Home › The Compound Microscope › The Compound Microscope: Sometimes

The Compound Microscope: Sometimes

Sometimes the cells run out longitudinally into cylinders, and attain the really enormous length of three inches; sometimes they become flattened, as the skin or epidermis of many plants; and often-times they push out their sides into arms or rays, like stars, and form the tissue which is technically called "stellate." Here is a specimen of stellate tissue taken from the pith of the common rush, wherein the rays are seen to be very regular: generally, however, the rays are extremely irregular, and require some little practice to detect them. Stellate tissue may be seen in the white portion of orange-peel, in the thick fleshy substance of many aquatic plants, in certain leaf-stalks, and in many similar objects.

We will now see how the soft cells which form the pulpy fruit of the strawberry can be changed into the hard timber of the oak or iron-wood tree. Wherever a cell is destined to form part of a permanent tissue, it is strengthened by receiving certain additions to its walls. These additions are technically known as "secondary deposit," and are made in various ways. Sometimes they extend in a thin layer over the whole cell-wall, leaving a number of little holes, which are called "pits," and earning the name of "pitted structures." Very frequently the secondary deposit is arranged in a series of rings, an example of which is given in the accompanying illustration. This object is taken from the mistletoe. Good examples of the ringed structures may be seen in the anthers of many plants, and in the leaf-stem of the common rhubarb, an example of which is shown in the next illustration. Another very common form of secondary deposit is the spiral, which is generally used where strength and elasticity are united. Two examples of the spiral form are given in the illustration; the first taken from the lily, and the second from the "rhizome," or subterranean stem of the water-lily.

Another beautiful form of secondary deposit is seen in the fern root. If the root be cut longitudinally, and the dark hard fibre dissolved carefully out with nitric acid, the deposit will seem to have assumed the shape of a winding staircase, and is then called "scalariform," or ladder-shaped. Similar structures may be found in asparagus.

The reader will see that the hardness of the structure depends entirely on the amount of secondary deposit, and we accordingly find that when the wood is hard and fit to be worked with tools the cells are almost wholly filled with the secondary deposit. In this state they are called "wood-cells." Examples of these cells may be seen in the accompanying illustration. In the first example, which is taken from the elder-tree, four cells are shown in order to display the manner in which their pointed ends are arranged. (The reader must remember that in all wood-cells the ends are pointed.) In the next example, which is taken from the chrysanthemum, the pitted structure is still retained; but in the last figure, which is drawn from the lime-tree, the entire cell is filled with secondary structure. The reader must understand that we can only give the veriest outline of the subject, and profess to do nothing more than indicate the method of observation, leaving the pupil to work out the details by himself.

Another curious development of the plant-cells is the formation of HAIRS. These objects alone afford an inexhaustible field for the microscopist, and any one who chooses to work out the subject will find himself repaid if he makes a good series of preparations. In their primary forms the hairs are seen merely as little projections on the epidermis, whether of the stem, leaf, or petal, and by degrees assume their varied and beautiful forms. In order to show the singular forms which hairs sometimes assume, an illustration is here given of the hairs of the lavender leaf. This is one of the hairs that give the leaf its silvery gloss. It consists of an upright stem, from the top of which a number of forked branches shoot out horizontally, much like a open umbrella held upright. The object of this remarkable form is, that the delicate vessels in which the perfume is held should escape injury. If the reader will refer to the second figure, which represents a much magnified view of the edge of the leaf, he will see the globular perfume-gland standing under the shelter of the branching hairs.

The following plants afford valuable examples of hair:--Arabis, marvel of Peru, sowthistle, tobacco, southernwood, hollyhock, snapdragon, pansy (in throat of flower), deutzia (under-side of leaf), verbena, alyssum, tradescantia, borage, cowhage, and many others. The beautiful effect produced by the petals of flowers is caused by the imperfect hairs with which their surfaces are studded.

The POLLEN of plants is always worth observing, and some specimens are of remarkably beautiful shapes. Take that of althaea, crocus, cactus, heath, violet, daisy, lily, snowdrop, wallflower, willow-herb (a very beautiful form), hollyhock, periwinkle, primrose, &c. Put some up in Deane's gelatine, and dry some, besides examining them all when fresh. The microscopist ought to examine the structures of WOOD by making sections in the directions transverse and longitudinal. A razor will answer very well for the purpose, and the wood should always be soaked inside, and the razor wetted before the section is made. It is often useful to make diagonal sections of several woods, especially those of the pine and juniper. All the forest trees should be examined, and their roots and bark should not be omitted. Cut sections of coconut-shell, vegetable ivory, sugar-cane (a most beautiful object when mounted opaque), bamboo, butcher's broom, &c. MOSSES are beautiful objects, and can always be found. Examine particularly the fruit or seed-vessel, and note the structure of its different parts. Put these on a slide, and breathe on them, noting at the same time any change which may take place.

