[Illustration: FIG. 79.--MARKINGS IN CELL WALLS OF WOOD FIBERS. _sp_, spiral; _an_, annular; _sc_, scalariform.] [Illustration: FIG. 80.--PITS IN THE CELL WALL. 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. [Illustration: 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.] [Illustration: 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. 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_. [Illustration: FIG. 83.--COLLENCHYMA IN WILD JEWELWEED OR TOUCH-ME-NOT (IMPATIENS).] [Illustration: FIG. 84.--GRIT CELLS.] =The outer bark= of old shoots consists of _corky_ cells that _protect_ from mechanical injury, and that contain a fatty substance (suberin) impermeable to water and of service to _keep in moisture_. There is sometimes a cork cambium (or phellogen) in the bark that serves to extend the bark and keep it from splitting, thus increasing its power to protect.