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ORGANS OF NUTRITION. 127 



the higher classes of plants the leaf usually contains, in addition 

 to the parenchyma, a framework or skeleton, consisting of wood- 

 cells or liber-cells, or both, and vessels, and all its structures 

 are in direct connexion with similar parts of the fibro-vascular 

 system of the stem. We distinguish therefore, in such leaves, 

 as in the stem, both a parenchymatous and a fibro-vascular 

 system, — the former constituting the soft parts, or the paren- 

 chyma of the leaf; the latter, the hard parts, which act as a 

 mechanical support, and which, by their ramification, form what 

 are called veins or nerves. 



The part of the stem or branch from which a leaf arises is called 

 a node, and the space between two nodes an internode. The 

 portion of the leaf next the stem is termed its base, and the oppo- 

 site extremity the apex. The lines connecting the base and apex 

 of the leaf are called the margins. The leaf is commonly of a 

 flattened nature, and has two surfaces ; but when the parenchyma 

 is greatly developed the leaf becomes thick and fleshy, and is 

 said to be succulent, and, in such cases, it has frequently more than 

 two surfaces. The terms upper and lower are applied to the 

 two surfaces of ordinary leaves, because in by far the greater 

 number of plants, such leaves are placed horizontally, so that one 

 surface is placed upwards, and the other downwards. We shall 

 find however hereafter, that there are certain leaves which are 

 placed vertically, as those of some species of Acacias and Eu- 

 calypti, in which case the margins are turned upwards and 

 downwards instead of the surfaces. The angle formed by the 

 yunion of the upper surface of the leaf with the stem is called 

 V the axil, and everything which arises out of that point is said 

 L^to be axillary; or, if from the stem above, or below the axil, 

 /^ it is extra-axillary ; or, as more generally described when above, 

 )( supra- axillary ; if below, infra-axiUary. 



The leaf varies as regards its duration, and receives different 



(names accordingly. Thus, when it falls soon after its appear- 

 ance, it is said to be fugacious or caducous ; if it lasts through- 

 out the season in which it is developed, it is deciduous or annual ; 

 or if beyond a single season, or until new leaves are developed, 

 So that the stem, is never without leaves, it is persistent, ever- 

 green, or perennial. 



When a leaf separates from the stem, it either does so by 



/decaying upon it, when it is said to be non-art icidated, or by an 

 articulation, in which case it is articidated. The remains of a 

 non-articulated leaf, as they decay upon the stem, are some- 

 \ times called reliquicB or induvice; and the stem is said to be 

 \ induviate. When a leaf separates by an articulation, it leaves a 

 scar or cicatrix. 



The leaf in the highest state of development, consists of three 

 i distinct parts ; namely, of an expanded portion, which is usually 

 J more or less flattened {fig. 247, 1), called the lamina, blade, or 



X 



