Fascicled Roots and Tap-Roots

Indian corn: such are denominated fascicled roots. To this class belong the grasses and very many herbaceous plants whose rapid growth depends largely upon their abundant roots, Such are the two main forms of primary roots—that is, roots growing directly from the seed ; but of course they are subject to many modifi- cations. We are to consider the beet, turnip, and carrot, as presented to us in cultivation, only as exaggerated forms of tap-roots, due chiefly to ‘Fro. 10,—A tarnip—tap-root, Fro. 11.—A dahlia—thickened fascicled roota, natural habit but largely also to man’s intelligent propagation. A some- what similar exaggeration of fascicled roots is seen in the common dahlia (Fig. 11). This plant starts on its course with a fascicle of roots which, later on, become thickened for some distance below their junction with the stem. As will be seen later, these various modifications of the primary forms of roots have an important bearing upon plant life, and, incidentally, often serve as the storehouses from which are drawn valuable medicinal agents, In addition to the primary roots growing from the seed, nature has en- 6 ELEMENTS OF BOTANY. dowed many plants with the power of putting forth roots as occasion may require from any part of the stem, or even, in some instances, from the leaves, thus giving them a double hold on life: such are denominated secondary or adventitious roots. The common strawberry of our gardens will serve as an illustration of one kind of adventitious roots, This plant produces “runners,” which put forth roots at the point where they touch ground, then a cluster of leaves, and a new plant is formed. The stems of the running blackberry (Rubus Canadensis L.) often root freely also, and form a netting for the feet of the unwary. Very many illustra- tions of plants endowed with this power might be found anywhere about Fra. 12—Common ivy (Hedera Heltz), with adventitious roots, us, but it remains for the skilful gardener to develop it to its fullest ex- tent, as his cuttings of roses, geraniums, and indeed of almost all his rarest and most beautiful blooming plants abundantly testify. Such adventitious roots are, however, true roots, which perform the ordinary functions of such organs. There are other adventitious roots of an entirely different character. The stem of the poison ivy (Rhus Toxicoden- dron L.), when growing beside some object to which it can cling, puts forth roots in innumerable numbers which do not contribute in any de- gree whatever to the nourishment of the plant, but merely afford mechan- ical support. Moreover, these roots appear to be produced in response to THE ROOT. 7 the stimulation of contact with a supporting surface, in the same manner as the adventitious (true) roots of the strawberry runner are called forth by contact with the damp earth. Of this kind of adventitious roots many illustrations might also be adduced ; one more will suffice, the common ivy (Hedera Helix L.) (Fig. 12). Some plants produce a kind of adventitious roots by which they attach themselves to other herbs or shrubs and draw their nourishment from them. The common dodder (Fig. 13) will serve as an illustration. The Fro, 18.—Dodder (Cuscuta), parasitic upon another plant. seeds of this plant germinate in the ground ; the stems grow until some suitable support is reached, then twine about it and put forth a sort of rootlets by means of which nourishment is drawn from the supporting plant. Then direct communication with the earth is cut off by the death of the stem below the adventitious rootlets, and the plant thenceforth lives as a true parasite, The general structure of the root does not differ greatly from that of the stem. In both there is bark, woody tissue, and pith, though the last- named is seldom very distinct in the root. In their modes of growth there is, however, considerable difference. As already shown, the stem branches according to a fixed and regular plan, and, as will be seen later, while still young elongates between the branches. In the root, on the contrary, branches issue irregularly and elongation is limited to the grow- ing extremity. In other words, a root of a given length, once laid in the . 