262 SUEFACE AND VEINING. [BOOK i. 



In speaking of the surface of a leaf it is customary to make 

 use of the word pagina. Thus, the upper surface is called 

 pagina superior; the lower surface, pagina inferior. The 

 upper surface is more shining and compact than the under, 

 and less generally clothed with hairs ; its veins are sunken ; 

 while those of the lower surface are usually prominent. The 

 epidermis readily separates from the lower surface, but with 

 difficulty from the upper. There are frequently hairs upon 

 the under surface while the upper is perfectly smooth ; but 

 there are few instances of the upper surface being hairy 

 while the lower is smooth. 



The ramifications of the petiole among the cellular tissue 

 of the leaf are called veins, and the manner of their distribu- 

 tion is termed venation. This influences in a great degree the 

 figure and general appearance of the foliage.* The vein 

 which forms a continuation of the petiole and the axis of the 

 leaf is called the midrib or costa : from this all the rest diverge, 

 either from its sides or base. If other veins similar to the 

 midrib pass from the base to the apex of a leaf, such veins 

 have been named nerves ; and a leaf with such an arrange- 

 ment of its veins has been called a nerved leaf. In speaking 

 of these parts, a leaf is said to be three, or five, or otherwise 

 nerved, if the so-called nerves all proceed from the very base 

 of the lamina, but it. is called triple, quintuple, &c. nerved, if 

 the nerves all proceed from above the base of the lamina. 

 If the veins diverge from the midrib towards the margin, 

 ramifying as they proceed, such a leaf has been called a 

 venous or reticulated leaf. This is the sense in which these 

 terms were used by Linnaeus; but Link and some others 

 depart from so strict an application of them, calling all the 

 veins of a plant nerves, whatever may be their origin or 

 direction. 



The veins are, however, improperly called nerves, either in 

 all cases, as by Link, or in certain cases only, when they have 



* Dr. Dickie is, however, of a different opinion, and thinks " that it cannot be 

 said that the forms of leaves in flowering plants have any dependence whatever 

 on their venation, since young leaves are lobed, &c., previous to the appearance 

 of the veins. The truth he believes to be, that the quantity of cellular tissue in 

 a leaf determines the development and position of the veins, and not the reverse." 

 (Annals of Natural History, xi. 322.) 



