LEAVES. 91 
120. Occurrence of Netted Veining and of Parallel Veining. 
—The student has already, in his experiments on germina- 
tion, had an opportunity to observe the difference in mode of 
veining between the leaves of some dicotyledonous plants and 
those of monocotyledonous plants. This difference is general 
throughout these great groups of flowering plants. What is 
the difference ? 
The polycotyledonous pines, spruces, 
and other coniferous trees have leaves 
with but a single vein, or two or three 
parallel ones, but in their case the veining 
could hardly be other than parallel, since 
the needle-like leaves are so narrow that 
no veins of any considerable length could 
exist except in a position lengthwise of 
the leaf. 
The fact that a certain plan of vena- 
tion is found mainly in plants with a 
particular mode of germination, of stem 
structure, and of arrangement of floral 
parts, is but one of the frequent cases in 
botany in which the structures of plants 
are correlated in a way which it is not Veiliing tart: 
easy to explain. Veins running 
No one knows why plants with two coty- om midrib to 
gin. 
ledons should have netted-veined leaves, 
but many such facts as this are familiar to every botanist. 
121. Simple and Compound Leaves. — The leaves so far 
studied are simple leaves, that is, leaves of which the blades 
are more or less entirely united into one piece. But while in 
the elm the margin is cut in only a little way, in some maples 
it is deeply cut in toward the bases of the veins. In some 
ledves the gaps between the adjacent portions extend all the 
way down to the petiole (in palmately veined leaves) or to 
Fig. 74. — Parallel 
