276 MORPHOLOGY. 



natural science, except in the pure study of motion, least of all in the 

 barren, empirical beginnings of our Botanical efforts. 



b. Structural Condition of the Foliar Organs. 



132. 1. The nascent leaf consists, like all nascent parts of 

 vegetables, of cellular tissue ; determinate cords of cellular tissue 

 are first gradually organised into vascular bundles, and in fact this 

 process proceeds from the vascular bundles of the axis, and advances 

 gradually into the leaf. In many foliar organs, especially the 

 parts of the flower, no vascular bundles are ever formed. The 

 vascular bundles of the leaves are distinguished by the most incon- 

 veniently chosen expressions, nerves or veins (nervi, vena}. In 

 Monocotyledons with undeveloped internodes, the whole of the 

 vascular bundles together (?) of the internode bounded above by 

 the leaf, pass into the leaf. In all other plants, many at least 

 of the vascular bundles entering the leaf are minor twigs of the 

 vascular bundles of the axis ; in the Dicotyledons proceeding exclu- 

 sively, in great part, from the borders of the loop of the vascular 

 bundles of the axis. The course of the vascular bundles in the 

 leaf depends essentially on the form of the latter. In flat leaves, 

 petioles, or vaginal portions, the vascular bundles lie in one plane ; in 

 relatively thick leaves, &c., they lie scattered (Palms) or in a circle 

 (species of^4loe, Mesembryanthemuni). The vascular bundles rarely 

 run separately through the whole leaf (as in the last named) : they 

 mostly anastomose in various ways with each other by lateral 

 branches ; frequently in the petiole, in such a manner that all the 

 vascular bundles entering it unite into a single one, and then 

 separate again in the blade of the leaf. The form of the combina- 

 tions is very varied : in many Monocotyledons the branches are 

 short, going off at right angles ; in others, and in most Dicotyledons, 

 more varied, so that a net with polygonal meshes is formed. 



De Candolle*, in particular, has devoted great pains to tracing up the 

 distribution of vascular bundles in the leaf to certain types, and to the 

 application of these to the division of plants into definite groups. I can- 

 not perceive any regularity in it. The mode of distribution is as manifold 

 as the form itself of the leaf upon which it is dependent, although De 

 Candolle strangely takes the matter in the opposite way. The nearest 

 allied plants often exhibit a different form of leaf, as also wholly different 

 modes of distribution of the vascular bundles, e. g. Alisma natans and 

 Plantago, Funkia and Hemerocattis, Hydrocharis and Vallisneria, Taxus 

 and Salisburia, Dortmanna and Isotoma, Sedum and Bryophyllum, 

 Peireskia and Opuntia, Salicornia and Beta, Dianthus and Lychnis, 

 &c. No general laws, therefore, can be deduced from these facts, 

 although it is right and useful most minutely to investigate and charac- 

 terise the individual groups, families, genera, and species, in this respect 

 as in all others. In many flat leaves we may distinguish one principal 



* Organographie vegetale, vol. i. p. 289, et seq. 