8 ELEMENTS OF BOTANY. soil, never elongates by intercellular growth, but only increases its length by additions to its growing extremity. Its increase in diameter is accom- plished in the same manner as that of the stem. Without entering too minutely into the structure of the growing extremity of the root, it is necessary to ob- serve that this is made up of a mass of cells which mul- tiply rapidly and are constantly building up tissue (Fig. 14) in a forward direction. They are, moreover, endowed with the power of rapid absorptiou, and are, in fact, the innu- merable mouths which feed the growing plant. In order to increase the absorbing surface of the roots, the younger ones are provided with elongated cells, commonly called Pani emia root-hairs. Though of microscopic size, these are pro- ofa root, with root- duced in such infinite numbers as to be of immense ser- hain Magnified. vice, As, however, they are only required during the pe- riod of active growth, they are not evident in autumn after the season’s work is accomplished. FUNCTIONS OF ROOTS. The mechanical functions of the roots in supporting the stem have already been alluded to. We have now to consider their other important offices, As animals feed upon plants, so plants feed upon minerals, And as they are not provided with organs for the mastication of their food, they must necessarily receive it in a state of subdivision suited to their needs and powers of absorption. In this form they find it in the damp earth which their roots penetrate. Every rootlet is a seeker for food and every growing cell is hungry. Water, with gases and mineral salts in solution, is greedily sucked up and carried toward the sunlight for elaboration. Roots are, moreover, endowed, to some extent, with the power of se- lecting the proper nourishment for the plant which they feed, and they will take this or nothing. The most careless farmer has learned that he cannot successfully raise the same crop on a field year after year without fertilizers. The explanation is simple. Suppose corn be planted year after year in the same ground. The roots of each succeeding crop find less and less nourishment, until finally partial or complete starvation re- sults. And this occurs, too, while there may be still plenty of food fit for other plants. Hence the rotation of crops, a principle at the foundation of successful farming, depends upon the selective powers of roots. But all plants are not fixed in the soil and do not draw their nourish- ment from it, Many aquatic plants float in the water; these find their proper food in that element. Others ure parasitic upon the stems or roots STEM AND BRANCHES, 9 of other plants, their roots penetrating the bark and sucking up the juices already elaborated for their needs. To this class belong the beech-drops (Epiphegus Virginiana Barton), parasitic upon the roots of the beech, and American mistletoe (Phoradendron flavescens Nuttall), a woody parasite upon the branches of forest trees, common in the Southern States. Still other plants are mainly nourished by roots which hang in the air. These, called air-plants, are almost exclusively inhabitants of warm, moist regions. Another important office of some roots is to serve as storehouses of nourishment for the future needs of the plant. The great mass of plants are annuals, living but a single season, during which they germinate from the seed, attain their full development, flower, produce fruit, and die. These have no need for a reserve store of nourishment, hence their roots are fibrous and not thickened. But many plants, termed biennials, germinate from the seed in spring, produce a cluster of radical leaves, and develop a very large tap- root during the first season. The next spring, drawing upon the store of nourishment laid up in the root, they send up vigorous flower-stems, pro- duce seed, and die. Many such roots, as the beet, carrot, and turnip, are of great importance as articles of food. Plants which endure for several years, termed perennials, not un- frequently have roots of the same character. These thickened roots in many instances contain the active medicinal principles of the planta, THE STEM AND BRANCHES. ‘We have seen that the stem is the ascending axis of a plant; that it grows upward toward the light at the same time that the root is develop- ing in an opposite direction. We have now to consider more particularly its mode of growth and some of its more common forms. In the case of the bean it was observed that after the seed-leaves came a pair of green leaves (Fig. 5) ; after these another pair, and so on. The points where these leaves appear are termed nodes or joints, and the spaces between them internodes. Now, during the earlier stages of growth the internodes increase both longitudinally and in diameter also by cell-proliferation, so that though two nodes of a growing shoot may, when their leaves first unfold, be quite close together, in the end we find them separated by an interval of perhaps several inches. In this particular, as stated above, the growth of the stem differs greatly from that of the root, Again, it was noted that in the axils of the leaves were buds which nor- mally developed into branches subject to the same laws of growth as the main stem. We have now only to suppose that these axillary buds keep pace with the development of the main stem, and every opposite-leaved 