706. The epidermis.—The epidermis proper consists of a single layer of external cells originating from the outer layer of parenchyma cells at the growing apex of the stem or root. These cells undergo various modifications of form. In many cases they lose their protoplasmic contents. In many cases the outer wall becomes thickened, especially in plants growing in dry situations or where they are exposed to drying conditions. The epidermal cells generally become considerably flattened, and are usually covered with a more or less well developed waterproof cuticle, a continuous layer over the epidermis. In many plants the cuticle is covered with a waxy exudation in the form of a thin layer, or of rounded grains, or slender rods, or grains and needles in several layers. These waxy coverings are sometimes spoken of as “bloom” on leaves and fruit.
707. Trichomes.—Trichome is a general term including various hair-like outgrowths from the epidermis, as well as scales, prickles, etc. These include root hairs, rhizoids, simple or branched hairs, glandular hairs, glandular scales, etc. Glandular hairs are found on many plants, as tomato, verbena, primula, etc.; glandular scales on the hop; simple-celled hairs on the evening primrose, cabbage, etc.; many-celled hairs on the primrose, pumpkin; branched hairs on the shepherd’s-purse, mullein, etc., stellate hairs on some oak leaves.
For stomates see [Chapter IV].
4. ORIGIN OF THE TISSUES.
708. Meristem tissue.—The various tissues consisting of cells of dissimilar form are derived from young growing tissue known as meristem. Meristem tissue consists of cells nearly alike in form, with thin cell walls and rich in protoplasm. It is situated at the growing regions of the plants. In the higher plants these regions in general are three in number, the stem and root apex, and the cambium cylinder beneath the cortex. Tissues produced from the stem and root apex are called primary, those from the cambium secondary. In most cases the main bulk of the plant is secondary tissue, while in the corn plant it is all primary.
Fig. 417.
Section through growing point of stem, d, dermatogen; p, plerome; periblem between. (After De Bary.)
709. Origin of stem tissues.—Just back of the apical meristem in a longitudinal section of a growing point it can be seen that the cells are undergoing a change in form, and here are organized three formative regions. The outer layer of cells is called dermatogen (skin producer), because later it becomes the epidermis. The central group of elongating cells is the plerome (to fill). This later develops the central cylinder, or stele, as it is called ([fig. 417]). Surrounding the plerome and filling the space between it and the dermatogen is the third formative tissue called the periblem, which later forms the cortex (bark or rind), and consists of parenchyma, collenchyma, sclerenchyma, or cork, etc., as the case may be. It should be understood that all these different forms and kinds of cells have been derived from meristem by gradual change. In the mature stems, therefore, there are three distinct regions, the central cylinder or stele, the cortex, and the epidermis.
