Diatomaceae of Philadelphia/Morphology and Development
MORPHOLOGY AND DEVELOPMENT
THE CELL
The cell membrane is composed of two usually equal parts, each of which consists of a valve and a girdle or zone formed of cellulose modified by silica deposited in an insoluble state from a very dilute aqueous solution. The valves are more siliceous and robust than the girdle. Both are in most species easily separable, or at least the bands of the girdle which may be more or less closely fastened to the valves have a motion over each other permitting the cell to enlarge at pleasure. The longitudinal diameter of the cell, or the distance between the centres of the two valves, will vary according to the convexity of the valve and the age of the frustule which may be often determined by the width or number of the girdle bands. These, owing to their diversity of form and arrangement, will be further described under the generic diagnoses.
The siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleoderm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less understood bodies.
The Cytoplasma is a thin skin of colorless plasma covering the entire inner surface of the cell. It is invisible in the living cell but is evident in plasmolysis. In long forms it is thickened at the ends and is condensed at the plasma bridge which frequently connects the two valves and divides the cell into two parts, each containing more or less protoplasm surrounding the vacuole in which are found the cell-sap and certain granules. In some forms, as Meloseira, the cytoplasma includes the entire mass of protoplasm.
The Endochrome is seen in the form of one or more bands or plates, of a yellowish or brownish color, on the inner side of the valves or connective zone, or in granules or irregular masses, more or less numerous, on the inner walls, or sometimes grouped near the centre. It consists of a mixture of chlorophyll and diatomine which differ in their relative solubility in alcohol and in their spectroscopic analyses. The color varies from green to a chocolate brown in proportion to the amount of diatomine. So far as the function of the endochrome is concerned it does not appear to differ from that of ordinary chlorophyll, absorbing, under the influence of light, the carbon, and disengaging the oxygen of the carbonic anhydride in the water. Diatoms do not live in absolutely pure or non-aërated water. The individual plates or granules of the endochrome are called chromatophores. Their number and significance will be referred to in the description of genera.
The Pyrenoids.—In the chromatophores of many species are found colorless, homogeneous bodies, strongly refractive, of various shapes, usually lenticular or fusiform, which are known as Pyrenoids (Schmitz). They are scarcely evident in the living cell, but are distinguished by the action of hæmatoxylin and other reagents. Flat forms occur in Surirella and Pleurosigma, lens forms in Pinnularia, Stauroneis, Synedra, Fragilaria and Nitzschia, while a spherical form is found in Cymbella cuspidata. The pyrenoids are always imbedded in the chromatophore. Their growth is by division. Schmitz considers them a part of the living chromatophore, and their substance as working material which in excess has become resolved into the nature of a crystal which its form sometimes resembles. Comparisons are made between them and crystalloids found in certain monocotyledons. The pyrenoid is evidently concerned in the formation of the chromatophore, or in its division. Much of the conjecture, however, is due to the behavior of pyrenoids in other plants.
Oil Globules.—It has been established by Pfitzer that starch and sugar, as assimilation products, are replaced by oil in the cells of diatoms ("da bekannlich Staerke und Zucker bei den Bacillariaceen nicht nachzuweisen sind"). The oil drops are more or less numerous, of various sizes, and are found in the cytoplasma, the cell-sap, and sometimes the chromatophores. Mereschkowsky describes certain globules as elæoplasts, which he divides into four kinds according to their number and position. Whether all of these are oil globules is a question not yet determined.
Other bodies, known as "Buetschli granules," or volutin, and described as "little blisters filled with a tolerably robust refractive substance," are considered by Lauterborn to be a nitrogen reserve store. They are found in the cytoplasma, or in the cell-sap, and can be fixed in picric acid and stained in methylene blue.
Note.—For a discussion of the morphology of diatoms and a valuable résumé of the investigations of Buetschli, Karsten, Lauterborn, Mereschkowsky, Mueller, Pfitzer, Schuett, and others, the student is referred to "Der Bau der Diatomzelle," by Dr. Otto Heinzerling, in "Bibliotheca Botanica," 1908.
