CHAP. II. Leaves'. 105 



different species of the same genus or order (provided there 

 is no articulation), it rarely happens that truly compound 

 leaves — that is to say, such as are articulated with their 

 petiole — are found in the same natural assemblage with 

 those in which no articulation exists. Alphonse De Candolle 

 remarks, indeed, and with perfect justice, that in Gleditschia, 

 whose leaves are mostly compound, we find some leaves with 

 their leaflets united, and therefore not articulated with their 

 midrib ; but this is a special case, and can hardly be con- 

 sidered to invalidate a general law. 



In speaking of the sin-Jace 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. Tlie upper surface 

 is more shining and compact than the under, and less generally 

 clothed with liairs ; its veins are sunken ; while those of the 

 lower surface are usually prominent. The cuticle readily 

 separates from the lower surface, but with difficulty from the 

 upper. There are frequently haii's upon the under surface 

 while the upper is perfectly smooth ; but there is scarcely any 

 instance 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 distribution 

 is termed venation. This influences in a great degree the 

 figure and genei'al appearance of the foliage, and requires a 

 more careful consideration than it generally receives in ele- 

 mentary works. 



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 

 arrangement of its veins has been called a nerved leaf. If the 

 veins diverge from the midrib towards the margin, ramifying 

 as they proceed, such a leaf has been called a venous or reticu- 

 lated 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 mav be their orinin or direction. 



Till within a few years the distribution of veins in the leaf 



