Читать книгу The Elements of Botany, For Beginners and For Schools - Gray Asa - Страница 12

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88. The Stem is the axis of the plant, the part which bears all the other organs. Branches are secondary stems, that is, stems growing out of stems. The stem at the very beginning produces roots, in most plants a single root from the base of the embryo-stem, or caulicle. As this root becomes a descending axis, so the stem, which grows in the opposite direction is called the ascending axis. Rising out of the soil, the stem bears leaves; and leaf-bearing is the particular characteristic of the stem. But there are forms of stems that remain underground, or make a part of their growth there. These do not bear leaves, in the common sense; yet they bear rudiments of leaves, or what answers to leaves, although not in the form of foliage. The so-called stemless or acaulescent plants are those which bear no obvious stem (caulis) above ground, but only flower-stalks, and the like.

89. Stems above ground, through differences in duration, texture, and size, form herbs, shrubs, trees, etc., or in other terms are

Herbaceous, dying down to the ground every year, or after blossoming.

Suffrutescent, slightly woody below, there surviving from year to year.

Suffruticose or Frutescent, when low stems are decidedly woody below, but herbaceous above.

Fruticose or Shrubby, woody, living from year to year, and of considerable size—not, however, more than three or four times the height of a man.

Arborescent, when tree-like in appearance or mode of growth, or approaching a tree in size.

Arboreous, when forming a proper tree-trunk.

90. As to direction taken in growing, stems may, instead of growing upright or erect, be

Diffuse, that is, loosely spreading in all directions.

Declined, when turned or bending over to one side.

Decumbent, reclining on the ground, as if too weak to stand.

Assurgent or Ascending, rising obliquely upwards.

Procumbent or Prostrate, lying flat on the ground from the first.

Creeping or Repent, prostrate on or just beneath the ground, and striking root, as does the White Clover, the Partridge-berry, etc.

Climbing or Scandent, ascending by clinging to other objects for support, whether by tendrils, as do the Pea, Grape-Vine, and Passion-flower and Virginia Creeper (Fig. 92, 93); by their twisting leaf-stalks, as the Virgin's Bower; or by rootlets, like the Ivy, Poison Ivy, and Trumpet Creeper.

Twining or Voluble, when coiling spirally around other stems or supports; like the Morning-Glory (Fig. 90) and the Hop.

Fig. 90. Twining or voluble stem of Morning-Glory.

91. Certain kinds of stems or branches, appropriated to special uses, have received distinct substantive names; such as the following:

92. A Culm, or straw-stem, such as that of Grasses and Sedges.

93. A Caudex is the old name for such a peculiar trunk as a Palm-stem; it is also used for an upright and thick rootstock.

94. A Sucker is a branch rising from stems under ground. Such are produced abundantly by the Rose, Raspberry, and other plants said to multiply "by the root." If we uncover them, we see at once the great difference between these subterranean branches and real roots. They are only creeping branches under ground. Remarking how the upright shoots from these branches become separate plants, simply by the dying off of the connecting under-ground stems, the gardener expedites the result by cutting them through with his spade. That is, he propagates the plant "by division."

95. A Stolon is a branch from above ground, which reclines or becomes prostrate and strikes root (usually from the nodes) wherever it rests on the soil. Thence it may send up a vigorous shoot, which has roots of its own, and becomes an independent plant when the connecting part dies, as it does after a while. The Currant and the Gooseberry naturally multiply in this way, as well as by suckers (which are the same thing, only the connecting part is concealed under ground). Stolons must have suggested the operation of layering by bending down and covering with soil branches which do not naturally make stolons; and after they have taken root, as they almost always will, the gardener cuts through the connecting stem, and so converts a rooting branch into a separate plant.

96. An Offset is a short stolon, or sucker, with a crown of leaves at the end, as in the Houseleek (Fig. 91), which propagates abundantly in this way.

Fig. 91. Houseleek (Sempervivum), with offsets.

97. A Runner, of which the Strawberry presents the most familiar and characteristic example, is a long and slender, tendril-like stolon, or branch from next the ground, destitute of conspicuous leaves. Each runner of the Strawberry, after having grown to its full length, strikes root from the tip, which fixes it to the ground, then forms a bud there, which develops into a tuft of leaves, and so gives rise to a new plant, which sends out new runners to act in the same way. In this manner a single Strawberry plant will spread over a large space, or produce a great number of plants, in the course of the summer, all connected at first by the slender runners; but these die in the following winter, if not before, and leave the plants as so many separate individuals.

98. Tendrils are branches of a very slender sort, like runners, not destined like them for propagation, and therefore always destitute of buds or leaves, being intended only for climbing. Simple tendrils are such as those of Passion-flowers (Fig. 92). Compound or branching tendrils are borne by the Cucumber and Pumpkin, by the Grape-Vine, Virginia Creeper, etc.

Fig. 92. A small Passion-flower (Passiflora sicyoides), showing the tendrils.

99. A tendril commonly grows straight and outstretched until it reaches some neighboring support, such as a stem, when its apex hooks around it to secure a hold; then the whole tendril shortens itself by coiling up spirally, and so draws the shoot of the growing plant nearer to the supporting object. But the tendrils of the Virginia Creeper (Ampelopsis, Fig. 93), as also the shorter ones of the Japanese species, effect the object differently, namely, by expanding the tips of the tendrils into a flat disk, with an adhesive face. This is applied to the supporting object, and it adheres firmly; then a shortening of the tendril and its branches by coiling brings up the growing shoot close to the support. This is an adaptation for climbing mural rocks or walls, or the trunks of trees, to which ordinary tendrils are unable to cling. The Ivy and Poison Ivy attain the same result by means of aerial rootlets (78).

Fig. 93. Piece of the stem of Virginia Creeper, bearing a leaf and a tendril. 94. Tips of a tendril, about the natural size, showing the disks by which they hold fast to walls, etc.

100. Some tendrils are leaves or parts of leaves, as those of the Pea (Fig. 35). The nature of the tendril is known by its position. A tendril from the axil of a leaf, like that of Passion-flowers (Fig. 92) is of course a stem, i.e. a branch. So is one which terminates a stem, as in the Grape-Vine.

101. Spines or Thorns (Fig. 95, 96) are commonly stunted and hardened branches or tips of stems or branches, as are those of Hawthorn, Honey-Locust, etc. In the Pear and Sloe all gradations occur between spines and spine-like (spinescent) branches. Spines may be reduced and indurated leaves; as in the Barberry, where their nature is revealed by their situation, underneath an axillary bud. But prickles, such as those of Blackberry and Roses, are only excrescences of the bark, and not branches.