Page:The Oak.djvu/130

 mass of wood drives it outwards, on the other. The other differences chiefly concern the individual elements now to be described.

All that was said of the medullary rays in the wood applies also to those in the bast; the cambium in keeping open or originating new medullary rays does so on both sides, and therefore the medullary rays are to be traced radially through the cambium from wood to cortex. The rays in the bast are termed “bast rays”; the broader ones contain isolated groups of sclerotic cells and cells containing crystals.

The changes which the radial rows of cells on the exterior of the cambium zone undergo to form the elements of the secondary phloëm are as follows:

(1) Bast parenchyma (Fig. 17, b p) is developed, like the wood parenchyma, from cambium cells which undergo a few transverse divisions and then pass over as longitudinal groups of cells, which retain their living contents, etc. From these longitudinal groups, accompanying the sieve-tubes as parallel series, they are called companion cells (cambiform cells).

(2) Sieve-tubes (Fig. 18, b p), which may be regarded as homologous with the vessels of the wood, and, like those, are constituted of series of segments. Each segment corresponds to a cambium cell, and is obliquely tapering at the end where it fits on to another segment. These dividing septa are not completely broken through, as in the case of the wood-vessels, however, but are pierced by a grating-like series of holes (the sieve)