Page:The Oak.djvu/80

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62
THE OAK.

ing of vessels and tracheids. They resemble tracheids, but have very few and small, scarcely bordered, oblique, slit-like pits: every stage can be detected between these and true fibers. They must be looked upon as, so to speak, abnormal, because their numbers are small compared with the typical elements among which they occur.

The wood-parenchma consists of vertical groups of short cells, each group having the fusiform shape of a tracheid (Fig. 16, w.p): hence the upper and lower cell of each group has a pointed end. Each group obviously arises from the transverse divisions of a long, prismatic cell, pointed at both ends—a cambium cell. The transverse section is round, and somewhat larger than that of a tracheid, and the walls are somewhat thinner. Where they abut on vessels and tracheids their walls have bordered pits, but where they stand in contact with similar groups, or with parenchyma rays, the pits are simple. During periods of rest they are loaded with starch grains.

The length of the groups—i.e., of the fusiform cells cut up into short cells—varies; the shorter ones have only one transverse division.

The wood-parenchyma is less abundant than the tracheids and fibers, and predominates in the more vascular parts; after two to four or more fibers in a radial row a single parenchyma cell may often be seen, but other arrangements occur. In the parts where fewer vessels occur it is not uncommon to find a series of radial rows of