10 ELEMENTS OF BOTANY. plant would assume a regular, symmetrical shape. But practically the vast majority of such plants are unsymmetrical, mainly because the ter- minal bud—that is, the bud terminating the main stem—is s0 much more vigorous than the others, that having the start in the beginning it keeps it and leaves the branches with comparatively little nourishment. In many plants the growth of the terminal bud is so vigorous that the axillary buds never unfold and the stem remains always simple—that is, not branched. But suppose some accident destroys the terminal bud: then the ax- illary buds, especially those nearest the seat of the injury, are quickened into activity, and a plant whose stem is naturally simple becomes branched. Fra. 15.—Solomon’s-seal—a subtorranean stem (rhézome). Leaf parallel-veined. Again, some plants in germination have not one or two seed-leaves only, but a cluster of them ; in these the branches normally assume a ver- ticillate character. This is the rule in the pine family (Conifer). Others still which start with their leaves in pairs, at a later stage produce them alternately ; here the branches are also alternate. And some plants have forking branches, the growing bud ceasing activity at a certain point and a pair of forking branches starting from the axils of the last developed leaves. This brief view of the growth of the stem and branches demonstrates sufficiently that every plant in its development obeys a fixed law of its being. Yet from a few primary forms arises endless diversity ! Many of the forms of stems have received distinctive names, which are made use of in botanical descriptions, as simple, not branched ; erect, growing straight up; ascending, arising obliquely ; twining, climbing STEM AND BRANCHES. 11 by twining about some support; prostrate, lying flat on the ground ; trailing, running along over the surface of the ground or other plants, etc, Thus far we have studied the stem as it ordinarily appears to us above ground, but there are stems of vast importance which are wholly subter- ranean and are commonly spoken of as roots. That of Solomon’s-seal (Fig. 15) will serve as an illustration of one form of such stems. Placed juat beneath the surface in a horizontal position, it appears as a thickened, fleshy, root-like body, bearing numerous rootlets, a terminal scaly bud at its anterior extremity, and sending up a flower-stem, behind which are scars left by the falling away of previous ones, Each year a new joint is added, while commonly the oldest one rots away ; hence the stem is slowly, year by year, creeping forward. Such an underground stem is called a rhizome. Fro. 16.—Hyacinth bulb. Fra, 17.—Vertical section of a hyacinth bulb. Rhizomes present many different forms. They are simple or branched, horizontal or ascending, fleshy, etc., and are perennial. Podophyllum, sanguinaria, and iris are among our active medicinal plants which have stems of this character. In general, rhizomes contain the more active principles of the plants, and in the greatest proportion. As such plants store away a great amount of nourishment in their fleshy stems for the succeeding year’s needs, they commonly make vigorous growth early in spring, and are mostly early bloomers. Another common form of subterranean stem is the bulb; that of the hyacinth (Fig. 16) affording a good illustration. A vertical section (Fig. 17) 12 ELEMENTS OF BOTANY. explains its character perfectly. It is made up of fleshy scales (reduced leaves), arranged in regular order, which gradually take on the form of true leaves toward the centre and enclose the flower-stem. The onion (Fig. 18) has a similar structure. Bulbs also present themselves in diverse forms, as the scaly and fleshy. One form, which is solid and of more or less homogeneous structure, not made up of scales, is termeda corm. That of the Indian turnip (Arisema triphyllum Tor- rey) is of this character. Many of these underground stems multiply by division or offshoots. The hyacinth, for example, forms bulblets in the axils of its scales, which develop into new plants. And many creeping rhi- zomes send up aérial stems from nearly every joint. It should not be forgotten, however, that all such plants grow from the seed originally, in the same manner ag those having only aérial stems. The common potato furnishes a curious example of a plant with aérial and underground stems, both well developed ; for the potato, termed a tuber, is really a thickened portion of a subterranean stem, and each of its so-called eyes a bud capable of developing into a new plant, Stems, like roots, are annual, biennial, or perennial. Naturally all an- nual roots support only annual stems, but all subterranean stems send up annual flowering stems and leaves. Perennial plants are spoken of as