The SPORE CASES of ferns are extremely beautiful, and should be carefully examined. The little brown dots or streaks that are seen on the under surface of the fronds are called "sori," and contain a large but variable number of the sporanges. These consist of stalked sacs or cases, and differ much in shape, according to the species of fern. If the fern be fresh from which the sorus is taken, the sporanges may be seen writhing and twisting like so many serpents, and sometimes it happens that one of the sporanges bursts, and suddenly covers the field of the microscope with minute black dots. These dots are the spores or seeds of the fern, and when magnified with a very high power, they are seen to be variously shaped. One of the most remarkable spores is that of the equisetum, or mare's tail of the water. This spore looks like a ball with something coiled round it. As soon as the spore is discharged from its case, four threads are seen to uncoil themselves from around it, and by their elasticity to cause the spore to jump about as if alive. These fibres are technically named elasters, and are prolongations of the outer coat of the spore.

FUNGI of all kinds should be examined. There is never any difficulty in finding fungi, though the autumn is the best time of year for this purpose. "Mould," as it is popularly called, is a form assumed by many species of fungus, which, though objectionable to the careful housewife, are full of interest to the microscopist. The well-known mushroom and toadstools are the highest of the fungi. The black spots on leaves are fungi, mostly belonging to the genus puccinia, and the best specimens are generally found on the wild rose or bramble. The black "smut" of wheat is another fungus, very pretty under the microscope, but very obnoxious to the farmer; and the "bunt" also belongs to the same vast tribe of plants, four thousand species of which are now known to exist.

The young observer should also look for the beautiful crystals which exist in many vegetable cells. The RAPHIDES, as these crystals are called, are of various forms, mostly shaped like curved needles, but often assuming very pretty and regular outlines. Raphides are plentifully found in the bulb of the onion, in the rhubarb, the lily, the iris, &c. They are best mounted as opaque objects and, if the reader can procure a binocular microscope, he will see the form of the raphides better than with the single-tube instrument. SEEDS of different plants should be carefully examined, especially those of small dimensions, which often exhibit some wonderful beauties of structure. The winged seed of various plants, such as the thistle, the dandelion, the valerian, and the willow-herb, are extremely interesting objects; while those of the yellow snapdragon, the mullein, the Robin Hood, and the bur-seed, are remarkably beautiful in form, though they have no parachute, as the feathery appendage is called.

Leaving dry land, we will devote a short time to the water. Let the reader take with him the simple collecting apparatus mentioned on page 430, and secure specimens of the water from different ponds, ditches, and streams. For collecting the larger objects a little net, which can be purchased cheap, is of very great use. It is easily made by any tinman, and if the young microscopist knows the use of solder, as all experimental philosophers ought to do, he can put it together in a few minutes. It is formed of a strip of zinc bent into the requisite form, and with a socket, to which a handle can be attached. A piece of coarse muslin, or, rather, fine "net," is then stretched over the bottom, and the apparatus is complete.

In the water is sure to be found one of the lowest forms of vegetable life--namely, the "confervoid algae." Look for these in bright, clear pools, placing the collecting bottle near any greenish film collected around the stems of plants, or spread over the stones on the bed of the pool. If this film be very carefully taken up, it will produce many interesting forms of vegetable life. One of the most remarkable of these vegetables is that which is called "volvox globator," a figure of which is here given. This wonderful object is about as large as the head of a very small pin, so that it is visible to the naked eye, and looks like a tiny globule passing through the water. When it is placed under a lens of moderate power, say of an inch focus, it exhibits some very strange peculiarities. It continually revolves, and by its revolution is able to enjoy a moderate degree of locomotion, though without any apparent object. Small dark spots are also seen upon it.

If a half-inch lens be now used, the structure of the volvox begins to be exhibited. The whole surface is covered with a network of very fine fibres, having a spot at the intersection of each mesh. On applying a still higher power, say the four-tenths of an inch, the structure is further elucidated, and the dots on the surface are seen to consist of greenish bodies, each furnished with a pair of delicate fibres, technically named cilia, which are constantly vibrating, and cause the revolution of the general mass. The dark spots are now seen to be the young plants in different stages of progress. From six to ten of these are inclosed within the parent, and when the latter has reached its full age, the membrane bursts asunder, and the little volvoces are liberated.

Another interesting form is the closterium, a genus which is sure to produce several good examples. We may mention that the ponds in Blackheath are very rich in these curious vegetables, and a very considerable series of confervoids may be obtained from them. The closteria are easily recognised by their resemblance to the Australian "boomerang."

As our space is rapidly waning, we must leave the vegetable, and proceed to the animal kingdom.

