II. VEGETABLE CELLS.—In the vegetable, as in the animal kingdom, the primary form of the cell is that of a sphere. There are, however, interfering influences, which usually alter or modify the primary form, of which the most important are, (1.) Special directions assumed in the development, in obedience to a law regulating the structure of the tissue in which the cell occurs; and (2.) Obstructions to.the expansion of the cell iu certain directions from the pressure of surrounding cells.
The most common forms referrable to the law of development are, (1.) The spherical or fundamental form; (2.) The cylindrical, in which there is a tendency to elongation in the direction of a vertical axis; and (3.) The tubular, in which there is an excess of development in the direction of the two transverse axes.
The secondary modifications of these forms are numerous. Thus,. in lax tissues, the spherical form may become an irregular spheroid, running out into lobed, and even stel late forms, as may be seen in the pith of rushes and the stems of various aquatic plants. Again, in seeds, the hard part of fruits, etc., the mutual pressure of the C. converts the spherical into polyhedral forms, of which the dodecahedron—giving a hexagonal section, and arising from equal pressure in all directions—is the most common, although cubic and many other forms occasionally occur.
The magnitude of the vegetable C. is very varied. In flax, the liber-cells have been found 4-, or even g of an inch in length, and the cylindrical C. of some of the confers-re are more than an inch long—although their transverse diameter is very minute—whilst, on the other hand, the spores of fungi are C. of a diameter of of an inch. The average diameter of the 0. in the parenchymatous tissues is about his inch.
Both the cell-wall and the contents differ from the corresponding parts in animal cells. In all young C. the wall is membranous, freely permeable by water, elastic, and flexible. In many cases it retains these properties, whilst in others it becomes much modified, as the cell grows older. It consists mainly of cellulose (q.v.). As the vital and chemical phenomena exhibited by plants depend primarily upon operations in the inte rior of the cell, the careful study of the cell contents is of the highest importance. Of
these contents, the most important are the primordial utricle, with the protoplasm, the nucleus, chlorophyll corpuscles, and starch granules.
The primordial utricle is a layer of substance of mucilaginous consistence (colored yellow by iodine), lining the entire wall of the young cell, but often disappearing at a comparatively early period. The protoplasm is a tough mucilaginous and frequently granular fluid, which fills up the space in the interior of the cell not occupied by the nucleus. The nucleus or cytoblast is a globular or lenticular body, identical in its char acter with the substance of the primordial utricle, and occurring in the protoplasm of most young cells. Little is known with certainty regarding the chlorophyll corpuscles, except that, under the influence of solar light, green coloring matter is developed from them. Of the starch granules, which are very commonly found in the cell contents, we need not speak, as they are sufficiently described in the article STARCH.
In addition to the above organized structures, we must mention as frequent constit uents of the cell-contents, fluid coloring matters, essential and fixed oils, resins, sugar, dextrine, gum, alkaloids, and mineral or organic salts, which are not unfrequently found in a crystalline form, when they are termed raphides.
There are two modes of cell-development in the vegetable kingdom—viz., (1) Cell ditision, where two or more new cells fill the cavity of the parent cell, and adhere to its membranes, appearing to divide it into compartments; and (2) Free cell-formation—not to be confounded with a process of the same name which is supposed to occur in the animal kingdom—in which the whole or part of the cell-contents become detached from the cell-wall and resolved into new loose C., which ultimately escape from the parent cell. The former mode universally occurs in the formation of the C. by which growth is effected; the latter occurs only in the production of C. connected with reproduction. For further information, we must refer the reader to Von Mohl's Principles of the Anat omy and Physiolog±J .(,),f the Vegetable ce '4 translated by ilenfrey, .LOndbn, 1852.