Common Objects of the Microscope

Curiously branched hairs are not at all uncommon, and some very good and easily obtained examples are given on Plate II.

Fig. 28 (#x8_x_8_i52) is one of the multitude of branched hairs that surround the well-known fruit of the plane-tree, the branches being formed by some of the cells pointing outward. These hairs do not assume precisely the same shape; for Fig. 29 (#x8_x_8_i52) exhibits another hair from the same locality, on which the spikes are differently arranged, and Fig. 30 (#x8_x_8_i52) is a sketch of another such hair, where the branches have become so numerous and so well developed that they are quite as conspicuous as the parent stem.

One of the most curious and interesting forms of hair is that which is found upon the lavender leaf, and which gives it the peculiar bloom-like appearance on the surface.

This hair is represented in Figs. 40 and 41. On Fig. 40 (#x8_x_8_i52) the hair is shown as it appears when looking directly upon the leaf, and in Fig. 41 (#x8_x_8_i52) a section of the leaf is given, showing the mode in which the hairs grow into an upright stem, and then throw out horizontal branches in every direction. Between the two upright hairs, and sheltered under their branches, may be seen a glandular appendage not unlike that which is shown in Fig. 16 (#x8_x_8_i52). This is the reservoir containing the perfume, and it is evidently placed under the spreading branches for the benefit of their shelter. On looking upon the leaf by reflected light the hairs are beautifully shown, extending their arms on all sides; and the globular perfume cells may be seen scattered plentifully about, gleaming like pearls through the hair-branches under which they repose. They will be found more numerous on the under side of the leaf.

This object will serve to answer a question which the reader has probably put to himself ere this, namely, Where are the fragrant resins, scents, and oils stored? On Plate I. Fig. 16 (#x8_x_8_i9), will be seen the reply to the first question; Fig. 41 (#x8_x_8_i52) of the present Plate has answered the second question, and Fig. 42 (#x8_x_8_i52) will answer the third. This figure represents a section of the rind of an orange, the flattened cells above constituting the delicate yellow skin, and the great spherical object in the centre being the reservoir in which the fragrant essential oil is stored. The covering is so delicate that it is easily broken, so that even by handling an orange some of the scent is sure to come off on the hands, and when the peel is stripped off and bent double, the reservoirs burst in myriads, and fling their contents to a wonderful distance. This may be easily seen by squeezing a piece of orange peel opposite a lighted candle, and noting the distance over which the oil will pass before reaching the flame, and bursting into little flashes of light. Other examples are given on the same plate.

Returning to the barbed hairs, we may see in Fig. 35 (#x8_x_8_i52) a highly magnified view of the “pappus” hair of a dandelion, i.e. the hairs which fringe the arms of the parachute-like appendage which is attached to the seed. The whole apparatus will be seen more fully on Plate III. Figs. 44 (#x9_x_9_i9), 45, 46. This hair is composed of a double layer of elongated cells lying closely against each other, and having the ends of each cell jutting out from the original line. A simpler form of a double-celled, or more properly a “duplex” hair, will be seen in Fig. 44 (#x8_x_8_i52). This is one of the hairs from the flower of the marigold and has none of the projecting ends to the cells.

In some instances the cell-walls of the hairs become greatly hardened by secondary deposit, and the hairs are then known as spines. Two examples of these are seen in Figs. 37 (#x8_x_8_i52) and 38, the former being picked from the Indian fig-cactus, and well known to those persons who have been foolish enough to handle the fig roughly before feeling it. The wounds which these spines will inflict are said to be very painful, and have been compared to those produced by the sting of the wasp. The latter hair is taken from the Opuntia. These spines must not be confounded with thorns; which latter are modified branches.

Fig. 10 (#x8_x_8_i52) represents the extreme tip of a hair from the hollyhock leaf, subjected to a lens of very high power.

Many hairs assume a star-like appearance, an aspect which may be produced in different ways. Sometimes a number of simple hairs start from the same base, and by radiating in different directions produce the stellate effect. An example of this kind of hair may be seen in Fig. 14 (#x8_x_8_i52), which is a group of hairs from the hollyhock leaf. There is another mode of producing the star-shape which may be seen in Fig. 45 (#x8_x_8_i52), a hair taken from the leaf of the ivy. Very fine examples may also be found upon the leaf of Deutzia scabra.

