The Romance of the Microscope / An interesting description of its uses in all branches of science, industry, agriculture, and in the detection of crime, with a short account of its origin, history, and development

by C. A. Ealand

Amongst the Wracks, of which there are a number of kinds, the methods of increase reach a higher stage. First of all let us describe the plants, so that we may know what to look for. They all belong to the group of brown Algæ. The “Channelled Wrack” is, when fully grown, about six inches long. It is much branched, often almost yellowish in colour and grows just below high-water mark. Along one side of the plant there is a moderately deep groove. Here we may note that the Wracks grow in zones from just below high water mark to low water mark. A little nearer the sea than the haunts of the Channelled Wrack, we shall find the Flat and Bladder Wracks. The former is but six inches or so in length, with flat, forked fronds, along the centres of which runs a single rib. The Bladder Wrack varies considerably in size. It may be smaller than either of the Wracks we have already mentioned or it may be two or three feet in length. It is the one seaweed familiar to everybody. Nearer to low tide mark we shall encounter the Knobbed Wrack, greenish brown in colour and often as much as six feet in length. It is so named because from the sides of its flat, leathery, strap-like fronds, there arise little stalked bladders. Right at, and often beyond, low tide mark there dwells the Notched Wrack; very similar to, though larger than, the Flat Wrack, from which it may easily be distinguished by the fact that the edges of its fronds are toothed, after the manner of a saw.

It is obvious that the structure of any one of these Wracks is much more complicated than is the case with Ectocarpus. The latter Alga was composed of a number of cells, similar to one another in every respect except size. If we tease a stem or a frond of one of the Wracks upon a slide and examine the result of our efforts under the microscope we shall see that the cells which compose the Wrack are not by any means similar to one another. Those of us who have mastered the, by no means difficult, art of section cutting, should cut sections of stem and frond and compare them with sections of leaf and stem of some higher plant. The comparison will show us that, although the seaweed does not exhibit the complicated structure of a flowering plant, it has at least three kinds of different cells—an outer layer, a central structure and an intermediate layer.

If we secure a specimen of the Channelled Wrack, at the end of the summer, we shall notice that the tips of certain of the fronds are swollen. Examination of these swellings under a low magnification will reveal a number of wart-like structures, and at the end of each wart there is a little pore. If we open up one of these little warts, very carefully with our mounted needles, we shall find that each little pore opens into a cavity, within which we can find two kinds of structures, hidden amongst a number of hair like growths. We shall see a number of dark, oval bodies, at the base of the hairs, these are the egg cells; more careful search will show us a number of much branched structures also partly concealed by the hairs, these are the male organs of the plants. The purpose of the hairs, by the way, is to keep the little chamber moist, when the plant is left high and dry. If we watch the pores of the other warts carefully we may be fortunate enough to see the process of increase taking place, for it occurs outside the plant and not within the chamber. The egg cell divides into two and its contents pass out of the chamber by way of the pore; each cell of the male organs gives rise to sixty-four oval little structures, each provided with a pair of minute whip-like threads by means of which it swims from the chamber and goes in search of the egg cells. Many of these little navigators are lost by the way but one of them will reach and fuse with each egg cell. After fusion the new-formed cell germinates at once into a new Channelled Wrack.

That this is an advance is shown by the fact that the little swimming bodies which fail to fuse with the egg cell, do not develop into new plants as in Ectocarpus nor does the egg cell, which has failed to fuse with a swimming body, germinate.

In the Bladder Wrack, a very similar process takes place. There are, however, certain important differences, differences which show that the plant is still more highly developed. If we examine the cavities, in the little warts of the Bladder Wrack we shall find that some of them contain egg cells, some contain male organs but none contain both. We noticed that the egg cell of the Channelled Wrack produced two eggs, that of the Bladder Wrack produces eight. In other respects the two plants behave similarly.

The methods of increase amongst the red seaweeds are rather more complicated and as our object is to interest and not to puzzle our readers we will content ourselves with a few general remarks. Microscopists who are anxious to probe more deeply into the subject will soon devise ways and means for themselves. The little swimming bodies which lend an added attraction to the study of the brown seaweeds are replaced, amongst the red Algæ, by organisms with but one whip-like structure apiece and that without the power of propelling its owner through the water. As with Ectocarpus, increase may take place in two ways. On these red plants we may find the now familiar swellings, which we have learned to know are spore cases, but instead of the multitudes of free swimming organisms which are set free on the bursting of the brown sea weed spore case, we now witness the expulsion of but four inert spores, which settle down in the water and immediately grow into new plants.

In the second method of increase, where male and female organs are concerned, we find that both these structures grow on the outside of the plant and not in cavities. Let us take the common, pink, much branched seaweed, known by the fearsome name of Callithamnion Corymbosum as our example. The male organs grow in little fungus like tufts about the branches of the plant and they give off enormous numbers of little organisms which have no power of swimming to the female organs. Either on the same or on another plant we shall find the female organs; we need not describe them in detail but there is one point of very great interest. From each of the female organs there grows a long jelly-like hair. As we have remarked, the organisms set free by the male organs cannot swim about but float aimlessly in the water. Obviously the majority of them simply perish, one perchance may touch a sticky hair to which it adheres, with which it fuses and passes down to the female cell, resulting in the production of a new seaweed.

It may be remembered that in writing about the pollen grains of flowering plants, we mentioned that those plants dependent upon wind for the distribution of their pollen, have stigmas ingeniously contrived for catching and retaining the grains. It is curious that the red seaweeds should have very similar contrivances for capturing and retaining the male cells.

The sea will also provide us with a rich harvest of those beautiful microscopic objects, the Diatoms. They may be sought on seaweeds, their yellowish brown colour often betrays their whereabouts, on rocks and sand and in mud. The salt water forms are as varied and as beautiful as their fresh water cousins.