697. Collenchyma.—This is a strengthening tissue often found in the cortex of certain shoots. It also is composed of living cells. The cells are thickened at the angles, as in the tomato and many other herbs ([fig. 414]).
698. Sclerenchyma, or stone-tissue.—This is also a strengthening tissue and consists of cells which do not taper at the ends and the walls are evenly thickened, sometimes so thick that the inside (lumen) of the cell has nearly disappeared. Usually such cells contain no living contents at maturity. Sclerenchyma is very common in the hard parts of nuts, and underneath the epidermis of stems and leaves of many plants, as in the underground stems of the bracken fern, the leaves of pines ([fig. 415]), etc.
Fig. 414.
Transverse section of portion of tomato stem. ep, epidermis; ch chlorophyll-bearing cells; co, collenchyma; cp, parenchyma.
Fig. 415.
Margin of leaf of Pinus pinaster, transverse section, c, cuticularized layer of outer wall of epidermis; i, inner non-cuticularized layer; c´, thickened outer wall of marginal cell; g, i´, hypoderma of elongated sclerenchyma; p, chlorophyll-bearing parenchyma; pr, contracted protoplasmic contents. ×800. (After Sachs.)
Fig. 416.