THEIR FORM, DIVISION, ETC. 165 



leaves, the simple veins or nerves arise from a prolongation of the 

 petiole in the form of a thickened midrib, instead of the base of 

 the blade, constituting the curvinerved leaves of De Candolle. 

 This structure is almost universal in the Ginger tribe, the Arrow- 

 root tribe, in the Banana, and other tropical plants ; and our com- 

 mon Pontederia, or Pickerel-weed (Fig. 194), affords an illustra- 

 tion of it, in which the nerves are curved backwards at the base, 

 so as to produce a cordate outline. 



281. As to the margin and particular outline of leaves, they ex- 

 hibit every gradation between the case where the blade is entire, 

 that is, with the margin perfectly continuous and even (as in Fig. 

 201), and those where it is cleft or divided into separate portions. 

 The convenient hypothesis of De Candolle connects these forms 

 with the abundance or scantiness of the parenchyma, compared 

 with the divergence and the extent of the ribs or veins ; on the 

 supposition that, where the former is insufficient completely to fill 

 up the framework, lobes, incisions, or toothings are necessarily 

 produced, extending from the margin towards the centre. Thus, 

 in the white and the yellow species of Water Ranunculus, there 

 appears to be barely sufficient parenchyma to form a thin covering 

 for each vein and its branches (Fig. 207, the lowest leaf) ; such 

 leaves are said to be filiformly dissected, that is, cut into threads ; 

 the nomenclature in all these cases being founded on the conven- 

 ient, but incorrect supposition, that a leaf originally entire is cut 

 into teeth, lobes, divisions, &c. If, while the framework remains 

 the same as in the last instance, the parenchyma be more abun- 

 dantly developed, as in fact happens in the upper leaves of the 

 same species when they grow out of water, and is shown in the 

 same figure, they are merely cleft or lobed. If these lobes grow 

 together nearly to the extremity of the principal veins, the leaf is 

 only toothed, serrated, or crenated ; and if the small remaining 

 notches were filled with parenchyma, the leaf would be entire. 

 The study of the development of leaves, however, proves that the 

 parenchyma grows arid shapes the outlines of the organ in its own 

 way, irrespective of the framework, which is, in fact, adapted to 

 the parenchyma rather than the parenchyma to it. The principal 

 terms which designate the mode and degree of division in simple 

 leaves may now be briefly explained, without further reference to 

 this or any other theory. 



282. A leaf is said to be serrate, when the margin is beset with 



