Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/109

] large leaves. At the end of spring these leaves die down, the bases of the lower ones alone remaining, and constituting thin brown scales around the conn (as at li). Meanwhile, the young bud conn (k&quot;) in the axil of the middle leaves grows rapidly at the expense of its parent conn (& ), but it does not attain a great size. In autumn it produces new leaves, which remain small, but from the axil of the two upper the flowering stem rises up and bears flowers ; whilst in the axil of its middle leaves a new bud-corm appears, which will the following autumn produce young leaves, flowering stem, and a new bud-corm, and thus the cycle goes on. The buds or new corms formed from the old conns may be produced either laterally, as in Colchicum autumnale, or terminally, as in Crocus and Gladiolus. The bulb is another form of underground stem or bud. The axis in this case is much shortened, and the internodes are hardly developed. The bases of the leaves rising from the stem are quite close together, and become succulent and enclose the axis. In the Lily the thick and narrow scales are arranged separately in rows, and the bulb is called scaly ; while in the Leek, Onion, Squill, and Tulip the scales are broad, and enclose each other in a concentric manner, the outer ones being thin and membranous, and the bulb is tunicated. In the axils of these fleshy scales new lateral shoots arise, forming new bulbs. The lateral buds or doves sometimes remain attached to the axis, and produce flowering stems, so that apparently the same bulb continues to flower for many years, as in the Hyacinth and Tulip ; at other times the young bulbs are detached, and form separate plants. In the axil of the leaves of Lilium bulbiferum, Dentaria bulbifera, and some other plants, small conical or rounded bodies are produced, called bulbils or bulblcts (fig. 71, 6). They resemble bulbs in their aspect, and con sist of a small number of thickened scales enclosing a growing-point. These scales are frequently united closely to gether, so as to form a solid mass. Bulbils are therefore transformed leaf- buds, which are easily detached, and are capable of producing young plants when placed in favourable circum stances. The scales in bulbs vary in number. In Gagea there is only one scale ; in the Tulip and Fritillaria im- perialis they vary from two to five ; while in Lilies and Hyacinths there are a great number of scales. In the Tulip a bud is formed in the axil of an outer scale, and this gives rise to a new flowering axis, and a new bulb, at the side of which the former bulb is attached in a withered state. Fig. 71. Stem of Bulbiferous Lily (Lilium bulbiferwn), showing bulbils or bulblcts b, produced in the axils of the leaves.

The forms of the stem having been considered, we now proceed to examine its anatomical structure. This structure consists of the elementary tissues combined and arranged in various ways. The arrangement of the fibre-vascular bundles in the mature stem or root is not the same in all plants. But we find that in most plants which have two seed-lobes in the embryo, i.e., Dicotyledons, a characteristic structure is apparent, quite distinct from what is found in the majority of plants in which only one seed-lobe is present in the embryo, i.e., in Monocotyledons; and these, again, have a different structure from that found in, Acotyleclons, or plants with no seed-lobe in their embryo. The three forms of stem here referred to have been usually distin guished as follows : (1.) Exogenous stem, in which the libro-vascular bundles are produced indefinitely in an out ward direction, and the stem increases in diameter by the annual formation of a new layer of woody matter on the outside of the preceding layers. This is the form found in most Dicotyledons, and they have hence been called Exoyens, or Outward-growers. Ordinary trees, such as tho Oak and Ash, furnish instances. (2.) Endogenous stem, in which the fibro- vascular bundles are definite, and are formed towards the centre, which becomes filled up with them in the progress of growth, so that the diameter of the stem increases in a great measure by the new matter pushing out that previously formed. This stem characterizes many Monocotyledons, which have thus been called Endogens, or Inward-growers. Palms supply examples. (3.) Acrogenous stem, in which the bundles of vessels are simultaneously developed, and the additions to the stem take place at the summit by the union of the bases of the leaves. Plants having this kind of stem are called Acrogens, or Summit- growers, and are Acotyledons. Tree-ferns furnish an example. Recent research, however, has shown that these terms cannot always be used as synonymous with Dicotyle don, Monocotyledon, and Acotyledon, as we find amongst Dicotyledons stems where the formation of new fibro-vascu- lar bundles is distinctly endogenous, and again amongst Monocotyledons stems with a provision for exogenous growth, and also amongst Acotyledons examples are not wanting in which a continuous increase in diameter is provided for. We shall consider the structure of the stem in Dicotyle dons, Monocotyledons, and Acotyledons successively.

Fig. 72. [ missing text ] Fig. 73. Young Dicotyledonous stem : tn, pith;/r, fibro-vas cular bundles; cb, connecting band of meristem, by which a cambium ring is f ormed ; r, cortex. (Sachs ) In the young stein of a Dicotyledon the fibro-vascular bundles first appear as a circle of wedge-shaped masses, by which the stem is divided into a central cellular portion, the pith, and a peripheral cortex, the space between the bundles being occupied by cellular tissue con stituting the medullary rays, and uniting the pith and cortex. Each fibro-vascular bundle increases by division of its own cambium cells. If eventually all the cambium cells become permanent tissue then the bundle becomes closed, and all further growth ceases. This is the complete structure of an annual herbaceous dicotyle donous stem, which thus consists of a central cellular pith, a circle of fibro-vascular bundles, a cel lular cortex united with the pith by medullary rays, and outside all an epidermis (fig. 72). In trees and shrubs with permanent woody stems, the young shoots given out Fig. 72. annually have a structure similar Young Dicotyledonous stem, m, .iff -i r -i T ith; fv, wedge-shaped fibro- tO that Of annual herbaceous vascular bundles; ^medullary Stems : but as the shoot grOWS, rays;, cortex. Outside all is, }. -I i -i t he epidermis. (Sachs.) further changes occur by which the diameter is increased, and the stem becomes more dense. After the first year s growth, the cells of the medullary rays, stretching between the fibro-vascular bundles and con tinuous with their cambium cells, be come converted into secondary meri- stem, and then an interfascicularcsLin- bium is formed, which eventually coalesces with the cambium cells of the fibro-vascular bundles, and thus a complete cam bium ring is formed (fig. 73). From this cambium ring new xylem or wood is formed on the inside, whilst new cortex 