118 



BRITISH PLANTS 



these organs, but in the course of its development it 

 became modified as a climbing organ, and gradually 

 assumed the form and functions of a tendril. 



The Morphology of Tendrils (a) Modified Stem or 

 Shoot Structures. These generally occur in the posi- 

 tion of flowering shoots, and so may be regarded as 

 modified floral branches. Their morphological nature 

 may be established from the fact that they arise in 

 the axils of leaves (passion-flower, Fig. 36), or them- 

 selves bear leaves or the vestiges of leaves, or even 



FIG. 36. PASSION-FLOWER. (REDUCED.' 



a, branch- tendril with reversal-point at b ; c, extra-floral nectary ; d, vegeta- 

 tive bud ; e, flower-bud ;/, stipule. 



flowers (vine). They may be branched (vine) or un- 

 branched (passion-flower.) In the Virginia- creeper, Am- 

 pelopsis Veitchii ( Vitis inconstans or Veitchii) , each branch 

 of the tendril bears at its tip a small adhesive pad, which 

 cements the tendril to the wall (Fig. 37). 



(b) Modified Leaves or Parts of Leaves (i.) Leaflets. 

 In peas (Lathyrus) and vetches (Vicia) some of the ter- 

 minal leaflets are modified into tendrils. In the ever- 

 lasting pea all but two leaflets are modified, and the stem 

 becomes winged and green (Fig. 38). In Lathyrus Aphaca 

 every leaflet becomes a tendril, and the work of assimila- 



