Page:The Oak.djvu/82

 comes in of deciding whether a continuation occurs at a higher or lower level.

The cells of the cambium, seen in transverse section, are rectangular in shape and arranged in regular radial rows, owing to the regular tangential divisions (Fig. 12, n, m). In longitudinal sections they are found to be like the tracheids in shape and size, so that they stand one behind the other at the same level. Regarding the tangential series in rings, however, they are less regular, because the tangential longitudinal divisions of two cells side by side do not lie in the same tangential plane. This regular radial arrangement would be found in the xylem also, and is so to a certain extent, but it is disturbed by the differences in diameter which the various elements attain later. The fibers are most apt to preserve the regularity, but in many cases growth in length, and the intercalation of oblique septa, disturb it.

In later years the length of the cambial cells increases, and hence the length of the elements in the wood.

The phloëm or bast of the individual bundle is separated from its neighbors by large rays of parenchyma, the cells of which agree with the secondary bast-parenchyma rays. As these pass into the cortex they widen, as they do at the pith (Fig. 12).

The oldest portion of the phloëm—that next the cortex—consists of a group of thick-walled bast fibers with their lumina nearly obliterated; these are long, spindle-shaped fibers much like the fibers of the wood.