Branching

Branching, in the widest sense of the term, applies to the production of any lateral structures by any organ of a plant. Unicellular plants, such as yeast, branch by gemmation, each cell being capable of putting out other cells as lateral pouch-like outgrowths, which may either be entirely separated by constriction, or may remain united so as to give rise to a branching chain of cells. Some of the simpler "filamentous" algae branch by producing innovations, one cell of the filament growing out laterally behind its junction with the next cell and outstripping that cell and undergoing cell-division. Such an innovation may become a new plant by the decay of its base of attachment. A similar mode of branching occurs in the far more highly organised stems of mosses. In Characeae (q.v.) the large apical cell divides transversely, each alternately formed half being nodal or internodal respectively. The internodal cell divides parallel with its circumference, so as to form a cortical layer; and the production of both leaves and branches depends upon the outgrowth of certain cells in this cortical layer. Leaves in this group differ from branches mainly in their branching only proceeding to a limited extent. In the axes of the Pteridophyta, or ferns and their allies, and in the leaves in some case, branching is chorisipodial (Greek, chorisis, division; pous, podos, a foot or basis), resulting from the repeated division of a large apical cell by oblique cell-walls, very commonly three in number. In flowering-plants the one large apical cell is replaced by an apical primary meristem (q.v.), or group of small similar cells capable of forming new tissue by repeated divisions. The lateral branches of roots in this group originate endogenously, i.e. beneath the thick cortical tissue; those of leaves, exogenously, or from outer tissues, and basipetally, they being structures of limited growth with their apices formed first; and those of stems, exogenously and mainly acropetally, or from below upwards towards the apex whilst their growing points or apical meristems are alvays protected by overlapping rudiments of leaves, forming a bud (q.v.). In arrangement (caulotaxis or ramification) the primary lateral branches of roots ("secondary roots" of many writers) are acropetal, all of them originating in the pericambium (q.v.) opposite the bundles of wood, which are limited in number, so that these branches occur in a limited number of vertical rows (orthostichies). Subsequently other roots are given off adventitiously, or in no definite order. Stems (as when they are pollarded or otherwise mechanically injured), and less commonly leaves, may also branch adventitiously; but the main branches of the stem of a flowering-plant normally produced, and consequently much of the general outline of the plant, owe their arrangement to that of the leaves. The stems of most monocotyledons, like those of ferns and cycads, are either unbranched or are chorisipodially dichotomous, as in Aloe dichotoma; but others, such as Asparagus and Ruscus, branch freely. In the Coniferae the indefinite growth of the main stem or "leader" forms much more wood than the lateral branches, many of which may die off if the trees are crowded, leaving "knots" in the timber, and the tree, at least when young, acquires a conical outline. The primary branches, though apparently in whorls, are truly at slight different levels. In Pinus short twigs of definite growth bear each two, three, or five needle-leaves, and in the larches similar branchlets bearing tufts of leaves elongate after these leaves have fallen. In the honeysuckle several branches spring from the axil (q.v.) of a single leaf; but as a rule among dicotyledons only one does so, and the various methods of gemmary, or bud-produced, branching in this group are divided into two main types, the racemose and the cymose. Racemose, indefinite, monopodial, or acropetal branching, such as that of conifers, the flower-clusters of the grape-vine, or the wallflowers, cabbages, mignonettes, etc., consists in the continuous growth of a main axis by the partial unfolding of a terminal bud and the successive development of lateral buds from below upwards. If the main axis branches once only, it is simple; if more than once, compound. Cymose, definite, polychasial or centrifugal branching consists in the unfolding of the terminal bud of a stem into a flower or some other early termination to the growth of main axis, which is thus definite, its growth being continued by lateral axes that overt of it, so that "the stem is lost in its branches," and many axes, or chasia, are produced from the centre outwards. Such branching may be multilateral. two (dichasium) or three (trichasium) lateral branches of equal vigour being produced, as in Stellaria, Cerastium, or Datura; or it may be unilateral and sympodial, one branch at each forking being more strongly developed than the other, whilst the primary axis and its successive stronger-growing lateral axes, secondary, tertiary, etc., form a pseudaxis or sympodium. Unilaterally cymose branching may be either cicinnal, where the stronger branch originates first to one side of the direction of the main axis, and then alternately to the other, or bostrychoid, where the stronger branches form a spiral round the main direction or pseudaxis, as in the inflorescence of Hemerocallis. Chorisipodial branching is similarly either polytomous, as in the stem of Marchantia or the stamens of Ricinus, or unilateral and sympodial; and in the latter case it is either cicinnal or scorpioid, as in Selaginella, or bostrychoid or helicoid, as in the fronds of Adiantum pedatum or other pedate leaves, such as those of the Christmas rose (Helleborus niger).