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Page:Mrs Beeton's Book of Household Management.djvu/908

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consists principally of cells of various shapes according to particular plants. The cells, which are generally very minute, are the elementary organs, and although the cell may vary in form, in its essential nature it is always the same. Most cells are inclosed by a cell-wall, and contain a watery cell-sap, and a mucilaginous semi-fluid substance called protoplasm, composed of different organic constituents; among these nitrogenous or albuminous matter is always present, and in the largest proportion. In some plants the protoplasm is not at first inclosed by cell-walls, but it is sooner or later enclosed in a more or less elastic membrane. No cell can exist in a living state or grow unless it contains protoplasm, which is therefore the basis of all vegetable life. In addition to protoplasm and the watery cell-sap, there exists in the cell various substances and gases in a state of solution, albumins, proteins, etc., and chlorophyll, the green colouring matter in plants, which always occurs combined with protoplasm. The action of chlorophyll in the life of a plant is important, as it breaks up the carbonic acid gas taken in by the plant into its two elements, oxygen and carbon, converting the carbon with the water in the plant into starch, and giving back the oxygen to the air. Light is indispensable for the production of chlorophyll; without light plants become bleached or etiolated, a circumstance utilized by the gardener to produce a blanched appearance on certain vegetables. Those parts of a plant which are not green, as the petals of flowers, owe their colour to the presence of peculiar pigments which give their tint to the blossom.

The forms of the cells are various; they are also subject to various transformations. Sometimes a number of cells are laid end to end, and, by the absorption of the transverse partitions, form a continuous tube, as in the sap vessels of plants, or in muscular and nervous fibre; and when the cells are thus woven together, they are called cellular tissue, which, in the human body, forms a fine net-like membrane, enveloping or connecting most of its structures. In pulpy fruits, the cells may be easily separated, one from the other; and within the cells are smaller cells, commonly known as pulp. Among the cells contents of some plants are beautiful crystals, called raphides. The term is derived from "raphis," a needle, on account of the resemblance of a crystal to a needle. They are composed of the phosphate and oxalate of lime; but there is a great difference of opinion as to their use in the economy of the plant. The differences between the highest form of crystal and the lowest form of organic life known, viz., a simple productive cell, are manifold and striking. In a layer of an onion, a fig, a section of garden rhubarb, in some species of the aloe, in the bark of many trees, and in portions of the cuticle of the medicinal squill, bundles of these needle-shaped crystals are to be found. Some of them are as large as 1-40th of an inch, others are as small, as 1-1000th. They are found in all parts of the plant—in the stem, bark, leaves, stipules, petals, fruit, roots, and even in the pollen, with some few