Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/94

Hexagonal complete cel lular tissue from the pith of the Elder. It exists in all plants and abounds in fleshy roots, stems, leaves, and in succulent fruits. It constitutes the pith and outer bark of trees, and is very abundant in the centre of the stem of the Aralia (Fatsia) papyrifera, whence Chinese rice-paper is derived by cutting it into thin sheets. By cultiva tion the Turnip, Carrot, Cabbage, and other esculent vegetables acquire much cellular tissue, and become tender and succulent. The cells of the tissue vary much in size. In a cubic inch of a leaf of the Carnation there are said to be upwards of three millions of cells. They are frequently seen li) 1 00 th,-g^ Tr th, and -g-^th of an inch in diameter. In some of the Cucum ber tribe, and in the pith of aquatic plants, cells -^th

and -^th of an inch in diameter occur.

Fig. 2.
[ missing text ]

In young cells the cell- wall is a thin membrane consist- i n g O f cellulose, with some water and a certain amount of incombustible material. It is permeable by water, is slightly extensible and elastic, and is colourless. It dissolves in, sulphuric acid, and upon addition of iodine and sulphuric acid assumes a deep blue colour. By intus susception of nutrient material, i.e., the interposition of new molecules between those pre-existing, the cell-wall increases both in surface-extent and in thickness. The resulting cell- wall is not, however, uniform in its structure, but is composed of lamellie of different refractive power, in which the cellulose is combined alternately with much and with little water. These alternating dense and watery layers, of which one set is concentric with the cell-wall, whilst two other series are vertical or oblique to the surface of the cell- wall, and cut the concentric ones throughout the whole thickness of the wall, under a high power of the micro scope present a series of mutually intersecting lines, and constitute respectively what are termed the stratification and striation of the cell-wall.

Independently of these changes in the structure of the cell-wall, consequent on its increase in surface-extent and thickness, which will be presently noticed, there are other changes of a chemical nature which take place during the growth of the cell, and which so affect its wall as to break it into distinct " shells," which differ both chemically and physically from the original cell-wall. Thus, in the epidermis or outer cellular covering of plants, the outer most portion of the outer wall of the cells becomes converted into an elastic substance, quite impervious to water, which acts as a protective covering. This substance is known as cork or cuticular matter. Another alteration is the conversion of the layers of the cell-wall into woody matter, by a process of lignification, or formation of wood. Or, again, layers of the cell-wall may be converted into mu cilaginous substance, i.e., absorbing water, and becom ing gelatinous, as in the cells of pith of Astragalus Tra- gacantha, which furnishes gum tragacanth, arid the outer cells of the seed of the Common Flax. Lastly, mineral matters may be deposited in the cell-wall, such as lime, silica, &c., so abundantly in some instances as to constitute, after burning, a perfect skeleton of the cell- wall. In all these cases, however, of alteration of layers of the cell-wall, an innermost layer, giving all the reactions of pure cellulose, may be observed. If growth in surface- extent proceeded uniformly over the whole of a cell-wall the resulting structure would be a more or less rounded vesicle ; but at different points portions grow more rapidly than at others, and thus cells, originally oval or spherical, may become cylindrical, conical, &c. The changes con sequent on unequal growth in thickness are, however, much more important, giving rise to altered appearances both on the outside and inside of the cell-wall. The external thick enings are most usually projections in the form of spines, knobs, &c., as in some pollen-grains, and in cilia connected with the reproductive cells of many Algte (tig. 3), or club- shaped hygrometric filaments as in Equisetum (figs. 4, 5) ; whilst the internal ones are more usually ridges annular, spiral, or reticulate (figs. 11 and 12) which may proceed far as almost to obliterate the cavity of the cell-wall.

Fig. 6.

FIGS. 3 and 4. Spore or reproductive cell of Equisetum, Horsetail, with two cla- vatc hygrometric filaments. In fig. 3 the filaments are expanded in a dry state; in 4 they are curled round the spore on the application of moisture. FIG. 5. Cell v, with vibratile filaments or cilia <, from Chrctophora. FIG. 6. Older cells than those rep;.esented in fig. 1. A, cell-wall; j>, protopiasm with nucleus and nucleolus ; .<, vacuolcs in the protoplasm filled with fluid cell- sap. (Sachs.)

The protoplasm, which lines the interior of the cell-wall, and which is the essential living portion of the cell, consists of albuminous substance mixed with water and some in combustible materials, and it also contains some organic compounds. It is a homogeneous, soft, gelatinous substance. As we usually find it in cells it has a granular and turbid appearance. This arises from an admixture of formative matters, to which the name metaplasm has been applied. It is coagulated by heat, and is soluble in a dilute solution of caustic potash ; iodine solution colours it yellow or brown, whilst strong sulphuric acid at first colours it rose-red, sxibsequently dissolving it. Usually, at points in the interior, drops of fluid become differen tiated as vacuoli (fig. 6), which may subsequently coalesce, and thus the protoplasm may be come a sac containing cell-sap ; and if growth of the cell-wall con tinues the protoplasm eventually forms a mere lining of the cell- wall constituting the primordial utricle of Von Mohl. The proto plasm in some cells exhibits pheno mena of movement within the cell- wall of a definite character. Thus in the internodal cells of Characege (fig. 7) a movement of protoplasm round the longest diameter of the cell is seen, and in the hairs of Tracles- cantia (Virginian Spiderwort) a cir culation of protoplasm occurs. These constitute the phenomena of rotation and circulation.

Fig. 7.
[ missing text ]

The nucleus (fig. 1, k} is present in

the cells of all the higher plants. It - - -- is a small rounded differentiated nor- . 5 , , . L , A small portion of ft Chara tion ot the protoplasm, and frequently magnified to show the Intra- contains vacuoles, which are termed nucleoli (fig. 1, M). It may be in the centre of the cell or close to the sides, but it may change its position. Portions of the protoplasm are also differentiated as grains or granules, to which colouring matters are attached ; but cellular circulation. The arrows mark the direction of the course of the proto plasm in the cells. The clear spaces are parts where there

is no movement