1 50 ORGAXOGRAPHT. 



always exposed to similar hygrometric conditions. Such leaves 

 Avhen removed from the Avater and exposed to the air, speedily 

 become dry and shrivelled ; this is owino; partly to the absence 

 of an epidermis to control the evaporation of their fluids, and 

 partly to the want of a fibro-vascular skeleton to act as a 

 support to the surrounding parenchyma. 



4. LAMINA OR BLADE. 



We have already seen that the leaf {figs. 254 and 255) may 

 consist of three parts; namely, of a lamina or blade; ^petiole or 

 staJk; and of a vaginal or stipular portion. We miist no\v describe 

 each of those portions, commencing -with the lamina or blade, 

 whicli is generally the most highly developed, and to which the 

 name of leaf is commonly applied. 



Venatiox. — We will first describe the distribution of the 

 veins of the lamina, to which the name of Venation lias been 

 given. The veins have also been called Nerves, and their dis- 

 tribution Nervation ; but the latter are bad terms, indicating, 

 as they do, an analogy between them and the nerves of animals, 

 which is by no means the case ; the former are those, therefore, 

 which are more commonly and properly used. 



In some plants, as Mosses, those living under water, &c., the 

 leaves have no fibi-o-vascular skeleton, and conscqueritly no 

 trne veins, and arc hence said to be veinless. In succulent 

 plants again, the veins are hidden more or less from vieAv, in 

 consequence of the great develo])ment of parenchyma. The 

 leaves of such plants are therefore termed hidden-veined. 



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

 subject to various modifications of arrangement, tlie more im- 

 portant of which need only be mentioned here. AVhen there is 

 but one large central vein, which proceeds from the base to the 

 apex of the lamina, and from which all the other veins proceed, 

 this is called the niidrib or costa {fig. 288). In other leaves 

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

 the base to the apex, they are then termed ribs {fig. 289). 

 The divisions or primary branches of these ribs are commonly 

 called reins, and their smaller ramifications veinlets. 



There are two ])rincipal modes in which the ribs and veins 

 are distributed thronghout tlie lamina. In the first i)lacc, the 

 fibro-vascular bundle as it enters the leaf from the petiole or 

 stem, may be continued as the midrib, or may divide into two 

 or more ribs; from these other branches arc given oft", and from 

 them, in like manner, other smaller ramifications arise, which 

 unite with one another, so as to form a kind of network, as in 

 figs. 288 and 289 : or, secondly, the fil)ro-vascular bundle may be 

 continued from the base to the ai)ex of the leaf, and give ort" from 

 its sides other veins, which run jiarallel to the margins, and 



