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 PlateIII. Fig.9, 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.

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, 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 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 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 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, 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 are represented some grains of starch from West Indian arrowroot, and Fig.8 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 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 is starch from sago, and Fig.12 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 exhibits the granules obtained from the root of the water-lily, and Fig.14 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 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 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.

In PlateIII. Fig.1, 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 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 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 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 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, and that of the yellow snapdragon in Fig.33; 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 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 PlateIII.

Fig.17 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 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 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 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.

The pollen-tube is nothing more than the interior coat of the grain, very much developed, and filled with a substance technically named “fovilla,” composed of “protoplasm” (the semi-liquid substance which is found in the interior of cells), very minute starch grains, and some apparently oily globules.

In order to examine the structure of the pollen-grains properly, they should be examined under various circumstances—some dry, others placed in water to which a little sugar has been added, others in oil, and it will often be found useful to try the effect of different acids upon them.

Fig.20 is the pollen of the common violet, and is easily recognisable by its peculiar shape and markings. Fig.21 is the pollen of the musk-plant, and is notable for the curious mode in which its surface is belted with wide and deep bands, running spirally round the circumference. Fig.22 exhibits the pollen of the apple, and Fig.23 affords a very curious example of the raised markings upon the surface of the dandelion pollen. In Fig.24 there are also some very wonderful markings, but they are disposed after a different fashion, forming a sort of network upon the surface, and leaving several large free spaces between the meshes. The pollen of the lily is shown in Fig.25, and is a good example of a pollen-grain covered with the minute dottings which have already been described.

Figs.26 and 27 show two varieties of compound pollen, found in two species of heath. These compound pollen-grains are not of unfrequent occurrence, and are accounted for in the following manner.

The pollen is formed in certain cavities within the anthers, by means of the continual subdivision of the “parent-cells” from which it is developed. In many cases the form of the grain is clearly owing to the direction in which these cells have divided, but there is no great certainty on this subject. It will be seen, therefore, that if the process of subdivision be suddenly arrested, the grains will be found adhering to each other in groups of greater or smaller size, according to the character of the species and the amount of subdivision that has taken place. The reader must, however, bear in mind that the whole subject is as yet rather obscure, and that further discovery may throw doubt on many theories which at present are accepted as established.

Fig.28 shows the pollen of the furze, in which are seen the longitudinal slits and the numerous dots on the surface; and Fig.29 is the curiously shaped pollen of the tulip. The two large yellow globular figures at each side of the Plate represent the pollen of two common flowers; Fig.36 being that of the crocus, and Fig.37 a pollen-grain of the hollyhock. As may be seen from the illustration, the latter is of considerable size, and is covered with very numerous projections. These serve to raise the grain from a level surface, over which it rolls with a surprising ease of motion, so much so indeed that if a little of this substance be placed on a slide and a piece of thin glass laid over it, the glass slips off as soon as it is in the least inclined, and forces the observer to fix it with paper or cement before he can place it on the inclined stage of the microscope. The little projections have a very curious effect under a high power, and require careful focusing to observe them properly; for the diameter of the grain is so large that the focus must be altered to suit each individual projection. Their office is, probably, to aid in fertilisation.

The seeds of plants are even easier of examination than the pollen, and in most cases require nothing but a pocket lens and a needle for making out their general structure. The smaller seeds, however, must be placed under the microscope, many of them exhibiting very curious forms. The external coat of seeds is often of great interest, and needs to be dissected off before it can be rightly examined. The simplest plan in such a case is to boil the seed well, press it while still warm into a plate of wax, and then dissect with a pair of needles, forceps, and scissors under water. Many seeds may also be mounted in cells as dry objects, after being thoroughly dried themselves.

A few examples of the seeds of common plants are given at the bottom of PlateIII.

Fig.38 exhibits the fruit, popularly called the seed, of the common goosegrass, or Galium, which is remarkable for the array of hooklets with which it is covered. Immediately above the figure may be seen a drawing of one of the hooks much magnified, showing its sharp curve (Fig.39). It is worthy of remark that the hook is not a simple curved hair, but a structure composed of a number of cells terminating in a hook.

Fig.40 shows the seed, or rather the fruit, of the common red valerian, and is introduced for the purpose of showing its plumed extremity, which acts as a parachute, and causes it to be carried about by the wind until it meets with a proper resting-place. It is also notable for the series of strong longitudinal ribs which support its external structure. On Fig.41 is shown a portion of one of the parachute hairs much more magnified.

The seed of the common dandelion, so dear to children in their play-hours, when they amuse themselves by puffing at the white plumy globes which tip the ripe dandelion flower-stalks, is a very interesting object even to their parents, on account of its beautiful structure, and the wonderful way in which it is adapted to the place which it fills. Fig.45 represents the seed portion of one of these objects, together with a part of the parachute stem, the remainder of that appendage being shown lying across the broken stem.

The shape of the seed is not unlike that of the valerian, but it is easily distinguished from that object by the series of sharp spikes which fringe its upper end, and which serve to anchor the seed firmly as soon as it touches the ground. From this end of the seed proceeds a long slender shaft, crowned at its summit by a radiating plume of delicate hairs, each of which is plentifully jagged on its surface, as may be seen in Fig.46, which shows a small portion of one of these hairs greatly magnified. These jagged points are evidently intended to serve the same purpose as the spikes below, and to arrest the progress of the seed as soon as it has found a convenient spot.

Fig.42 is the seed of the foxglove, and Fig.43 the seed of the sunspurge, or milkwort. Fig.47 shows the seed of the yellow snapdragon; remarkable for the membranous wing with which the seed is surrounded, and which is composed of cells with partially spiral markings. When viewed edgewise, it looks something like Saturn with his ring, or, to use a more homely but perhaps a more intelligible simile, like a marble set in the middle of a penny. Fig.48 is a seed of mullein, covered with net-like markings on its external surface. These are probably to increase the strength of the external coat, and are generally found in the more minute seeds.

On Fig.50 is shown a seed of the burr-reed; a structure which is remarkable for the extraordinary projection of the four outer ribs, and their powerful armature of reverted barbs. Fig.51 shows another form of parachute seed, found in the willow-herb, where the parachute is not expanded nearly so widely as that of the valerian; neither is it set upon a long slender stem like that of the dandelion, but proceeds at once from the top of the seed, widening towards the extremity, and having a very comet-like appearance. Two more seeds only remain, Fig.49 being the seed of Robin Hood, and the other, Fig.52, that of the muskmallow, being given in consequence of the thick coat of hairs with which it is covered.

Many seeds can be well examined when mounted in Canada balsam.