Longitudinal section of wall at b, showing pit borders at o, o.
The cambium, as we have seen, consists of cells whose function is growth. These 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.
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.
Fig. 82.—Thick-walled Bast Cells.
The bark consists of inner or fibrous bark or new bast (these fibres 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 fibres (Fig. [82]) in the bast that serve for support. (3) There is also some parenchyma in the new bast; it is now in part a storage tissue. Sometimes the walls of parenchyma cells in the cortex thicken at the corners and form brace cells (Fig. [83]) (collenchyma) for support; sometimes the whole wall is thickened, forming grit cells or stone cells (Fig. [84]; examples in tough parts of pear, or in stone of fruits). Some parts serve for secretions (milk, rosin, etc.) and are called latex tubes.
Fig. 83.—Collenchyma in Wild Jewelweed or Touch-me-not (Impatiens).