herbaceous, suffruticose, or woody, according to whether they have annual stems, those that are partly woody and do not die entirely down to the ground, or those of wood sufficiently vigorous to resist the winter. ‘Woody plants under about twenty feet in height are called shrubs; when of greater height they are known as trees. This distinction is, of course, somewhat arbitrary, and a given specimen may be spoken of as a shrub or small tree. In structure stems are composed of bark, wood, and pith; and the manner in which these three are arranged, with their relations to each other, serve as the basis of the division of flowering plants into two great classes, namely, the exogenous and the endogenous. Exogenous plants have their bark, wood, and pith each distinct, as shown in the cross-section of the stem of an oak (Fig. 19), in which the cen- tral stellate portion is the pith, the external dark zone the bark, and the intermediate part the wood. The proportions of the three vary greatly in different plants, but their relative positions are always the same in exogen- ous stems, uy \ ro. 18.—Vertical section of an onlon, STEM AND BRANCHES, 13 Their structure deserves more attention than we can give in this place, but must receive at least a passing glance. Pith is but an aggregation of thin-walled cells, originally spherical in Fra, 19.—Crom-section of the stem of an oak, Fro, 90.—Cellular tissue (pith). Magnified. shape but become polyhedral by mutual compression (Fig. 20). In other words it is merely cellular tissue, with feeble vitality and short-lived. Though active in the young and growing shoot, it soon becomes inert, and not unfrequently decays long before the plant reaches its term of existence. It is commonly more abundant proportion- ately in herbs and suffruticose plants than in woody perennials. In some rapidly growing woody perennials, how- ever, the young stems have a very large proportion of pith, as seen in the ailan- _ thus (Fig. 21). As it exists in most Fro, 21,—Obliqae €XOGenous stems, it might very properly section of oneyear- be viewed solely as a relic of their in- ‘old stem of allanthus. fancy. During the stage of its active growth, the pith of some plants abounds in mucilaginous principles, that of the young shoots of sassafras being especially marked in this respect, and being considerably used in medicine on this account. Wood also possesses a cellular structure, but the cells are of a different shape from those of the pith, and are differently arranged. They are commonly elongate- cylindrical, tapering at each end, placed side by side, and overlapping at the ends (Fig. 22) in such manner as to form more or less tough, strong fibres. In early | Fro, 22.—Wood-cells. ‘Magnified. youth they have transparent walls, and thus permit the ready ingress and 14 ELEMENTS OF BOTANY. egress of liquids by osmose. Later their walls become thickened by the deposition of cellulose, etc., and lose their transparency. The wood-cells of some plants are of a characteristic form, which may serve, as in the conifers, for the identification of the order. But inspection of a cross-section of almost any exogenous stem will show that the woody tissue is not uniformly solid throughout, but is trav- ersed by many small canals (Fig. 23). In some stems, as that of the grape- vine, these are so large that one can readily draw water through them by suction with the lips. These canals are called ducts or vessels, and are formed from large cells placed end to end, the ceH-walls at their point of contact afterward being absorbed. ‘Fra, 28,—Horfzontal and vertical section of the stem of a maple. Magnified. Nor is thisall. A longitudinal section of many stems shows glistening plates of tissue traversing the wood from the pith toward the bark ; these plates, called medullary rays, are formed, like the pith, of cellular tis- sue, and serve as a means of communication between the pith and the ex- ternal growing surface of the stem (Fig. 24). The medullary rays of oak and sugar-maple are highly developed, forming the so-called satin grain of the wood. Inspection of a cross-section of any exogenous stem of a few years’ growth will show that the wood is made up of concentric rings (Fig. 19). These rings represent annual accessions to the wood previously formed, each one comprising the growth of a year. Each is complete in itself, and, though more or less strongly adherent to the one which it encir- cles, evidently is, at the time of its growth, the only growing part of the stem. Once formed, these rings afterward undergo but slight changes in char- acter, and never any involume. As years pars by they become more dense, and generally more or less deeply colored by the deposition of coloring matters. Hence, after a few years’ growth most exogenous stems present,