Median longitudinal section of the apex of a root of the barley, Hordeum vulgare. k, calyptrogen; d, dermatogen; c, its thickened wall; pr, periblem; pl, plerome; en, endodermis; i, intercellular air-space in process of formation; a, cell row destined to form a vessel; r, exfoliated cells of the root cap. (After Strasburger.)
Fig. 421.
Cross-section of fibrovascular bundle in adventitious root of Ranunculus repens. w, pericycle; g, four radial plates of xylem; alternating with them are groups of phloem. This is a radial bundle. (After De Bary.)
Function of the root cap.—The root cap serves an important function in protecting the delicate meristem or cambium at the tip of the root. These cells are, of course, very thin-walled, and while there is not so much danger that they would be injured from dryness, since the soil is usually moist enough to prevent evaporation, they would be liable to injury from friction with the rough particles of soil. No similar cap is developed on the end of the stem, but the meristem here is protected by the overlapping bud-scales. One of the most striking illustrations of a root cap may be seen in the case of the Pandanus, or screw-pine, often grown in conservatories (see [fig. 447]). On the prop roots which have not yet reached the ground the root caps can readily be seen, since they are so large that they fit over the end of the root like a thimble on the finger.
713.
Descriptive Classification of Tissues.
| Epidermal System.···· | Epidermis. | ||
| Trichomes. | Simple hairs. | ||
| Many-celled hairs. | |||
| Branched hairs, often stellate. | |||
| Clustered, tufted hairs. | |||
| Glandular hairs. | |||
| Root hairs. | |||
| Prickles. | |||
| Guard cells of stomates. | |||
| Fibrovascular System.···· | Xylem (wood). | Spiral vessels. | |
| Pitted vessels. | |||
| Scalariform vessels. | |||
| Annular vessels. | |||
| Tracheides. | |||
| Wood fibers. | |||
| Wood parenchyma. | |||
| Cambium (fascicular). | |||
| Phloem (bast). | Sieve tubes. | ||
| Bast fibers. | |||
| Companion cells. | |||
| Bast parenchyma. | |||
| Fundamental System.···· | Stem and root. | Cortex.···· | Cork. |
| Collenchyma. | |||
| Parenchyma. | |||
| Fibers. | |||
| Milk tissue. | |||
| Pith-ray.·· | Parenchyma. | ||
| Intrafascicular cambium. | |||
| Pith.······ | Parenchyma. | ||
| Sclerenchyma. | |||
| Bundle-sheath. | |||
Endodermis. | |||
| Leaves. | Palisade tissue. | ||
| Spongy parenchyma. | |||
| Reproductive Organs (mainly fundamental). | |||
714. Physiological Classification of Tissues.
Formative Tissue.