=The Several Plant Cells and their Functions.=—In the =wood= there are some parenchyma cells that have thin walls still, but have lost the power of division. They are now _storage cells_. There are also wood fibres which are thick-walled and rigid (h, Fig. 76), and serve to _support_ the =sap-canals= or _wood vessels_ (or tracheids) that are formed by the absorption of the end walls of upright rows of cells; the canals pass from the roots to the twigs and even to ribs of the leaves and serve to transport the root water. They are recognized (Fig. 79) by the peculiar thickening of the wall on the inner surface of the tubes, occurring in the form of spirals. Sometimes the whole wall is thickened except in spots called _pits_ (g, Fig. 76). These thin spots (Fig. 80) allow the sap to pass to other cells or to neighbouring vessels. [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 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_. [Illustration: FIG. 83.—COLLENCHYMA IN WILD JEWELWEED OR TOUCH-ME-NOT (IMPATIENS).]