Fundamental or Ground Tissue System.—This system constitutes the groundwork of plants, and is the system through which the vasal bundles are distributed. The fundamental tissues are composed largely, though not wholly, of parenchyma, and are chiefly concerned in the metabolic work of plant life.
Ground tissue includes, besides ordinary parenchyma, collenchyma, selerenchymatous parenchyma, fibrous tissue, cork, laticiferous and glandular tissues. To the fundamental system also belongs the chlorophyll cells of leaves, the thin-walled cells of the pith and medullary rays, the cells of the cortex of stems and roots, and most of the soft cellular tissues in all plants.
The lower plants consist almost entirely of fundamental tissue. In the herbaceous forms of the higher plants the ground tissues largely predominate, while in woody plants they are present in much smaller proportion, the vascular tissues being the most abundant. In aquatic plants generally, the fundamental tissues constitute the principal system.
The hypoderma occurs immediately beneath the epidermis, and consists of several layers of cells. A collenchymatous hypoderma is found in the stems and petioles of most herbaceous dicotyls, and frequently occurs next the mid-rib of leaves, where it forms a strengthening tissue. A sclerenchymatous hypoderma occurs either as a continuous layer beneath the epidermis, as in the stems of some ferns, Pteris aquilina, and in leaves of the pine; or it may form numerous isolated strands beneath the epidermis, as in the stems of horsetails and in certain Umbelliferæ.
Fig. 314.
a. Tangential section of Taxus baccata (Yew), showing the woody fibre; b. Vertical section of same, spiral fibres, and ducts; c. Vertical section of Elm, showing ducts and dotted cells.
The endodermis is the innermost layer of the extra-stelar fundamental tissues, and always abuts on the stele or steles. In monocotyls it marks the boundary between the cortex and the central cylinder, and it is sometimes spoken of as the nucleus sheath.
In stems the endodermal cells are usually thin-walled and unlignified, having a suberous thickening band extending round the upper, lower and lateral surfaces, which in cross-section appears as a black dot on the radial wall ([Fig. 314], c.)
According to its position in the stele, the conjunctive tissue is divided into three principal portions, viz., that portion which invests the vasal bundles, the pericycle; that portion which lies between the bundles of the stele, the interfascicular conjunctive tissue; and that which occupies the centre of the stele, the medullary conjunctive tissue. The pericycle, formerly called the pericambium, is the outermost layer of the conjunctive tissue of the stele.