Popular Science Monthly/Volume 41/June 1892/What are Diatoms?
|WHAT ARE DIATOMS?|
By EMILY L. GREGORY,
OF BARNARD COLLEGE.
SINCE the microscope has become so familiar in our homes and ordinary places of resort, many terms are frequently heard which have an unfamiliar sound. For example, a lady asked the other day, with a laugh over the open confession of ignorance: "What are diatoms? I hear the word used very frequently, and with such an air of acquaintanceship and familiarity, that one must suppose they are the most common, every-day affairs, and yet I must confess I have never seen one and don't know really what they are."
Thinking possibly there might be others interested in a brief description of this curious plant, the following story is told of a visit paid this summer to a gentleman said to know all about diatoms. The plants in question are so small as to be seen only with the aid of the microscope; those of ordinary size, when magnified about three hundred and fifty diameters, appear about a quarter of an inch long. Others are much larger. They are curious little plants with a silica shell, which, in certain places, is provided with little apertures through which living parts of the plant protrude. In this way they are enabled to move about freely in the water by which they are generally surrounded, for, though they are not all strictly water plants, they all need considerable water to enable them to thrive, and so are always found in wet places.
Owing to their freedom of motion they were at one time supposed to be animals. Now it is known that they are plants, as they can perform all the functions of plants, and no animal, with all his superiority, high nature, etc., is able to do this. They are found everywhere in all inhabited countries, and in fact all over the seas, so it may be readily granted that a plant so common and wide-spread as this should be quite familiar to every one.
Again, not only are the living plants so wide-spread and common, but the shells of the dead ones remain intact for many years; and in certain localities these tiny shells are so numerous as to form a large portion of the soil. Some of the best known of these localities are the sites of Richmond, Va., and Berlin in Germany. It is often said that the city of Berlin rests on a foundation of
|Fig. 1.—Pleurosigma Formosum.||Fig. 2.—Pinnularia Major.||Fig. 3.—Stauroneis Phœnicenteron.||Fig. 4.—Navicula Didyma.|
diatom shells. The little plant dies and decays, leaving the shell, which retains its shape for many years. These cells are most beautifully marked with very delicate tracery. No tools can be made to perform such work as this. Some shells with the most regular forms of markings are used for testing lenses, such as Pleurosigma, shown in Fig. 1. Some of the most common forms are represented by Figs. 2, 3, and 4, while another less frequent and with more curious markings is shown in Fig. 5.
Now, though it is so easy to obtain large numbers of these plants—only a spoonful of mud from the bank of a stream or edge of a pool, a bit of sea-weed thrown up on the shore will contain thousands of them and a great many different forms—though it is quite easy to find them almost everywhere, it is still very difficult to make out their manner of existence. For example, how they perform the feat of locomotion is not well understood. There are two ways of explaining this: one is, that the diatom moves from place to place, owing to the osmotic changes constantly taking place inside the shell; the other and perhaps better authenticated opinion is connected with the peculiarity already referred to—that is, the presence of little apertures in the wall through which portions of the protoplasmic contents protrude. Those who believe in the osmotic theory claim that no such apertures exist, and consequently no protoplasm finds its way to the outside of the shell. In connection with this point comes the story of the visit.
While working in the Botanical Laboratory of Berlin this past summer, the writer was invited to visit a gentleman having the reputation of knowing more about diatoms than any other person now living. It is rather a strange fact that this gentleman is not a learned professor who has spent a long life over scientific problems, but a retired book-seller who owns a beautiful villa in the suburbs of Berlin, and has for many years been gathering information of various kinds about this wonderful little plant. He has nearly all the literature treating this subject, several large volumes of which are now out of print, and for which he told me he had been obliged to pay exorbitant prices.
