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division to anotlier. As we meet with more lowly differentiated types, it becomes, in fact, more and more impossible to discuss properly, apart from each other, structure aiul function.
The introductory remarks deal with the distinctions between plants and animals. First and foremost is, of course, their fundamental reUition to the inorganic world. The most philosophical plan is to show how all other differences can be made to How from this one. Too much stress is perliaps laid on the absence of nitrogen from the permanent parts of plants ; they are as incapable as animals of existing without it, and, in- deed, it is obvious that if they were devoid of nitrogen, animals must be so also, as all animal nitrogen is derived ultimately from the plant world. The greater proportion of nitrogen required by animals seems to depend on their greater activity. There is no activity without chemical change in the tissues of the living organism, and as all organic nitrogenous com- pounds are unstable, it is not unreasonable, with Mr. Herbert Spencer, to correlate the mutability of these compounds with the presence of nitrogen. Plants are in the main passive, not active ; hence the presence of any large proportion of nitrogen in them would be useless. As we descend in the animal world, we are not surprised to find indications of plant peculiarities ; hence, in some Molluscoida, we have a substance not very different from cellulose, and fiually, in the Racliolaria, actual starch, indis- tinguishable from that of vegetable origin.*
It seems to be an accepted principle to restrict botanical text-books to recent plants, and to say as little as possible about fossil botany. In the present volume the subject is altogether omitted. This seems a matter for regret, because in the case of any particular group, the great end in view should always be to reach the most generalized conception of its structure, and fossil plants may supply in part or even wholly the key to this. There is a general impression, for example, that acrogenous plants only grow at their summits ; yet the gradual elongation of the leaf-scars and their separation from one another as we proceed from the newer to the older parts of the stem, prove that in the Lepidodeiidrons, as in existing tree-ferns, growth never absolutely ceased in a longitudinal di- rection at any part of the axis ; again, in Lepldudendron, the vascular tissue of the stem was not at first produced to its full extent, but was continually added to from a Cambium layer, an arrangement of which, among allied plants, hoeles is the only existing representative. No ac- count, however, of the little-noticed structure of the corm of this cru-ious plant is to be found in the present volume. The necessity, in any really- general study of morphology, of taking into account both existing and recent types of plants, is well illustrated by the structure of Beuneliitfs, a genus of fossil Cycndere, which has been shown by Mr. Carruthersf to be- long to an entirely new group of the Order, standing to some extent in the same relation to other Cycads that Taxns does to other Coniferce. Besides the possession of a trunk ovoid in outline, the woody cylinder is pierced by large meshes for the passage of the whole of the fibro-vasrular buiulles j)assing to each leaf, — an arrangement which can only find its parfllel in the caudex of tree-ferns In all other Cycads a number of small bundles perforate the woody cylinder separately to pass collectively into each petiole ; and here, it must be observed, that in describing the structure
- See 'Nature,' vol. ii. p. 178.
t Trans. Linu. Soc. vol. xsvi. p. 695.
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