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 plaii 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 

 easy to explain. 



No one knows why plants with two coty- 

 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 

 leaves the gaps between the adjacent portions extend all the 

 way down to the petiole (in palmately veined leaves) or to 



Fig. 74. — Parallel 

 Veining in Canna. 

 Veins running 

 from midrib to 

 margin. 