Before proceeding to the inspection of the laboratories, specimens, models, etc., coffee and cakes were served in the garden, a distinctively German hospitality which no scientific interests are allowed to interfere with. We then began in the preparing laboratory, a small but very completely fitted room, where the material for investigation is stored, treated, and classified for use. Here are the chemicals used in preparing the plant for examination. Some processes serve to preserve the form and general structure of the living part within the shell, so that this may be studied; other reagents, on the contrary, destroy the living portion, whereby the shell may be more easily examined. In this laboratory was a microscope of somewhat older style than our recent ones, but a very good, reliable instrument, which he told me he used in the coarser manipulations, but, owing to the presence of chemicals in this room, none of his finer instruments were kept here. He explained his methods of treatment in clearing the soil and dirt from the land specimens, and also showed me the little silk nets which he uses when fishing for the water forms. Certain kinds grow only on the surface of the sea; so, to collect these, it is necessary to go out in a small boat and row very slowly, for the cloth of which the net is made must be very fine in order to prevent the escape of the tiny plants through its meshes. Owing to this extreme closeness of the meshes, the water drains through very slowly. The form of the net is quite like those used to catch butterflies, but is held with the handle up and close to the boat. At very short intervals of time it must be taken up and the water poured out; the inside of the cloth is then carefully rinsed in clear water, which is kept in a jar or bottle for that purpose. The most difficult part of the process is to row slowly and steadily enough to prevent tearing the nets.
These diatoms found on the surface of the water are furnished with long arms or projections, from which protrude hairlike bodies, which apparatus he conjectures is for the following purpose: One means of deciding that this little organism is a plant, is that it performs the function of assimilation, as it is called, by which it gives out oxygen. Now, this gentleman thinks the little hair-like outgrowths are for the purpose of holding the oxygen in their meshes so as to enable the plant to float.
After looking through this room and learning as much as possible about his methods of treatment, we went into another much larger and more elegantly furnished apartment, where all the nice and delicate work of studying forms and making models was done. Here were kept the books, all that have been written on this plant, and they filled a case of considerable size; also a very complete collection of microscopical apparatus. All that modern artisans can do in the way of fine and delicate instruments may be found here. Nothing less than the best oil-immersion lenses can be used in the study of form necessary to understand the inner structure of these plants. Several models have been made by this gentleman, and he told me of the hours of patient labor which it cost to bring out so much, as one little turn of the inner canal whose windings hold the living and active part of the plant, and also the explanation of the manner in which it moves. He has studied the mechanism of several forms and made models of plaster of Paris, and others of wire. Pinnularia major (see Fig. 2) is the plant from which the most conclusive results were obtained, and he claims to have demonstrated the existence of apertures on the surface of the shell through which the protoplasm may protrude. He does not, however, claim to have actually seen the protoplasm on the outside of the shell, but holds that, according to other known facts, it must be forced out, though in very small quantities. These apertures do not open directly into the interior, but by a series of winding canals whose action prevents the too easy expulsion of the contents. The movement of this protoplasm along the lines between the openings causes the movement of the diatom in a similar manner to the action of the fins of fishes. All this labor, after all, has reference only to a certain class of these plants; there are many others of such different forms that much study will yet have to be expended on them before their secrets are laid bare.
There are some curious little forms which grow in clusters on stem-like bodies which are often fastened by their other extremities to some object in the water. Some of these are shown in Figs. 6 and 7; and, finally, a variety of miscellaneous forms may be seen in Fig. 8.
There are large collections of these plants in nearly all the large herbaria of Europe, and the manner of preparing them for such collections may almost be said to form a special branch of industry. Experts are able to mount and arrange in order hundreds of these little organisms under a circular cover-glass of about five eighths of an inch in diameter. The dexterity which these experts acquire in the use of instruments is something almost as marvelous as the organisms themselves. It must be remembered, however, that this mechanical labor has nothing to do with the work of the scientist who studies the plant. It would be impossible for an investigator to give enough time to enable him to acquire this skill. A gentleman in Wedel, Holstein, has acquired a great reputation in this kind of work, and has plates holding from four to sixteen hundred different forms. These cost from twenty dollars upward, and he has recently finished a plate on which are mounted four thousand diatoms, with which there is a printed catalogue. This single plate is the result of four years of continued labor, and the price he fixes for it is twenty thousand marks, or about five thousand dollars. I inquired if there was any probability of the owner selling such an expensive collection, and he said very quickly: "Oh, yes! Some rich English or American gentleman will probably purchase it; no German ever will." According to some of the latest systematic
Fig. 8. Microscopic View of Richmond Infusorial Earth. (By Ehrenberg.)
authorities, there are about eight thousand different species so this plate may be considered as representing half of the known species.
It may be doubtful whether this story will help relieve the embarrassment of those who do not exactly understand what a diatom is. It serves to show, however, that very few people do know all about it; and this, together with the thought that it is considered of sufficient importance to warrant spending years of patient labor over it, will surely prove a consolation to those who have been puzzling over the meaning of the word.