Hairs are often covered with curious little branches or protuberances, and present many other peculiarities of form which throw a considerable light upon certain problems in scientific microscopy.

Fig. 33 (#x8_x_8_i52) represents a hair of two cells taken from the flower of the well-known dead-nettle, which is remarkable for the number of knobs scattered over its surface. A similar mode of marking is seen in Fig. 31 (#x8_x_8_i52), a club-shaped hair covered with external projections, found in the flower of the Lobelia. In order to exhibit these markings well, a power of two hundred diameters is needed. Fig. 21 (#x8_x_8_i52) shows this dotting in another hair from the dead-nettle, where the cell is drawn out to a great length, but is still covered with these markings.

Fig. 20 (#x8_x_8_i52) is an example of a very curious hair taken from the throat of the pansy. This hair may readily be obtained by pulling out one of the petals, when the hairs will be seen at its base. Under the microscope it has a particularly beautiful appearance, looking just like a glass walking-stick covered with knobs, not unlike those huge, knobby club-like sticks in which some farmers delight, where the projections have been formed by the pressure of a honeysuckle or other climbing plant.

A hair of a similar character, but even more curious, is found in the same part of the flower of the Garden Verbena (see Fig. 27 (#x8_x_8_i52)), and is not only beautifully translucent, but is coloured according to the tint of the flower from which it is taken. Its whole length is covered with large projections, the joints much resembling the antennæ of certain insects; and each projection is profusely spotted with little dots, formed by elevation of the outer skin or cuticle. These are of some value in determining the structure of certain appearances upon petals and other portions of the flowers, and may be compared with Figs. 33 to 35 on Plate III (#x9_x_9_i9).

Fig. 26 (#x8_x_8_i52) offers an example of the square cells which usually form the bark of trees. This is a transverse section of cork, and perfectly exhibits the form of bark cells. The reader is very strongly advised to cut a delicate section of the bark of various trees, a matter very easily accomplished with the aid of a sharp razor and a steady hand.

Fig. 24 (#x8_x_8_i52) is a transverse section through one of the scales of a pine-cone, and is here given for the purpose of showing the numerous resin-filled cells which it displays. This may be compared with Fig. 16 (#x8_x_8_i9) of Plate I. Fig. 25 (#x8_x_8_i52) is a part of one of the “vittæ,” or oil reservoirs, from the fruit of the caraway, showing the cells containing the globules of caraway oil. This is rather a curious object, because the specimen from which it was taken was boiled in nitric acid, and yet retained some of the oil globules. Immediately above it may be seen (Fig. 23 (#x8_x_8_i52)) a transverse section of the beechnut, showing a cell with its layers of secondary deposit.

In the cuticle of the grasses and the mare’s-tails is deposited a large amount of pure flint. So plentiful is this substance, and so equally is it distributed, that it can be separated by heat or acids from the vegetable parts of the plant, and will still preserve the form of the original cuticle, with its cell-walls, stomata, and hairs perfectly well defined.

Fig. 7 (#x8_x_8_i52), Plate II., represents a piece of wheat chaff, or “bran,” that has been kept at a white heat for some time, and then mounted in Canada balsam. I prepared the specimen from which the drawing was made by laying the chaff on a piece of platinum, and holding it over the spirit-lamp. A good example of the silex or flint in wheat is often given by the remains of a straw fire, where the stems may be seen still retaining their tubular form but fused together into a hard glassy mass. It is this substance that cuts the fingers of those who handle the wild grasses too roughly, the edges of the blades being serrated with flinty teeth, just like the obsidian swords of the ancient Mexicans, or the shark’s-tooth falchion of the New Zealander.

These are but short and meagre accounts of a very few objects, but space will not permit of further elucidation, and the purpose of this little work is not to exhaust the subjects of which it treats, but to incite the reader to undertake investigation on his own account, and to make his task easier than if he had done it unaided.