As is the case with vegetables, the animal structure is composed of cells, though they cannot be so easily traced as in the examples which we have already noticed. The young observer may readily perceive the animal cell, in its largest and simplest form, by placing a little of the yolk of egg under the microscope. CARTILAGE, or gristle, is easily seen to be composed of cells. The nails of the fingers afford good objects for the microscopist in search of animal-cells. If a thin section be placed under the microscope, none but an experienced observer will be able to make out the presence of cells at all; but if the section be soaked in "liquor potassae," the cells immediately swell up, and their shape is at once made plain. Take the BONE of a young chicken or rabbit, and make a thin section that embraces both the bone and cartilage, and there will then be a beautiful object for the microscopist, showing how the cartilage is changed by degrees into bone.

Sections of bone should also be made, both transverse and longitudinal. The BLOOD is another object which must be carefully examined. The "corpuscles" which give the colouring matter to the blood are cells of different size, according to the creature from which they are taken. The dimensions of the animal exercise no apparent influence on the corpuscles, for those of "proteus anguinus," a little creature not larger than a lamprey, are many times larger than those of the ox. In the accompanying illustration is shown a series of specimens, in order to show the great difference in their shape and size, all being drawn to scale and magnified by the same lens. The circular corpuscles in the left-hand upper corner are those of man; immediately below is a single corpuscle from the pigeon. The great central corpuscle is taken from the proteus; the two in the lower right-hand corner are from the frog, one of these being viewed edgeways; and of the remaining two, that on the left hand belongs to the tortoise, and that on the right to a fish. The insect tribes are an inexhaustible source of objects for the microscopist, who may find that even a single fly will give him employment for many months. The scales from the butterfly's wing, the wonderful compound eyes with which insects are gifted, the structure of their feet, and their entire anatomy, are always at the service of any microscopist who really cares for his work. It would, of course, be impossible to give even a list of the interesting portions of the different insects; so one or two examples must suffice us. Take the ANTENNAE of the insect tribes, and see how beautifully they are formed, how graceful is the shape, and how elaborate the structure. A low power will be useful for exhibiting their general shape and outline, but it is not until we know how to use the higher powers that the real beauty of these curious organs is seen. In the accompanying illustration is given part of an antenna of the common blue-bottle fly, in order to show the remarkable cavities which exist within the antennae, and which are thought by some anatomists to be organs of hearing, and by others to be organs of smell. The WINGS of insects are also most remarkable, and possess many peculiarities of structure which cannot be detected without the aid of a microscope. Take, for example, the wings of any hymenopterous insect, say those of a humble-bee, and see how beautiful is the structure which causes the four wings to be united into two when the insect is about to fly. In the illustration may be seen a pair of these wings, together with the row of hooks which bind them together. A still more magnified representatation of the hooks is placed near the wings.

It is now ascertained that the wings of insects are connected with the breathing apparatus, and that the respiration of the insect extends even to the very tips of these singular organs, which are not modifications of existing limbs, as in the birds, but additional structures. The circulation of insects may often be seen by placing a portion of a transparent wing under a moderately high power. We have often seen it in the wings of great water-beetle. A series of very beautiful preparations may be made in order to show the distinction between the wings of different insects; and as the orders of insects are founded upon their wings, there ought to be at least one example of each order. The proboscis of insects is always worthy of careful examination.

As to the breathing apparatus itself, the best mode of examining it is to open a cater-pillar, remove a part of the large breathing tube which runs along each side, and place it under the microscope. It should always be taken so as to include one the spiracles, or breathing-holes. An example of a breathing-tube, taken from a silk-worm, is given in the illustration. HAIRS of animals are very curious and interesting objects. They should be mounted in three modes--namely, dry transparent, dry opaque, and in Canada balsam, transparent. Be sure to procure some hair of the bat, the sheep, the mouse, the deer, the mole, and any of the weasel tribe. Many insects have very beautiful hair, but the most lovely hair in the animal kingdom is that which is obtained from the sea-mouse. Fish scales should also be procured, and specimens should be taken from the lateral line.

MOLLUSCS of all kinds afford many beautiful objects, and the observer should be very careful to examine the wonderful tongue-ribbon of the snail, the slug, the periwinkle, the whelk, and other similar molluses. If meant to be examined by polarized light, the tongue-ribbon should be mounted in Canada balsam. CRYSTALS should always form part of a collection. Take those of common salt, nitre, sugar, chlorate of potash, salicine, &c.; indeed, anything that will crystallize should be prepared and mounted, as such objects will often be most useful when examining unknown substances.

ZOOPHYTES must of course find a place in the cabinet, and the young microscopist ought to put up a few specimens of the "bird's-head" processes which are found in the bugularia and other inhabitants of the sea. The pretty noctiluca, to which is mostly owing the phosphorescence of the sea, should be preserved, and the extraordinary appendages to the skin of certain star-fish and sea urchins should be examined. These are called pedicillariae, and a sketch of them is given in the illustration.