Cell Division
The growth of diatoms follows the usual method of cell division as described by Sachs (Text Book of Botany, 2nd ed., p. 16): "The nucleus of a cell which is about to divide becomes broader, assuming the form of a biconcave lens, and its nucleolus breaks up into irregular granules which together with its other granular contents begin to form a nuclear disc in the equatorial plane. A delicate striation is now apparent in what is becoming the long axis of the nucleus, at right angles to the nuclear disc, and the characteristic nuclear spindle is gradually produced. The nuclear disc splits into two halves lying side by side, each of which travels to the corresponding pole of the nucleus; thus two nuclei are constituted which are connected by fibrillæ."
The cell-wall and the chromatophore bands divide, each nucleus passes to the centre, and two new cells are formed. In the meantime, to permit of this division, the two siliceous valves separate, the girdle bands slipping over each other, and opposite the larger or enclosing valve a new valve is formed, the girdle band of which is seen later within the girdle of the mother valve. Opposite the smaller valve of the original cell and adjoining the new valve, another valve is formed which also produces a girdle within the girdle of the smaller valve. As a result of division we have, therefore, the valves of the original, or mother cell, the two new valves and four girdle bands. (Pl. 40, Figs. 18 and 19.)
In the process of division, the continual formation of new valves, enclosed in the older girdle bands, will naturally cause a reduction in the size of the frustule. While this reduction, owing to the elasticity of the girdle, does not always occur, I believe, yet, in most cases, the diameter is so reduced that a rejuvenescence of growth is required. This is caused by the production of auxospores which may appear without conjugation. In this process, the beginning of which, in certain species, may be noticed by the increase in the size of the girdle as in reduplication, the two valves separate and within is formed a more or less spherical mass about twice the size of the original frustule and which forms on its circumference two large and often shapeless valves. These valves form others which assume the appearance of the original valves, but larger, and proceed to grow in the usual way. The reduction in size of the frustule seldom proceeds further than about half the size of the type form, so that, as a general rule, it may be stated that diatoms are not often smaller than half the larger size.
Reproduction
The process of reproduction has been observed in many cases, but the conclusions reached are somewhat at variance with each other. The auxospore formation is simply a method of rejuvenescence. When, however, the auxospores are thrown off from filamentous diatoms, it is probable that two may conjugate, their contents dividing each into two daughter cells which unite into two zygospores. The usual method is the union of two frustules, which, throwing off the old valves, coalesce into a single mass of protoplasm which produces an auxospore, sometimes called a sporangial frustule. It is stated that in some cases two frustules coalesce and produce two auxospores.
The existence of spores in diatoms is a much-disputed point. While they have never been seen, the inference that they exist is very great, as otherwise it becomes difficult to understand the sudden growth of species in localities and under conditions that seem to preclude the actual presence of the living frustule. It is a matter of common observation that, in examining collections of living forms, minute frustules or brownish globules appear to resemble larger diatoms. In gatherings of Gomphonema, when many specimens are sessile on the same object, numerous intermediate sizes, varying from minute globules to the type, are seen, yet not positively demonstrable as the same.
Conjugation, the formation of auxospores, and the actual process of cell division are seldom seen, as they occur during the night or at least in darkness. It is advisable in order to observe reduplication to obtain the material about midnight and place it in very dilute alcohol. In filamentous forms, however, the cell division is easily observed at any time in its various stages. By immersing in picric acid (saturated solution), transferring to very dilute alcohol which is gradually increased in strength, and then passing through oil of cloves and finally to the mounting medium, excellent preparations can be made. By staining with gold chloride alone the nucleus is made apparent without further treatment.
