Page:EB1911 - Volume 21.djvu/797

Rh in the Cyanophyceae. These substances can, however, be dissolved out in water, and the green colouring matter of the chloroplast then becomes visible. The chloroplast consists of two parts, a colourless ground substance, and a green colouring matter, which is contained either in the form of fibrils, or in more or less regular spherical masses, in the colourless ground-mass. The chloroplasts increase in number by division, which takes place in higher plants when they have attained a certain size, independent of the division of the cell. In Spirogyra and allied forms the chloroplast grows as the cell grows, and only divides when this divides. The division in all cases takes place by constriction, or by a simultaneous splitting along an equatorial plane. Chloroplasts are very sensitive to light and are capable in some plants of changing their position in the cell under the stimulus of a variation in the intensity of the light rays which fall upon them. In the chromatophores of many Algae and in the Liverwort Anthoceros there are present homogeneous, highly refractive, crystal-like bodies, called pyrenoids or starch-centres, which are composed of proteid substances and surrounded by an envelope of starch-grains. In Spirogyra the pyrenoids are distinctly connected by cytoplasmic strands to the central mass of cytoplasm, which surrounds the nucleus, and according to some observers, they increase exclusively by division, followed by a splitting of the cytoplasmic strands. Those chromatophores which remain colourless, and serve simply as starch-formers in parts of the plant not exposed to the light, are called leucoplasts or amyloplasts. They are composed of a homogeneous proteid substance, and often contain albuminoid or proteid crystals of the same kind as those which form the pyrenoid. If exposed to light they may become converted into chloroplasts. The formation of starch may take place in any part of the leucoplast. When formed inside it, the starch-grains exhibit a concentric stratification; when formed externally in the outer layers, the stratification is excentric, and the hilum occurs on that side farthest removed from the leucoplast. As the starch-grains grow, the leucoplasts gradually disappear.

Chromoplasts are the yellow, orange or red colour-bodies found in some flowers and fruits. They arise either from the leucoplasts or chloroplasts The fundamental substance or stroma is colourless and homogeneous. The colour is due to the presence of xanthophyll, or carotin or both. The colouring matters are not dissolved in the stroma of the chromoplast, but exist as amorphous granules, with or without the presence of a protein crystal, or in the form of fine crystalline needles, frequently curved and sometimes present in large numbers, which are grouped together in various ways in bundles and give the plastids their fusiform or triangular crystalline shape. Such crystalline plastids occur in many fruits and flowers (e.g. Tamus communis, Asparagus, Lonicera, berries of Solaneae, flowers of Cacalia coccinea, Tropaeolum, bracts of Strelitzia, &c.), and in the root of the carrot. In some cases the plastid disappears and the crystalline pigment only is left. In the red variety of Cucurbita pepo these crystals may consist of rods, thin plates, flat ribbons or spirals. Starch grains may often be seen in contact with the pigment crystals. The crystalline form appears to be due entirely to the carotin, which can be artificially crystallized from an alcohol or ether solution. In addition to the plastids, there are found in some plant-cells, e.g. in the epidermal cells of the leaf of species of Vanilla (Wakker), and in the epidermis of different parts of the flower of Funkia, Ornithogalum, &c. (Zimmermann), highly refractive bodies of globular form, elaioplasts, which consist of a granular protein ground-substance containing drops of oil. They are stained deep red in dilute solution of alkanin.

Substances contained in the Protoplasm.—Starch may be found in the chlorophyll bodies in the form of minute granules as the first visible product of the assimilation of carbon dioxide, and it occurs in large quantities as a reserve food material in the cells of various parts of plants. It is highly probable that starch is only produced as the result of the activity of chromatophores, either in connexion with chromoplasts, chloroplasts or leucoplasts. Starch exists, in the majority of cases, in the form of grains, which are composed of stratified layers arranged around a nucleus or hilum. The stratification, which may be concentric or excentric, appears to be due to a difference in density of the various layers. The outer layers are denser than the inner, the density decreasing more or less uniformly from the outside layers to the centre of hilum. The outermost, newly formed layer is composed of a more homogeneous, denser substance than the

inner one, and can be distinguished in all starch-grains that are in process of development. The separate layers of the starch-grain are deposited on it by the activity of the chromatophore, and according to Meyer the grain is always surrounded by a thin layer of the chromatophore which completely separates it from the cytoplasm. The layers appear to be made up of elements which are arranged radially. These are, according to Meyer, acicular crystals, which he calls trichites. The starch grain may thus be regarded as a crystalline structure of the nature of a sphere crystal, as has been suggested by many observers.

