24 INTRODUCTION. 



Leaves are membranous organs of various forms growing from the stem 



or branches, and situated imme- 

 Fig. 16. diately below the buds. They 



may be considered to consist 

 of an extension of the skin or 

 cuticle of the plant into a flat 

 expanded surface, which is sup- 

 ported by a network of fibres 

 and vessels derived from the 

 medullary sheath. Leaves, ac- 

 cording to arrangement, are 

 alter mate, opposite, or verticil- 

 late, the first distribution being 

 considered the most normal. 

 When leaves arise immediately 

 from the stem they are called 

 sessile, and when the fibres are 

 clustered together for a certain 

 distance before they expand, 

 this is termed -a petiole. The leaf 

 therefore consists of two parts, 



Different ascidia. a Sarracenia. b Nepenthes, e Cephalotus. the Stalk or petiole and the ex- 

 pansion or lamina, blade ordisc. 

 Sometimes, as in the Australian Mimosas, the lamina is wanting, and the petiole 

 is dilated into a kind of nerved leaf called a phyllodium. Other plants, instead 

 of leaves, have ascidia or pitchers, and these also are usually altered petioles. 

 These ascidia are of various forms, and their true functions are not under- 

 stood. 



The projecting lines on the under 

 surface of a leaf are termed veins or 

 nerves, and their distribution venation, 

 and the form of the leaf depends, in a 

 great measure, on the distribution of 

 these veins. The general outline is re- 

 gulated by their division and direction ; 

 but the form of the margin is dependent 

 on the greater or less quantity of pa- 

 renchyma in which the veins are distri- 

 buted ; hence with the same venation, 

 there may be several different forms of 

 leaves. Most leaves are divided into 

 two lateral halves by a prominent vein 

 extending from the base to the summit, 

 called the midrib ; from it the other 

 veins proceed and ramify in all direc- 

 tions. In Exogens these veins form a 

 kind of network, frequently anasto- 

 mosing with each other ; these are re- 

 ticulated leaves. In Endogens the veins 

 or nerves proceed directly from the base 

 to the apex, or diverge regularly from 

 the midrib, are parallel to each other, 

 nora. d Bterctilia planifolia. /'Dichondra. g and have few ramifications ; these are 



parallel veined leaves. 



Fig. 17. 



a Rrauncalni aqaiti 



c Passi- 



