Root

Root, the descending axis of the plant, growing towards the centre of gravity. Its functions are (1) to fix the plant, whether in soil, in water, by weighting it, as in the case of the duckweed, or on another plant, as in that of epiphytes and parasites; (2) to absorb liquid nutriment; and (3) in some cases, chiefly among biennials and perennials, to act as a reservoir of nutriment. True roots only occur in plants above the grade of mosses, these latter.and more lowly-organised plants having merely cellular rhizoids. Roots originate in deep-seated layers of tissue, and are, therefore, termed endogenous. Pushing and corroding their way through the more superficial tissues, they are generally surrounded at their base by a torn sheath, or coleorhiza, whilst their apex is protected by the root-cap, or pileorhiza, of dead cells, which have originated from the division of a special region of the dermatogen, known as the calyptrogen. Increase in length takes place by division of cells behind this cap. The epidermis is delicate, and without stomates. It may be absorbent, but is generally furnished with numerous unicellular root-hairs, which absorb liquid food, with gases dissolved in the same, by the process known as osmose.

Roots may exude acid solvent substances, and are without root-hairs when surrounded by the mycorhiza, or mycelium of certain fungi. Roots develop mostly in those directions in which moisture occurs, and thus drain the land while appearing to search for water; and two kinds of plants growing in the same soil exercise a selective power by which they take in different substances, and thus differ markedly in the proportions of their chemical constituents. Similarly, the same species grown year after year in any soil will impoverish it of certain substances, thus necessitating manure, or a system of rotation of crops, or fallowing, to allow of natural regeneration of the soil. Internally the root consists generally of a stele, or fibro-vascular axis, consisting of alternating bundles of xylem and phloem, both centripetal in their development, surrounded by the pericycle, in which the lateral roots originate, and, in old roots, by a thick impermeable cortex. Roots do not as a rule bear leaves, and, with the exception of the aerial roots of epiphytes, are seldom green. In many cases they will grow equally well under diverse conditions, a young oak, a hyacinth, a willow, or an alder, growing equally well, for instance, in water or in soil, whilst some Himalayan rhododendrons, epiphytes in their moist native climate, in England grow well in soil.

Roots may be divided into three classes - primary, normal, or tap-roots; secondary or lateral; and adventitious. The primary root is developed directly from part of the embryo, and in its earliest stage is known as the radicle. In palms it only persists for four or five years; but in gymnosperms and dicotyledons it is commonly well developed, and, from its tapering form, is known as a tap-root. In nursery-gardens it is usual to cut off the taproots of apple, hawthorn, holly, oak, and other trees, to stimulate the formation of side-roots and facilitate transplanting. The conical root of the carrot and parsnip, thefusifonn or spindle-shaped root of the radish, and the napiform root of the turnip are examples of tap-roots enlarged to serve as food-reservoirs. Lateral, or secondary roots, spring from the primary root opposite the xylem bundles, and are consequently in a limited number of vertical rows (orthostichies) down the tap-root. They sometimes reach a 'great length, those of the ash extending over thirty yards from the stem. They may themselves branch repeatedly, as in the wallflower, until we have the so-called fibrous roots, which superficially resemble clusters of slender unbranched adventitious roots. Secondary roots occasionally bear necklace-like swellings, as in Pelargonium triste, when they are known as monilvform. Adventitious roots are those that are borne in no definite order, springing from any part of the plant, as from the base of the embryo in monocotyledons, from bulbs and other underground stems, the nodes of trailing stems and runners, as in the ground-ivy and the strawberry, the climbing stems of ivy, or the bases of cuttings. They are generally clustered and unbranched, but may be fteshily enlarged, as in the nodulose root of the dropwort, Spiraea Filipendula, the fasciculate roots of Dahlia, and the1 tuherculate roots of our terrestrial orchids.