12. THE LEAF. 41 



simpler. In many water-plants, the submerged leaves are different 

 from those which float at the surface; thus, in many species of 

 Potamogeton, the submerged leaves are narrow and ribbon-like, 

 whereas the floating leaves are broadly elliptical ; in many aquatic 

 species of Ranunculus, the former are finely divided, whereas the 

 latter have a circular lamina. Again, the submerged leaves of 

 Salvinia are filamentous, whereas the floating leaves are flattened 

 and oval. 



The simultaneous occurrence of two forms of foliage-leaf on a plant 

 is termed Jietcrophylly . 



In certain plants the foliage-leaves assume remarkable forms in 

 connection with their adaptation for catching small animals or for 

 collecting water (e.g. Nepenthes, Cephalotus, Sarracenia, Utricularia, 

 Dischidia, etc.). In these the lamina 

 is metamorphosed into a pitcher or 

 ascidium. The development of the 

 pitcher begins in very much the same 

 way as that of the lamina of a peltate 

 leaf ; but instead of remaining flat, it 

 becomes tubular by continued basal in- 

 tercalary growth (see p. 34). The leaf 

 may, as in Sarracenia and Darlingtonia, 

 be sessile ; or it may be petiolate, as 

 in Cephalotus and Nepenthes : in Ne- 

 penthes (Fig. 28) the petiole is winged 

 for some distance in its lower portion. 

 The lid, when present, is a development 

 of the apical, or sub-apical (Nepenthes), 

 portion of the lamina ; as its first de- 

 velopment it adheres firmly to the rim 

 of the ascidium, from which it eventu- 

 ally separates, except at the point of 

 attachment ; the lid is bilobed. 



2. Leaf -Tendrils (see p. 32) are 

 leaves or parts of leaves which have 

 a somewhat filamentous form, and which possess the property of 

 twisting spirally round foreign bodies, thus fixing the plant (see 

 Part III.). In species of Clematis, Tropseolum, etc., this function 

 is performed by the petiole of the foliage-leaf ; but in the Vetches 

 and Peas there is a division of labour of this kind, that the anterior 

 leaflets of the pinnate leaf are modified into tendrils (Fig. 19 C, 



FIG. 28. Pitchered leaf of Ne- 

 penthes : a organic apex of leaf ; 

 b leaf-base ; pet petiole, winged 

 in its basal portion ; as ascidium ; 

 I its lid; /r fringe of ascidium 

 (reduced). 