Evolution of Forms
It may be assumed that diatoms originated in the sea; to deny this requires evidence of the existence of fresh-water species previous to the Miocene period which is entirely marine. In those subject to fluctuations of the waves, as pelagic diatoms, their existence appears to be contingent upon the methods by which the separate frustules can cohere. Various devices, including hooks, spiral bundles, horns and processes exuding threads of plasma, exist for holding together the frustules. When marine forms are found in quiet waters some of these devices, being no longer of any value, cease to grow, although free swimming diatoms are rare. They either occur in long chains or are stipitate or sessile. If it is further assumed that the fresh-water diatoms are found in greater abundance in later periods, the action of running streams makes necessary the provision of some means by which the species may continue to colonize. This may be recognized in the occurrence of linear forms chiefly in streams. Circular forms, such as Cyclotella which have no raphe, are found in quiet waters, such as pools or ditches, and never exist living in running streams. Those forms only would be able to live in water having a more or less swift current under one of three conditions: they must, as in Gomphonema, be adherent to surrounding objects by a stipe; or be enclosed in a gelatinous tube, as in Homœocladia; or have an independent motion powerful enough to overcome the influence of the current. It is true that many forms with a raphe have no apparent motion. In the case of Mastogloia provision is made in a gelatinous cushion in which the frustules are preserved. In Cocconeis, with a true raphe in one valve only, in Epithemia, with a partial raphe, or in certain Eunotiæ with a trace of one, we find species evidently degenerate and parasitic. The long Synedræ, having only a median line, live in running streams, since they are attached at one end to other algae. Forms with a true raphe appear to be more highly developed, since they are able to seek locations favorable to growth. Given, therefore, the structure of the valve, the habitat may be inferred.
The Motion of Diatoms
The erratic backward and forward movement of certain diatoms, especially those of the Naviculoid group, or the slow, rolling motion of Surirella, has been discussed in so many ways without definite conclusions that a brief statement will be sufficient. Osmosis, the amœboid movement of the coleoderm, the protrusion of protoplasm or protoplasmic threads through the raphe, the existence of actual organs of locomotion or cilia, and the lack of synchronism in the chemical action occurring at the ends of the cell which is sometimes divided by the plasma bridge, have been offered in explanation. The chief objection to the theory of cyclosis appears to be that the resultant motion is so greatly in excess of the rotation of protoplasm in the cell. More or less motion is observed in various kinds of free cells, but the movement of diatoms is not evident in those without either a raphe or a keel upon which and apparently by which the phenomena are produced.
Mr. T. Chalkley Palmer, in various articles in the Proceedings of the Delaware County Institute of Science, especially in Vols. 1 and 3, gives the results of exhaustive experiments. "Nothing, it would seem," he says, "could be more conclusive as to the essential sameness of the nature of motion in monads and diatoms, than the fact that both monads and diatoms require oxygen in order to perform motion, that they come to rest when oxygen becomes scarce, and that they resume their motion when oxygen is again supplied."
He also thinks "that the living substance of the cell, more or less deeply overlaid with coleoderm substance of varying consistency, and itself assuming that degree of fluidity which best meets the requirements of the situation, permeates the raphes, circulates in the keels, or in some cases protrudes quite beyond the silica, and functions as the actual propulsive agent."
The Function of Diatoms
Of all forms of vegetation, the Diatomaceæ are, perhaps, the most ubiquitous. Where-ever a sufficient amount of moisture, heat and light are found, they grow. It was during the Miocene period that they first appeared, and, as marine forms, reached their greatest development, both as to size and beauty of marking, while their prevalence throughout the world in enormous quantities has been often mentioned. The Miocene beds of Richmond and Maryland continued over the Cretaceous formations of New Jersey have outcropped in certain localities within our district, but are not considered in this discussion.
The function of diatoms is not essentially different from that of other algæ in providing food for aquatic animals, such as Salpæ and oysters, but it is, however, in other respects that they are not only important but necessary factors in the preservation of life.
"Full nature swarms with life; one wondrous mass
Of animals, or atoms organized,
Waiting the vital breath, when parent heaven
Shall bid his spirit blow. The hoary fen,
In putrid streams, emits the living cloud
Of pestilence. Thro' subterranean cells
Where searching sunbeams scarce can find a way,
Earth animated heaves."
I am not certain if Thomson fully understood the matter, but he has remarkably described the facts. When "the vital breath" of returning spring animates the earth, the "subterranean cells" of diatoms, the "atoms organized," through the liberation of vast quantities of oxygen, immediately begin the purification of the "putrid streams." Were these streams not so purified, the accumulation of animal and vegetable débris would eventually cause an enormous bacterial growth fatal to animal life.