78 THE LEAF 



illustration. Such parallel veins Linnaeus called nerves, and parallel- 

 veined leaves are still commonly called nerved leaves, while those of 

 the other kind are said to be veined, — terms which it is convenient to 

 use, although these " nerves " and " veins " are all the same thing, and 

 have no likeness to the nerves and little to the veins of animals. 



124. Netted-veined leaves belong, with comparatively few excep- 

 tions, to the dicotyledonous plants ; while parallel-veined or nerved 

 leaves belong in general to the Monocotyledons. So that a mere glance 

 at the leaves generally tells what the structure of the embryo is, and 

 refers the plant to one or the other of these two grand classes. For 

 when plants differ from each other in some one important respect, 

 they usually differ correspondingly in other respects also. 



125. Parallel-veined leaves are of two sorts, — one kind, and the 

 commonest, having the ribs or nerves all running from the base to 

 the point of the leaf, as in the examples already given; while in 

 another kind they run from a midrib to the margin, as in the common 

 Pickerel weed of our ponds, in the Banana, in Calla, and many similar 

 plants of warm climates. 



126. Xetted-veined leaves are also of two sorts, as in the examples 

 already referred to. In one case the veins all rise from a single rib 

 (the midrib), as in Fig. 61. Such leaves are c?i\\ed feather-veined or 

 pinnately veined ; both terms meaning the same thing, namely, that 

 the veins are arranged on the sides of the rib like the plume of a 

 feather on each side of the shaft. 



127. In the other case (as in Fig. 15), the veins branch off from 

 three, five, seven, or nine ribs, which spread from the top of the leaf- 

 stalk, and run through the blade like the toes of a web-footed bird. 

 Hence these are said to be palmatehj or digitately veined, or (since the 

 ribs diverge like rays from a center) radiate-veined. 



128. Since the general outline of leaves accords w^ith the frame- 

 work or skeleton, it is plain that feather-veined leaves will incline to 

 elongated shapes ; while in radiate-veined leaves more rounded forms 

 are to be expected. 



129. The shape of the blade. — Infinite variety is ex- 

 hibited b}^ plants as regards the figure of the blade. Some 

 of the chief influences to which the forms are owing are 

 (1) the character of the natural surroundings, (2) the 

 mode of folding and of growth in the bud, and (3) the 

 advantage of certain shapes in respect to the equal illumi- 

 nation of all the leaves. 



130. Natural surroundings. — As examples of the influ- 

 ence of the natural surroundings, or habitat^ we may take 

 aquatic plants with submerged, and again others with 