CHAPTER V

Starch, its Growth and Properties—Surface Cells of Petals—Pollen and its Functions—Seeds.

The white substance so dear to the laundries under the name of starch is found in a vast variety of plants, being distributed more widely than most of the products which are found in the interior of vegetable cells.

The starch grains are of very variable size even in the same plant, and their form is as variable as their size, though there is a general resemblance in those of the same plant which allows of their being fairly easily identified after a moderate amount of practice. Sometimes the grains are found loosely packed in the interior of the cells, and are then easily recognised as starch grains by their peculiar form and the delicate lines with which they are marked; but in many places they are pressed so closely together that they assume an hexagonal shape under the microscope, and bear a close resemblance to ordinary twelve-sided cells. In other plants, again, the grains never advance beyond the very minute form in which they seem to commence their existence; and in some, such as the common oat, a great number of very little granules are compacted together so as to resemble one large grain.

There are several methods of detecting starch in those cases where its presence is doubtful; and the two modes that are usually employed are polarised light and the iodide of potassium. When polarised light is employed—a subject on which we shall have something to say presently—the starch grains assume the characteristic “black-cross,” and when a plate of selenite is placed immediately beneath the slide containing the starch grains, they glow with all the colours of the rainbow. The second plan is to treat them with a very weak solution of iodine and iodide of potassium, and in this case the iodine has the effect on the starch granules of staining them blue. They are so susceptible of this reaction that when the liquid is too strong the grains actually become black from the amount of iodine which they imbibe.

Nothing is easier than to procure starch granules in the highest perfection. Take a raw potato, and with a razor cut a very thin slice from its interior, the direction of the cut not being of the slightest importance. Put this delicate slice upon a slide, drop a little water upon it, cover it with a piece of thin glass, give it a good squeeze, and place it under a power of a hundred or a hundred and fifty diameters. Any part of the slice, provided that it be very thin, will then present the appearance shown in Plate III. Fig. 9 (#x9_x_9_i9), where an ordinary cell of potato is seen filled loosely with starch grains of different sizes. Around the edges of the slice a vast number of starch granules will be seen, which have been squeezed out of their cells by pressure, and are now floating freely in the water. As cold water has no perceptible effect upon starch, the grains are not altered in form by the moisture, and can be examined at leisure.

III.

III.

On focusing with great care, the surface of each granule will be seen to be covered with very minute dark lines, arranged in a manner which can be readily comprehended from Fig. 4 (#x9_x_9_i9), which represents two granules of potato starch as they appear when removed from the cell in which they took their origin. All the lines evidently refer to the little dark spots at the end of the granule, called technically the “hilum,” and represent the limits of successive layers of material deposited one after another. The lines in question are very much better seen if the substage condenser be used with a small central stop, so as to obtain partial dark-field illumination. Otherwise they are often very difficult of detection.

In the earliest stages of their growth the starch granules appear to be destitute of these markings, or at all events they are so few and so delicate as not to be visible even with the most perfect instruments, and it is not until the granules assume a comparatively large size that the external markings become distinctly perceptible.

We will now glance at the examples of starch which are given in the Plate, and which are a very few out of the many that might be figured. Fig. 2 (#x9_x_9_i9) represents the starch of wheat, the upper grain being seen in front, the one immediately below it in profile, and the two others being examples of smaller grains. Fig. 6 (#x9_x_9_i9) is a specimen of a very minute form of starch, where the granules do not seem to advance beyond their earliest stage. This specimen is obtained from the parsnip; and although the magnifying power is very great, the dimensions of the granules are exceedingly small, and except by a very practised eye they would not be recognisable as starch grains.

Fig. 3 (#x9_x_9_i9) is a good example of a starch grain of wheat, exemplifying the change that takes place by the combined effects of heat and moisture. It has already been observed that cold water exercises little, if any, perceptible influence upon starch; but it will be seen from the illustration that hot water has a very powerful effect. When subjected to the action of water at a temperature over 140° Fahr., the granule swells rapidly, and at last bursts, the contents escaping in a gelatinous mass, and the external membrane collapsing into the form which is shown in Fig. 3 (#x9_x_9_i9), which was taken out of a piece of hot pudding. A similar form of wheat starch may also be detected in bread, accompanied, unfortunately, by several other substances not generally presumed to be component parts of the “staff of life.”

