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, w^e 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 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, 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 cuticle 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 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 general appearance of the foliage, and requires a 
more carefid 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 casta : 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 t<3wards 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 may be their origin or direction. 
Till within a lev/ years the distribution of veins in the leaf 
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