ORGANS OF NUTRITION. ' 145 



In those leaves where the veins are well marked, they are 

 subject to various modifications of arrangement, the more im- 

 portant of which need only be mentioned hei'e. Thus, when 

 there is but one large central vein, proceeding from the base to 

 the apex of the lamina, and from which all the other veins pro- 

 ceed, such a vein is called the midrib cr costa {fig. 282) ; when 

 there are three or more large veins, which thus proceed from the 

 base to the apex(^^. 283), or to the margins {fig. 28-4), of the 

 lamina, the separate veins are then termed ribs. The divisions or 

 primary branches of the midrib, or of the separate ribs, are 

 commonly called veins, and their smaller ramifications vcmlcts. 



There are two principal modifications in which the ribs and 

 veins are distributed throughout the lamina. In the first modifi- 

 cation, the fibro-vascular bundle as it enters the lamina is either 

 continued as the midrib, or it divides into two or more ribs ; 

 and from these ribs or main branches other veins are given off; 

 and from them, in like manner, other smaller ramifications or 

 veinlets arise, which unite with one another, so as to form a 

 kind of network {figs. 282 and 284) : or, in the second modifi- 

 cation, the fibro-vascular bundle is either continued from the 

 base to the apex of the lamina, and gives off from its sides other 

 veins, which run parallel to the margins, and which are simply 

 connected by unbranched veinlets {fig. 285, b)-, or it divides at 

 once into several veins or ribs, which proceed from the base to 

 the apex {Hg. 290) or margin {fig. 291) of the blade, more or 

 less parallel to each other, and are in like manner connected only 

 by simple unbranched veinlets {fig. 285, a). The leaves which 

 exhibit the first modification of venation are called reticulated 

 or netted-veined leaves, and occur universally in Exogenous or 

 Dicotyledonous Plants; and those which present the second 

 modification are termed jparallel-vcined leaves, and are cha- 

 racteristic of Endogenous or MonocotyledonoTis Plants. 



These two modifications are also subject to others, some 

 which must be now noticed. 



1. Varieties of Beticulated Venation. 



There are two principal varieties of this kind of venation, 

 namely, the feather-veined ov pinnately-veined, and the radiated- 

 or 'palmately-veined. 



1. Feather-veined or Pinnately-veined. — In this A-ariety the 

 midrib either gives off lateral veins that proceed at once to 

 the margins, and which are connected by numerous branching 

 veinlets, as in the leaves of the Beech, Spanish Chestnut {fig. 

 286), Holly, Oak {fig. 287) ; or the midrib gives off branches 

 from its sides, which proceed at first towards the margins, and 

 then curve towards the apex, terminating finally within the 

 margins, with which they are connected by small veins, as in 

 I, 



