Common Objects of the Microscope. John George Wood

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СКАЧАТЬ do so by cutting a very thin slice from the fruit, putting it on a slide, covering it with a piece of thin glass (which may be cheaply bought at the optician’s, together with the glass slides on which the objects are laid), and placing it under a power of two hundred diameters. Should the slice be rather too thick, it may be placed in the live-box and well squeezed, when the cells will exhibit their forms very distinctly. In their primary form the cells seem to be spherical; but as in many cases they are pressed together, and in others are formed simply by the process of subdivision, the spherical form is not very often seen. The strawberry, being a soft and pulpy fruit, permits the cells to assume a tolerably regular form, and they consequently are more or less globular.

      Where the cells are of nearly equal size, and are subjected to equal pressure in every direction, they force each other into twelve-sided figures, having the appearance under the microscope of flat six-sided forms. Fig. 8, in the same Plate, taken from the stem of a lily, is a good example of this form of cell, and many others may be found in various familiar objects.

      We must here pause for a moment to define a cell before we proceed further.

      The cell is a close sac or bag formed of a substance called from its function “cellulose,” and containing certain semi-fluid contents as long as it retains its life. In the interior of the cell may generally be found a little dark spot, termed the “núcleus,” and which may be seen in Fig. 1, to which we have already referred. The object of the nucleus is rather a bone of contention among the learned, but the best authorities on this subject consider it to be the vital centre of the cells, to and from which tends the circulation of the protoplasm, and which is intimately connected with the growth and reproduction of the cell. On looking a little more closely at the nucleus, we shall find it marked with several small light spots, which are termed “nucléoli.”

      On the same Plate (Fig. 2) is a pretty group of cells taken from the internal layer of the buttercup leaf, and chosen because they exhibit the series of tiny and brilliant green dots to which the colour of the leaf is due. The technical name for this substance is “chlorophyll,” or “leaf-green,” and it may always be found thus dotted in the leaves of different plants, the dots being very variable in size, number, and arrangement. A very fine object for the exhibition of this point is the leaf of Anácharis, the “Canadian timber-weed,” to be found in almost every brook and river. It also shows admirably the circulation of the protoplasm in the cell.

      In the centre of the same Plate (Fig. 12) is a group of cells from the pith of the elder-tree. This specimen is notable for the number of little “pits” which may be seen scattered across the walls of the cells, and which resemble holes when placed under the microscope. In order to test the truth of this appearance, the specimen was coloured blue by the action of iodine and dilute sulphuric acid, when it was found that the blue tint spread over the pits as well as the cell-walls, showing that the membrane is continuous over the pits.

      Fig. 7 exhibits another form of cell, taken from the Spargánium, or bur-reed. These cells are tolerably equal in size, and have assumed a square shape. They are obtained from the lower part of the leaf. The reader who has any knowledge of entomology will not fail to observe the similarity in form between the six-sided and square cells of plants and the hexagonal and square facets of the compound eyes of insects and crustaceans. In a future page these will be separately described.

      Sometimes the cells take most singular and unexpected shapes, several examples of which will be briefly noticed.

      In certain loosely made tissues, such as are found in the rushes and similar plants, the walls of the cells grow very irregularly, so that they push out a number of arms which meet each other in every direction, and assume the peculiar form which is termed “stellate,” or star-shaped tissue. Fig. 3 shows a specimen of stellate tissue taken from the seed-coat of the privet, and rather deeply coloured, exhibiting clearly the beautiful manner in which the arms of the various stars meet each other. A smaller group of stellate cells taken from the stem of a large rush, and exemplifying the peculiarities of the structure, are seen in Fig. 4.

      The reader will at once see that this mode of formation leaves a vast number of interstices, and gives great strength with little expenditure of material. In water-plants, such as the reeds, this property is extremely valuable, as they must be greatly lighter than the water in which they live, and at the same time must be endowed with considerable strength in order to resist its pressure.

      A less marked example of stellate tissue is given in Fig. 11, where the cells are extremely irregular, in their form, and do not coalesce throughout. This specimen is taken from the pithy part of a bulrush. There are very many other plants from which the stellate cells may be obtained, among which the orange affords very good examples, in the so-called “white” that lies under the yellow rind, a section of which may be made with a very sharp razor, and placed in the field of the microscope.

Looking toward the bottom of the Plate, and referring to Fig. 27, the reader will observe a series of nine elongated cells, placed end to end, and dotted profusely with chlorophyll. These are obtained from the stalk of the common chickweed. Another example of the elongated cell is seen in Fig. 14, which is a magnified representation of the rootlets of wheat. Here the cells will be seen set end to end, and each containing its nucleus. On the left hand of the rootlet (Fig. 13) is a group of cells taken from the lowest part of the stem of a wheat plant which had been watered with a solution of carmine, and had taken up a considerable amount of the colouring substance. Many experiments on this subject were made by the Rev. Lord S. G. Osborne, and may be seen at full length in the pages of the Microscopical Journal, the subject being too large to receive proper treatment in the very limited space which can here be given to it. It must be added that later researches have caused the results here described to be gravely disputed.

      Fig. 9 on the same Plate exhibits two notable peculiarities—the irregularity of the cells and the copiously pitted deposit with which they are covered. The irregularity of the cells is mostly produced by the way in which the multiplication takes place, namely, by division of the original cell into two or more new ones, so that each of these takes the shape which it assumed when a component part of the parent cell. In this case the cells are necessarily very irregular, and when they are compressed from all sides they form solid figures of many sides, which, when cut through, present a flat surface marked with a variety of irregular outlines. This specimen is taken from the rind of a gourd.

      The “pitted” structure which is so well shown in this figure is caused by a layer of matter which is deposited in the cell and thickens its walls, and which is perforated with a number of very minute holes called “pits.” This substance is called “secondary deposit.” That these pits do not extend through the real cell-wall has already been shown in Fig. 12.

      This secondary deposit assumes various forms. In some cases it is deposited in rings round the cell, and is clearly placed there for the purpose of strengthening the general structure. Such an example may be found in the mistletoe (Fig. 5), where the secondary deposit has formed itself into clear and bold rings that evidently give considerable strength to the delicate walls which they support. Fig. 10 shows another good instance of similar structure; differing from the preceding specimen in being much longer and containing a greater number of rings. This object is taken from an anther of the narcissus. Among the many plants from which similar objects may be obtained, the yew is perhaps СКАЧАТЬ