The Elements of Botany, For Beginners and For Schools. Gray Asa
the main stem of Firs and Spruces, unless destroyed by some injury, is carried on in a direct line throughout the whole growth of the tree, by the development year after year of a terminal bud: this forms a single, uninterrupted shaft—an excurrent trunk, which cannot be confounded with the branches that proceed from it. Of such spiry or spire-shaped trees, the Firs or Spruces are characteristic and familiar examples. There are all gradations between the two modes.
Section V. ROOTS.
65. It is a property of stems to produce roots. Stems do not spring from roots in ordinary cases, as is generally thought, but roots from stems. When perennial herbs arise from the ground, as they do at spring-time, they rise from subterranean stems.
66. The Primary Root is a downward growth from the root-end of the caulicle, that is, of the initial stem of the embryo (Fig. 5–7, 81). If it goes on to grow it makes a main or tap-root, as in Fig. 37, etc. Some plants keep this main root throughout their whole life, and send off only small side branches; as in the Carrot and Radish: and in various trees, like the Oak, it takes the lead of the side-branches for several years, unless accidentally injured, as a strong tap-root. But commonly the main root divides off very soon, and is lost in the branches. Multiple primary roots now and then occur, as in the seedling of Pumpkin (Fig. 27), where a cluster is formed even at the first, from the root-end of the caulicle.
Fig. 81. Seedling Maple, of the natural size; the root well supplied with root hairs, here large enough to be seen by the naked eye. 82. Lower end of this root, magnified, the root seen just as root-hairs are beginning to form a little behind the tip.
67. Secondary Roots are those which arise from other parts of the stem. Any part of the stem may produce them, but they most readily come from the nodes. As a general rule they naturally spring, or may be made to spring, from almost any young stem, when placed in favorable circumstances—that is, when placed in the soil, or otherwise supplied with moisture and screened from the light. For the special tendency of the root is to avoid the light, seek moisture, and therefore to bury itself in the soil. Propagation by division, which is so common and so very important in cultivation, depends upon the proclivity of stems to strike root. Stems or branches which remain under ground give out roots as freely as roots themselves give off branches. Stems which creep on the ground most commonly root at the joints; so will most branches when bent to the ground, as in propagation by layering; and propagation by cuttings equally depends upon the tendency of the cut end of a shoot to produce roots. Thus, a piece of a plant which has stem and leaves, either developed or in the bud, may be made to produce roots, and so become an independent plant.
68. Contrast between Stem and Root. Stems are ascending axes; roots are descending axes. Stems grow by the successive development of internodes (13), one after another, each leaf-bearing at its summit (or node); so that it is of the essential nature of a stem to bear leaves. Roots bear no leaves, are not distinguishable into nodes and internodes, but grow on continuously from the lower end. They commonly branch freely, but not from any fixed points nor in definite order.
69. Although roots generally do not give rise to stems, and therefore do not propagate the plant, exceptions are not uncommon. For as stems may produce adventitious buds, so also may roots. The roots of the Sweet Potato among herbs, and of the Osage Orange among trees freely produce adventitious buds, developing into leafy shoots; and so these plants are propagated by root-cuttings. But most growths of subterranean origin which pass for roots are forms of stems, the common Potato for example.
70. Roots of ordinary kinds and uses may be roughly classed into fibrous and fleshy.
71. Fibrous Roots, such as those of Indian Corn (Fig. 70), of most annuals, and of many perennials, serve only for absorption: these are slender or thread-like. Fine roots of this kind, and the fine branches which most roots send out are called Rootlets.
72. The whole surface of a root absorbs moisture from the soil while fresh and new; and the newer roots and rootlets are, the more freely do they imbibe. Accordingly, as long as the plant grows above ground, and expands fresh foliage, from which moisture largely escapes into the air, so long it continues to extend and multiply its roots in the soil beneath, renewing and increasing the fresh surface for absorbing moisture, in proportion to the demand from above. And when growth ceases above ground, and the leaves die and fall, or no longer act, then the roots generally stop growing, and their soft and tender tips harden. From this period, therefore, until growth begins anew the next spring, is the best time for transplanting; especially for trees and shrubs.
73. The absorbing surface of young roots is much increased by the formation, near their tips, of Root-hairs (Fig. 81, 82), which are delicate tubular outgrowths from the surface, through the delicate walls of which moisture is promptly imbibed.
74. Fleshy Roots are those in which the root becomes a storehouse of nourishment. Typical roots of this kind are those of such biennials as the turnip and carrot; in which the food created in the first season's vegetation is accumulated, to be expended the next season in a vigorous growth and a rapid development of flowers, fruit, and seed. By the time the seed is matured the exhausted root dies, and with it the whole plant.
75. Fleshy roots may be single or multiple. The single root of the commoner biennials is the primary root, or tap-root, which begins to thicken in the seedling. Names are given to its shapes, such as
Conical, when it thickens most at the crown, or where it joins the stem, and tapers regularly downwards to a point, as in the Parsnip and Carrot (Fig. 84);
Turnip-shaped or napiform, when greatly thickened above, but abruptly becoming slender below; as the Turnip (Fig. 83); and
Spindle-shaped, or Fusiform, when thickest in the middle and tapering to both ends; as the common Radish (Fig. 85).
76. These examples are of primary roots. It will be seen that turnips, carrots, and the like, are not pure root throughout; for the caulicle, from the lower end of which the root grew, partakes of the thickening, perhaps also some joints of stem above: so the bud-bearing and growing top is stem.
Fig. 86. Sweet-Potato plant forming thickened roots. Some in the middle are just beginning to thicken; one at the left has grown more; one at the right is still larger.
Fig. 87. Fascicled fusiform roots of a Dahlia: a, a, buds on base of stem.
77. A fine example of secondary roots (67), some of which remain fibrous for absorption, while a few thicken and store up food for the next season's growth, is furnished by