cells are thin-walled and filled with protoplasm. During the growing season they are continually adding to the wood within and the bark without; hence the layer moves outward as it deposits the new woody layer within.
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Fig. 81.—Sieve-tubes, s, s;
p shows a top view of a sieve-plate, with a companion cell. c, at the side; o shows sieve-plates in the side of the cell. In s, s the protoplasm is shrunken from the walls by reagents.
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Fig. 82.—Thick-walled Bast Cells.
The bark consists of inner or fibrous bark or new bast (these fibers in flax become linen), the green or middle bark which functions somewhat as the leaves, and the corky or outer bark. The common word "bark" is seen therefore not to represent a homogeneous or simple structure, but rather a collection of several kinds of tissue, all separating from the wood beneath by means of cambium. The new bast contains (1) the sieve-tubes (Fig. 81) which transport the sap containing organic substances, as sugar and proteids, from the leaves to the parts needing it (s, Fig. 76). These tubes have been formed like the wood vessels, but they have sieve-plates to allow the dense organic-laden sap to pass with sufficient readiness for purposes of rapid distribution. (2) There are also thick-walled bast fibers (Fig. 82) in the bast that serve for support. (3) There is also some parenchyma (parent tissue) in the new bast; it is now in part a storage tissue. Some-
