94 THE LEAVES OF PLAXTS. 



certain thick fleshy leaves, as those of the aloe, the veins are altogether concealed. 



The first are called veinless, and the last hidden veined. 



The venation of endogeneoug plants offers a wide contrast to the foregoing, sine? 

 there are no reticulations, and the veins run in nearly parallel 

 lines (Fig. 151). Such leaves are found in grasses, in palms, 

 and in many exotics grown in hot-houses. They are termed 

 straight-veined, and their venation consists simply of a series of 

 primary veins running parallel with, and proceeding from, the 

 base of the midrih, and by a transverse arrangement of proper 

 veinlets. An endogenous plant may therefore be distinguished 

 from an exogenous one by the absence of all reticulation in the 

 venation of its leaf. 



There is a kind of exogenous leaf, which closely resembles 

 the straight- veined or endogenous leaf viz., the ribbed, in 

 which three or more midribs spring from, or near to, the base of 

 the leaf; but it differs in having a reticulation of small veins 



Fig. 151. The venation between the ribs. When the midribs proceed from the base, the 

 of an endogeneous ... . ... , r , ^ , . 



leaf, showing its leaf is said to be three (or more) ribbed ; but when they originate 

 straight _vdns. a j^tle above the base the distinguishing term triple-ribbed is 



2. The primary veins, given. 



lets ThC proper Vein " There are other arrangements of veins, as the equal-veined of 

 ferns, the netted, the curve-veined, the radiating, and feather- 

 veined ; but they are not of sufficient importance to merit further notice. 



Whatever may be the precise distribution of the veins, they all tend towards 

 the edge or border of the leaf, and do not there terminate, but are reflected back upon 

 themselves, so as to be accurately applied to the under surface of the one now described. 

 So perfectly is this effected that an observer could not detect the double distribution of 

 veining in any leaf attached to the tree, and it is only when the leaf is greatly decayed 

 that the two layers become separate. If such a leaf be handled, so that the veins on 

 the two surface be drawn asunder, a distinct division of the structure will be perceived. 

 This division may be accounted for in two ways : first, that there is such a process as 

 that just described ; secondly, that both sets are formed at the same time, and, from the 

 earliest moment, are connected together by their extremities ; and that as the leaf 

 increases in size both sets of vessels elongate equally at the same moment. It must 

 not be supposed that there is any substance intervening between the two sets of vessels, 

 for it is highly probable that the two sets form but one bundle. 



The importance of clearly establishing the existence of a double set of vessels is, 

 that there is clearly a double current ; one by which the sap is carried to the leaf, and 

 the other removing it from that organ. The former occupies the upper, and the latter 

 the under surface of the leaf. 



We have already intimated that the veins consist of bundles of woody fibre and 

 spiral vessels, with a prolongation of the cellular pith of the stem. 



The cellular structure of the leaf is somewhat peculiar, and is admirably adapted to 

 the lung-like functions of that organ. It is divisible into two portions, a cuticular and 

 a parenchymatous (Fig. 152). We have already fully explained the structure of the 

 cuticle in leaves, and shall only further add, that the cuticular cells vary greatly in 

 size, figure, number of layers, thickness, and hardness ; but that as a rule there are two 

 layers of cuticular cells on each surface of a leaf. The parenchyma of the leaf consists 



