8CET. 11 



PHYSIOLOGY 



289 



support is enhanced. During this time they remain soft and flexible, while the 

 turgor rigidity of their apices is maintained only by collenchyma. In this con- 

 dition 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. 



The tendrils of many plants (Cobaea, Eccremocarpus, Cissus) are irritable and 

 capable of curving on all sides ; others (tendrils of Cucurbitaceae and others with 



FHJ. 233. Portion of a cl 



shoot of A mpdopsis Veitch ii. The 

 tendrils (R) have fastened them- 

 selves to a smooth wall by nn-ans 

 of holdfasts. 



Flo. 232. Portion of a stem of S-icytm angvlutua with 

 tendril ; x, point of reversal in the coiling of the tendril. 



hooked tips) are, according to FITTING, sensitive 

 on all sides but only curve when the under side 

 is touched ; if the upper' surface is at the same 

 time stimulated curvature is arrested. In some 

 cases the tendrils quickly grasp the support 

 (Passiflora, Sicyos, Bryonia) ; while in other 

 tendrils the supports are very slowly grasped 

 (Smilax, Vitis). 



Tendril-climbers are not, like twining plants, restricted to nearly vertical 

 supports, although, on account of the mariner 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 heliotropic, and 

 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. Through their sticky excretions 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 supporting wall that it is easier to break the lignified tendrils 

 than to separate the hold-fasts from the wall. Fig. 233 represents the tendrils of 

 Quiiiaria (Ampelopsis) Veitchii. The suckers occur on its young tendrils in the 

 form of knobs. In Quinaria quinquefolia and Qu. radicantissima the suckers are 



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