In Fig. 7 (#x9_x_9_i9) are represented some grains of starch from West Indian arrowroot, and Fig. 8 (#x9_x_9_i9) exhibits the largest kind of starch grain known, obtained from the tuber of a species of canna, supposed to be C. edúlis, a plant similar in characteristics to the arrowroot. The popular name of this starch is “Tous les Mois,” and under that title it may be obtained from the opticians, or chemists.

Fig. 10 (#x9_x_9_i9) shows the starch granules from Indian corn, as they appear before they are compressed into the honeycomb-like structure which has already been mentioned. Even in that state, however, if they are treated with iodine, they exhibit the characteristics of starch in a very perfect manner. Fig. 11 (#x9_x_9_i9) is starch from sago, and Fig. 12 (#x9_x_9_i9) from tapioca, and in both these instances the several grains have been injured by the heat employed in preparing the respective substances for the market.

Fig. 13 (#x9_x_9_i9) exhibits the granules obtained from the root of the water-lily, and Fig. 14 (#x9_x_9_i9) is a good example of the manner in which the starch granules of rice are pressed together so as to alter the shape and puzzle a novice. Fig. 16 (#x9_x_9_i9) is the compound granule of the oat, which has already been mentioned, together with some of the simple granules separated from the mass; and Fig. 15 (#x9_x_9_i9) is an example of the starch grains obtained from the underground stem of the horse-bean. It is worthy of mention that the close adhesion of the rice starch into those masses is the cause of the peculiar grittiness which distinguishes rice flour to the touch.

Whilst very easily acted on by heat, starch-granules are very resistent to certain other reagents. Weak alkalies, in watery solution, readily attack them, but by treating portions of plants with caustic potash dissolved in strong spirit, the woody and other parts may be dissolved away; and after repeated washing with spirit the starch may be mounted. This, however, must never be in any glycerine medium, except that given on p. 172 (#x16_x_16_i18).

In Plate III. Fig. 1 (#x9_x_9_i9), may be seen a curious little drawing, which is a sketch of the laurel-leaf cut transversely, and showing the entire thickness of the leaf. Along the top may be seen the delicate layer of “varnish” with which the surface of the leaf is covered, and which serves to give to the foliage its peculiar polish. This varnish is nothing more than the translucent matter which binds all the cells together, and which is poured out very liberally upon the surface of the leaf. The lower part of this section exhibits the cells of which the leaf is built, and towards the left hand may be seen a cut end of one of the veins of the leaf, more rightly called a wood-cell.

We will now examine a few examples of surface cells.

Fig. 5 (#x9_x_9_i9) is a portion of epidermis stripped from a Capsicum pod, exhibiting the remains of the nuclei in the centre of each cell, together with the great thickening of the wall-cells and the numerous pores for the transmission of fluid. This is a very pretty specimen for the microscope, as it retains its bright red colour, and even in old and dried pods exhibits the characteristic markings.

In the centre of the Plate may be seen a wheel-like arrangement of the peculiar cells found on the petals of six different flowers, all easily obtainable, and mounted without difficulty.

Fig. 30 (#x9_x_9_i9) is the petal of a geranium (Pelargonium), a very common object on purchased slides. It is a most lovely subject for the microscope, whether it be examined with a low or a high power,—in the former instance exhibiting a most beautiful “stippling” of pink, white, and black, and in the latter showing the six-sided cells with their curious markings.

In the centre of each cell is seen a radiating arrangement of dark lines with a light spot in the middle, looking very like the mountains on a map. These lines were long thought to be hairs; but Mr. Tuffen West, in an interesting and elaborate paper on the subject, has shown their true nature. From his observations it seems that the beautiful velvety aspect of flower petals is owing to these arrangements of the surface cells, and that their rich brilliancy of colour is due to the same cause. The centre of each cell-wall is elevated as if pushed up by a pointed instrument from the under side of the wall, and in different flowers this elevation assumes different forms. Sometimes it is merely a slight wart on the surface, sometimes it becomes a dome, while in other instances it is so developed as to resemble a hair. Indeed, Mr. West has concluded that these elevations are nothing more than rudimentary hairs.

