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lary rays, and so forms a complete thin cylinder, concentric to the pith—from which it is separated by the breadth of the xylem—and the cortex, from which it is separated by the breadth of the phloem.

The cells of this cambium cylinder go on dividing continuously during the whole summer, until the cylinder is, say, ten times as thick as it was before. Now suppose it to rest during the winter and go on again next season, and so on during each successive period of growth. Obviously this would realize one fact in the process we are considering—namely, that the stem would grow in thickness year by year, its diameter being increased by twice the thickness of the added cylinder.

But to make the above supposition accord with the facts, we must further picture to ourselves that when the thickening cylinder has attained a certain thickness, a large proportion of those of its cells which lie on the inside—i. e., nearest the pith, and therefore abutting on, lose their cambial nature and the xylem—become converted into elements of the wood; while a smaller proportion of those on the outer side (beneath the phloëm) become new phloem elements. In this way it will be seen that the thin cylinder of active cambium cell travels outwards; ever receding radially farther from the pith, and leaving xylem between itself and the primary Vascular bundles next the pith, and ever driving outwards the primary phloem and cortex, adding new phloëm elements (but in far less proportion) to the inside of the phloëm. Each winter it pauses in this process, and