Whether the formation of the starch grain is due to a secretion from the plastid (Meyer, 1895) or to a direct transformation of the proteid of the plastid (Timberlake, 1901) has not been definitely established.

Aleurone—Aleurone is a proteid substance which occurs in seeds especially those containing oil, in the form of minute granules or large grains. It may be in the form of an albumen crystal sometimes associated with a more or less spherical body—globoid—composed of a combination of an organic substance with a double phosphate of magnesium and calcium. Albumen crystals are also to be found in the cytoplasm, in leucoplasts and rarely in the nucleus.

Glycogen, a substance related to starch and sugar, is found in the Fungi and Cyanophyceae as a food reserve. It gives a characteristic red-brown reaction with iodine solution. In the yeast cell it accumulates and disappears very rapidly according to the conditions of nutrition and is sometimes so abundant as to fill the cell almost entirely (Errera, 1882, 1895: Wager and Peniston, 1910).

Volutin occurs in the cytoplasm of various Fungi, Bacteria, Cyanophyceae, diatoms, &c., in the form of minute granules which have a characteristic reaction towards methylene blue (Meyer). It appears to have some of the characteristics of nucleic acid, and according to Meyer may be a combination of nucleic acid with an unknown organic base.

Numerous other substances are also found in the cytoplasm, such as tannin, fats and oil, resins, mucilage, caoutchouc, guttapercha, sulphur and calcium oxalate crystals. The cell sap contains various substances in solution such as sugars, inulin, alkaloids, glucosides, organic acids and various inorganic salts. The colours of flowers are due to colouring matters contained in the sap of which the chief is anthocyanin.

Reference must also be made here to the enzymes or unorganized ferments which occur so largely in the cytoplasm. It is probable that most, if not all, the metabolic changes which take place in a cell, such as the transformation of starch, proteids, sugar, cellulose; and the decomposition of numerous other organic substances which would otherwise require a high temperature or powerful reagents is also due to their activity. Their mode of action is similar to that of ordinary mechanical catalytic agents, such as finely divided platinum (see Bayliss, The Nature of Enzyme Action, and J. R. Green, The Soluble Ferments).

The Nucleus.—The nucleus has been demonstrated in all plants with the exception of the Cyanophyceae and Bacteria, and even here structures have been observed which resemble nuclei in some of their characteristics. The nucleus is regarded as a controlling centre of cell-activity, upon which the growth and development of the cell in large measure depends, and as the agent by which the transmission of specific qualities from one generation to another is brought about. If it is absent, the cell loses its power of assimilation and growth, and soon dies. Haberlandt has shown that in plant cells, when any new formation of membrane is to take place in a given spot, the nucleus is found in its immediate vicinity; and Klebs found that only that portion of the protoplasm of a cell which contains the nucleus is capable of forming a cell-wall; whilst Townsend has further shown that if the non-nucleated mass is connected by strands of protoplasm to the nucleated mass, either of the same cell or of a neighbouring cell, it retains the power of forming a cell-membrane.

The Structure of the Nucleus.—In the living condition the resting nucleus appears to consist of a homogeneous ground substance containing a large number of small chromatin granules and one or more large spherical granules—nucleoli—the whole being surrounded by a limiting membrane which separates it from the cytoplasm. When fixed and stained this granular mass is resolved into a more or less distinct granular network which consists of a substance called Linin, only slightly stained by the ordinary nuclear stains, and, embedded in it, a more deeply stainable substance called Chromatin. The nucleolus appears to form a part of the Linin network, but has usually also a strong affinity for nuclear stains. The staining reactions of the various