318 



BOTANY 



PAKT I 



support, but the tendrils themselves acquire greater elasticity and are euabled to 

 withstand the injurious effects of a sudden shock. 



Advantageous changes also take place in the anatomical structureof the tendrils 

 after they are fastened to the supports. The young tendrils, during their rapid 

 elongation, which under favourable conditions may amount to 90 per cent of their 

 length, exhibit active nutations, and thus the probability of their tiudiiig a 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 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 liguifies and becomes so strengthened by sclerenchymatous 



foi'mations 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 otf, but in some 

 eases they first coil themselves 

 into a spiral. 



The tendrils of many plants 

 {Cobaen, Eccrenioco.rptis, Cissus) 

 are irritable and capable of curv- 

 ing on all sides ; others (tendrils 

 of Cucurbitaceae and others 

 with hooked tips) are, according 

 to Fitting, sensitive on all sides 

 bat 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 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 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 holdfasts from the wall. Fig. 236 represents the tendrils of 

 Quinaria (AiiqKlopsis) Veitchii. The suckers occur on its young tendrils in the 

 form of knobs. In Quinaria quinqucfolia and Qu. rudicantissima the suckers are 

 only produced as the result of contact, and the tendrils of these plants are able 

 also to grasp tliin supports. 



Sometimes, as in the case oi Lophos-permum scandens (Fig. 237), the leaf-stalks, 

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

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



Fig. 235. — Portion of a stem of Sicyos uugxdaiusyiiUx tendril; 

 X, point of reversal in the coiling of the tendril. 