The dark radiating lines are shown by the same authority to be formed by wrinkling of the membrane forming the walls of the elevated centre, and not to be composed of “secondary deposit,” as has generally been supposed.

Fig. 31 (#x9_x_9_i9) represents the petal of the common periwinkle, differing from that of the geranium by the straight sides of the cell-walls, which do not present the toothed appearance so conspicuous in the former flower. A number of little tooth-like projections may be seen on the interior of the cells, their bases affixed to the walls and their points tending toward the centre, and these teeth are, according to Mr. West, formed of secondary deposit.

In Fig. 32 (#x9_x_9_i9) is shown the petal of the common garden balsam, where the cells are elegantly waved on their outlines, and have plain walls. The petal of the primrose is seen in Fig. 34 (#x9_x_9_i9), and that of the yellow snapdragon in Fig. 33 (#x9_x_9_i9); in the latter instance the surface cells assume a most remarkable shape, running out into a variety of zigzag outlines that quite bewilders the eye when the object is first placed under the microscope. Fig. 35 (#x9_x_9_i9) is the petal of the common scarlet geranium.

In several instances these petals are too thick to be examined without some preparation, and glycerine will be found well adapted for that purpose. The young microscopist must, however, beware of forming his ideas from preparations of dried leaves, petals, or hairs, and should always procure them in their fresh state whenever he desires to make out their structure. Even a fading petal should not be used, and if the flowers are gathered for the occasion, their stalks should be placed in water, so as to give a series of leaves and petals as fresh as possible.

We now pass from the petal of the flower to the pollen, that coloured dust, generally yellow or white, which is found upon the stamens, and which is very plentiful in many flowers, such as the lily and the hollyhock.

This substance is found only upon the stamens or anthers of full-blown flowers (the anthers being the male organs), and is intended for the purpose of enabling the female portion of the flower to produce fertile seeds. In form the pollen grains are wonderfully diverse, affording an endless variety of beautiful shapes. In some cases the exterior is smooth and marked only with minute dots, but in many instances the outer wall of the pollen grain is covered with spikes, or decorated with stripes or belts. A few examples of the commonest forms of pollen will be found on Plate III.

Fig. 17 (#x9_x_9_i9) is the pollen of the snowdrop, which, as will be seen, is covered with dots and marked with a definite slit along its length. The dots are simply tubercles in the outer coat of the grain, and are presumed to be formed for the purpose of strengthening the membrane, otherwise too delicate, upon the same principle which gives to “corrugated” iron such strength in proportion to the amount of material. Fig. 18 (#x9_x_9_i9) is the pollen of the wall-flower, shown in two views, and having many of the same characteristics as that of the snowdrop. Fig. 19 (#x9_x_9_i9) is the pollen of the willow-herb, and is here given as an illustration of the manner in which the pollen aids in the germination of plants.

In order to understand its action, we must first examine its structure.

All pollen-grains are furnished with some means by which their contents when thoroughly ripened can be expelled. In some cases this end is accomplished by sundry little holes called pores; in others, certain tiny lids are pushed up by the contained matter; and in some, as in the present instance, the walls are thinned in certain places so as to yield to the internal pressure.

When a ripe pollen-grain falls upon the stigma of a flower, it immediately begins to swell, and seems to “sprout” like a potato in a damp cellar, sending out a slender “pollen-tube” from one or other of the apertures already mentioned. In Fig. 19 (#x9_x_9_i9) a pollen-tube is seen issuing from one of the projections, and illustrates the process better than can be achieved by mere verbal description. The pollen-tubes insinuate themselves between the cells of the stigmas, and, continually elongating, worm their way down the “style” until they come in contact with the “ovules.” By very careful dissection of a fertilised stigma, the beautiful sight of the pollen-tubes winding along the tissues of the style may be observed under a high power of the microscope.