Page:The American Cyclopædia (1879) Volume XIII.djvu/594

 578 PLANT and in the axils of these are a greater or less number of axillary buds ; from the lower part of this reduced stem the roots are produced. The similarity between the underground stem of the scaly bulb, with its shortened inter- nodes and fleshy scales, and that of the simi- larly shortened above-ground stem of a house- leek or an echeveria, bearing short fleshy leaves, is very striking. All bulbs are not scaly, some, as in the onion, having the bases of the leaves in concentric layers. The Leaf. This, the remaining organ of vegetation, is described as to its various forms and its func- tions under LEAF. In most of the higher orders of plants the root absorbs nutriment from the soil, and the leaves digest it and prepare it for increasing the growth of the plant, and the stem serves as a medium of communication between the root and the leaves. Some of the many shapes assumed by the root and the stem are here pointed out, and those of the leaf are given elsewhere, at least a sufficient number to show that the wonderful variety presented by plants may be produced by very simple modifications of the root, stem, and leaf ; and it has been suggested (of which fact careful observers will find nu- merous other illustrations) that these organs may perform more than one office, as the leaf may serve both as leaf and tendril, and that they may serve a purpose widely different from their ordinary one, as when roots aid the plant in climbing. Internal Structure. The organs of vegetation have been treated only as to their external forms, except the leaf, which has its internal structure described in another article. (See LEAF.) When the em- bryo is fertilized a new cell is formed; the future growth of the embryo is due to the subdivision and multiplication of that cell; and when growth is resumed in germination, the future plant results from the multiplica- tion of the cells contained in the embryo. The microscope shows the plant to be made up in all its parts of cells, which vary great- ly in size and form, but are all regarded as modifications of the simple cell. The cell when not pressed upon by others is of a more or less spherical form, such as has been described in the simple cellular plants; it is some- times marked by a lining which, being thin in places, gives a dotted appearance ; it is sometimes near- ly filled with a de- posit, as in the gritty cells so common in the fruit of the pear. An aggregation of simple cells forms cellular tissue, like that found in the pulpy portions of the leaf, in young stems, in fruits, and in other soft parts of the plant; a cross section FIG. 4. Cellular Tissue and Intercellular Spaces (magni- fied). of the young stem shows the cellular tissue to be polyhedral, and the cells have their longest diameter in the direction of the growth. The walls exhibited by a cross section of cellular tissue are structurally double, and may often be separated ; and frequently, where the union of contiguous cells is not complete, intercellu- lar spaces are left. The size of the simple cell varies greatly ; in the fruit of the orange fam- ily it is from one fourth to one half of an inch long, but they are ordinarily much smaller, varying from T ^Vs- to ^-g- of an inch. The woody part of a plant shows cells of a different form ; they are many times longer than wide, and are placed length- wise parallel with the axis of the stem ; their smallest diam- eter ^ is from T ^TT to y ^ of an inch, but in some cases much larger; they are taper- ing at the ends, and overlap one another in such a manner as to make the woody tissue they compose very strong ; the term fibro-vascular is also used for this tissue. A cross sec- tion of woody tissue shows that their walls are thick and tough, though with numerous thin spaces ; in old or heart wood, the cells are nearly filled with a woody incrustation ; the wood cells or woody fibres in the pine family are remarkably large, and are marked by con- spicuous dots, which are thin spaces in the cell wall ; these are so charac- teristic that the wood of the pine family, even in the fossil state, may be at once recognized by the mi- croscope. The cells of the inner bark are usually longer, finer, and tougher than those of ordinary FIG. 5. Woody Fibre and Inter- cellular (magnified)". FIGS. 6 and 7. Vessels and Ducts from the Stem of a Melon. 1. Reticulated. 2. Dotted. 3. Annular. 4. Spi- ral Vessels. 5. Dotted Fibre of Clematis (magnified).