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zones round the axis: these zones are intersected transversely by medullary rays radiating from the central pith. Occasionally, as above hinted, increase of thickness does not take place by means of annual Zones, the wood at whatever age appearing to consist of a single homogeneous zone. Dr. Lindley has taken advantage of this circumstance and brought together most of the families in which it occurs to form his group of Homogens distinguished by the Endogenous strue- ture of their wood. Descending still lower in the scale we come to two groups of cellular plants, the Rhyzanths mushroom like plants, and the Podostemons, sea-weed like plants, agreeing with algæ in almost every thing except their fructification.
The leaves of Dicotyledons are, usually attached to, and separate from the stem by an articulation and are reticulated, that is their veins anastomose and form a net work, but this is not quite absolute as it is wanting in the leaves of most of the Gymnosperms.
The flowers are for the most part quinary in the number of their parts and are generally furnished with both calyx and corolla; but departures from both these rules are frequent: most of the Homogens have ternary, and many fami- lies quaternary flowers, while numbers have no corolla.
The seed is usually enclosed in a pericarp, but here also a striking ex- ception occurs, the whole of the coniferus family, forming Lindley's Gymnosperms having naked ovules and seed, a privation combined with some interesting peculi- arities of the Anatomical structure of the whole plant. The seed itself is either per- fect or imperfect, that is, is furnished with an Embryo having two or more op- posite cotyledons, or is sporulose: imperfectly developed as in Rhyzanths. The Embryo also is perfect or imperfect, with or without albumen. The albumenous ones are intra or extra albumenous, enclosed within the albumen like the yoke within the white of an egg, or placed on the outside of it, as in the case of the curvembryate orders.
From this description, brief and imperfect as it is, we find there are five mo- difications of structure, as regards vegetation, forming so many distinct groups. Ist Exogens as generally understood with the wood in Zones or concentric circles: 2d Homogens, first associated as a distinct group by Dr. Lindley: 3d Gymnogens or coniferæ: 4th Rhyzanths having more the structure of Fungi than perfect plants and 5th Podostemons which seem to have an anatomical structure nearly allied to algæ, but which Mr Griffith has determined, from actual dissection of the seed, to be dicotyledonous. Then as regards the structure of the seed, there are exal- bumenous and two modifications of albumenous Embryos; and a fourth where it is imperfect. The albumen, moreover, greatly varies in quantity, being sometimes very abundant with a minute Embryo, varying thence to a large embryo and very sparing albumen.
