MODIFICATIONS OF FORM 



21/ 



in the Grape-Vine ; and probably a like interpretation may be 

 applied to those of the Passion Flower and the Cucurbits. Thus 

 various parts of the shoot, or the whole of one, may in different cases 

 develop as structures called tendrils, and act as prehensile organs. 



Once they are attached, tendrils strengthen their tissues. As 

 growth ceases, the part between the distal attachment and the base 

 is usually thrown into spiral curves ; and as both ends are fixed, these 

 are necessarily equal in number in reverse directions. The elastic 

 tissues of the spirally coiled tendril act like a spring in resisting wind, 

 and recover when the pressure is relieved (Fig. 145). 



Adhesive Cliynbers attach themselves by application of some part 

 of their surface very closely to the surface of the support, following 

 its minute irregularities. The result is that 

 they are affixed so closely that they will 

 often break before quitting hold. Roots 

 require little adaptation to this function. 

 The Ivy is a native type of a number of 

 plants of other lands, often large and 

 woody, which attach themselves in this 

 way to tree-trunks, rocks, etc. Such roots 

 of attachment are " adventitious," that is, 

 they are formed not from the root-system, 

 but at points on the shoot, which are usually 

 determined by the external conditions. 

 The roots sometimes lap round the support, 

 with a prehensile action, as in many of 

 the large Aroids. 



The familiar case of the Virginia Creeper 

 (Ampelopsis) is morphologically identical 

 with its relative the Grape-Vine, but the 

 tendrils are attached by adhesive discs. 

 The tips of the branched tendril move away 

 from the light, and this leads to contact 



,, Climbing shoot of Ampelopsis 



with the support : a rock, wall, or tree- Veitchn. The tendrils (R) have 



..... attached their adhesive discs to the 



trunk. After Contact each tip Widens into wall-surface behind them. (After 



a disc, which at first secretes an adhesive tra6 urger ' 

 cement. This together with its very close application to the inequali- 

 ties of surface gives a firm attachment. Subsequently the tissues 

 harden, and the tendril may assume spiral curves, which give a 

 spring-like resistance (Fig. 146). 



k lis 



■ 



Fig. 146. 



