41] 



CHAPTER I. GENERAL PHYSIOLOGY. 



175 



petioles of various plants (e.g. Tropaeolum, Clematis) ; sometimes 

 by the whole phyllopodium (Lygodium) ; in many cases leaves 

 possessing this sensitiveness are modified into leaf-tendrils (see 

 p. 41 ; as in Cucurbitacese, etc.) ; leaves of this kind serve as 

 organs of attachment for climbing. 



Foliage-leaves are sometimes modified into pitchers or ascidia 

 (p. 41) : these serve the purpose in some cases (e.g. Nepenthes) 

 of capturing insects and of digesting and absorbing them: in 

 other cases (e.g. Dischidia) they collect water and organic detritus ; 

 in Dischidia adventitious roots are developed, which lie in the 

 pitchers and absorb water, together with dissolved substances, 

 therefrom. 



Leaf-spines, like stem-thorns, 

 appear to be exclusively protec- 

 tive against the attacks of her- 

 bivorous animals. 



(2) Cataphyllary or Scaly 

 Leaves (p. 42) serve to protect 

 growing-points and young 

 leaves of buds, and in this they 

 are assisted by the secreting- 

 hairs (colleters, p. 101) which 

 they frequently bear : they 

 sometimes serve as depositories 

 of reserve plastic materials (e.g. 

 scales of Onion-bulb). 



(3) Floral Leaves. 



a. Hypsophyllary Leaves. 

 The leaves included under this 

 head are the bracts (and bracteoles) and the perianth-leaves 

 (p. 57). 



When green, the bracts perform the ordinary functions of foliage- 

 leaves ; but when they are collected around a flower (epicalyx) or 

 an inflorescence (e.g. involucre of Compositse, Euphorbia, etc.) they 

 serve to protect the floral organs during their development. When 

 highly-coloured (e.g. in Aracese, Euphorbiacese, Nyctaginacese), 

 they serve to attract insects to visit the otherwise inconspicuous 

 flowers. 



The sepals, like the bracts, are commonly green, and then they 

 perform the ordinary functions of foliage-leaves, and also serve to 

 protect the other floral organs : when petaloid (e.g. many Ranun- 



FIG. 127 (after Darwin). Petiole of Solomon 

 jasminoides clasping a stick. 



