CYTOLOGY sap. It is sometimes differentiated into a clearer outer layer, of hyaloplasm, commonly called the ectoplasm, and an inner granular endoplasm. In some cases it shows, when submitted to a careful examination under the highest powers of the microscope, and especially when treated with reagents of various kinds, traces of a more or less definite structure which is sometimes very complicated. This exhibits itself in the form of a meshwork which consists of a clear homogeneous substance containing numerous minute bodies known as microsomes, the spaces being filled by a more fluid ground-substance. This structure, which is visible both in living cells and in cells treated by reagents, has been interpreted by many observers as representing a network of threads embedded in a homogeneous ground-substance. Biitschli, on the other hand, interprets it as an extremely finely vacuolated foam-structure or emulsion, comparable to that which is observed when small drops of a mixture of finely powdered potash and oil are placed in water, the vacuoles or alveoli being spaces filled with liquid, the more solid portion representing the mesh-work in which the microsomes are placed. According to Strasburger, the cytoplasm consists of trophoplasm, or nutritive plasm, and kinoplasm, which is concerned in the division of the cell. The former is of alveolar, the latter of fibrillar, nature. The structure of the protoplasm as revealed by the action of reagents is regarded with scepticism by many observers, and it has been shown, by Fischer especially, that some of the appearances observed in dead cells can be produced artificially by treating albumen and other substances of a similar nature with various reagents. But whilst due weight must be attached to such observations, it would not be wise to push them too far. The objects which are actually visible in the living cell are reproduced with striking fidelity in the fixed and hardened cell, and one might expect that their finer structure would also be to some extent preserved. Living protoplasm is alkaline or neutral, and, so far as is known at present, is a mixture of complicated chemical compounds. By the gross analysis of simple organisms which consist mainly of protoplasm, it has been found that there are two main groups of compounds, in one of which phosphorus is present, in the other not. The phosphorus is especially abundant in the nucleus, where it enters into the composition of that important substance known as nuclein. It may also be present in smaller quantities in other parts of the cell. By means of a simple micro-chemical test Macallum finds that it occurs in the chromatin of all nuclei, in nucleoli, and in pyrenoids; and that it is present in small quantities in cytoplasm and chromatophores, in the central body (nucleus 1) of the Cyanophycese and in Yeast, where it is sometimes located in a definite organ of the cell which is commonly regarded as of a nuclear nature. The chromatophores or plastids are protoplasmic structures, denser than the cytoplasm, and easily distinguishfrom it by their colour or 0greater refractive Ch I’mti n / able ower phores " P - They are spherical, oval, fusiform, or rod-like, and are always found in the cytoplasm, never in the cell-sap. They appear to be permanent organs of the cell, and are transmitted from one cell to another by division. In young cells the chromatophores are small, colourless, highly refractive bodies, principally located around the nucleus. As the cell grows they may become converted into leucoplasts (starchformers), chloroplasts (chlorophyll-bodies), or chromoplasts (colour-bodies). And all three structures may be converted one into the other (Schimper). The chloroplasts are generally distinguished by their green colour, which is due to the presence of chlorophyll; but in many Algae
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this is masked by another colouring matter—Phycoerythrin in the Florideae, Phycophcein in the Phaeophyceae, and Phycocyanin in the Cyanophyceae. These substances can, however, be dissolved out in water, and the green colouring matter of the choloroplast then becomes visible. Its structure is difficult to make out, but the views put forward by various observers lead to the conclusion that 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. 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 Solanete, flowers of Cacalia coccinea, Tropceolum, 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 j}lates, 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 plantcells, 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-
