenchyma. In this condition they 

 are easily ruptured, and have but little sustaining capacity. As soon, however, as 

 a support is grasped, the coiled-up portion of the tendril thickens and hardens, 

 while the other part lignifies and becomes so strengthened by sclerenchymatous 

 formations that the tendril can finally sustain a strain of many pounds. When 

 the tendrils do not find a support they usually dry up and fall off, but in some 

 cases they first coil themselves into a spiral. 



Tendril -climbers are not, like twining plants, restricted 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 them- 

 selves to smooth walls. Their tendrils are then negatively phototropic, and 



FIG. 288. Lophospermum scandens climbing by means of its tendril-like petioles. 

 (After NOLL.) 



provided at their apices with small cushion-like outgrowths, which may either 

 develop independently on the young tendrils, or are first called forth by contact 

 irritation. These cushions become fastened to the wall by their sticky excretions 

 and then grow into disc-like suckers, the cells of which come into such close 

 contact with the supporting wall that it is easier to break the lignified tendrils 

 than to separate the holdfasts from the wall. Fig. 210 represents the tendrils of 

 Parthenodssus tricuspidata. The suckers occur on its young tendrils in the form 

 of knobs. In other species of Wild Vine the suckers are only produced as the 

 result of contact, and the tendrils of these plants are also able to grasp thin 

 supports. 



Sometimes, as in the case of Lophospermum scandens (Fig. 288), the leaf-stalks, 

 although bearing normal leaf -blades, are irritable to contact stimuli and 

 function as tendrils. Of leaf-stalks which thus act as tendrils, good examples are 

 afforded by Tropaeolum, Maurandia, Soldnum jasminoides, Nepenthes, etc. In 

 other cases 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 stalks of the 



