R. Fraser Bastow, F.R.M.S.
The REV. R. FRASER BASTOW'S papers on diatoms have been published recently by several British Scientific Societies; and in collaboration with DR. FR. HUSTEDT, the greatest diatomist of all time, as well as with the cooperation of the British Museum and the RoyalSociety, he has good reason to hope that "THE FRESHWATER DIATOM FLORA OF THE BRITISH ISLES" will soon be available for worldwide distribution. Dr. Hustedt has promised that this will be one of his greatest works.
It is probable that everyone, who possesses a microscope and a few slides, has diatoms in his collection, and knows something at least of the skill that is required to show them to the best advantage. In most cases they will be slides that have been bought, and which were probaply made by some professional mounter many years ago, the prices now paid for them being in no way commensurate with their original value, which would have to provide a fitting reward for so much skill and patience. There may yet be one or two enthusists, here and there, who delight in setting up these exquisite creatures in similar array, but let it not be supposed that this is the ideal of every diatom enthusiast.
I am much too impatient, and much too clumsy, ever to become an artistic mounter; my choice is in another direction. The collection and identification of diatoms is a matter of first rate importance to the study of ecology, which study is an urgent requirement of modern times. Data, respecting the distribution of these ubiquitous organisms, is likely to provide a wealth of information in the field of botanical research. But besides this, it might be difficult,to name a science that lends itself so easily to ecological study. The latest phase in the study has to do with environment.
The abode of diatoms is wherever moisture is found: and the presence of species has much to do with its chemical constituents and physical condition. If records, therefore, are intended to be of the utmost value, they should in every case give the chlorine (salt) and acidity values (PH.) of the waters in which they are found. These appear to be the two prime factors of their environment and their values are easy to obtain.
Then there are also distinct genera and species to be found in mosses, others on the roots of hepaticae and ferns, some also on dripping rocks; but their principal habitat is amongst the algae, wherever these are found.
The latest researches have classified the diatoms as having about eighty-five genera; the species, varieties and forms are of course multitudinous, but, carrying the observations along certain well marked lines, the differentiation of the genera is not at all difficult, nor, in general, that of the species and forms.
But it is only fair to say that the identification of some diatoms will almost baffle the ingenuity of the most patient and 'careful observer. To the advanced student this may be one of their greatest charms.
It has long been the practice to specify diatoms as marine, brackish, freshwater, fossil, sub-fossil, sub-aerial etc., but it is doubtful whether these appellations are altogether justifiable, and whether such particular distinctions can be maintained, except in so far as any have never been known to exist in living form elsewhere.
Marine diatoms are probably more met with in collections, because many of the popular species are easier to mount in picturesque form, and also able to be resolved in moderate fashion with inexpensive objectives. The true resolution of many freshwater species still requires even better objectives than are yet to be acquired, and still defy even the most up-to-date methods of illumination and observation. There is much to be said, however, for grouping the diatoms together into one whole, irrespective of their peculiar habitats.
Localities of fossil diatoms in the British Isles are very few indeed; none have been observed in Devon, where I have been working. This paper therefore has no bearing whatever on that which appeared in Vol. 4. , No. 2. of these notes.
It should not be assumed that a characteristic diatom flora is anywhere to be found, though one region will most certainly be found to be richer than another, with regard to the species of certain genera. The local conditions that may be supposed to give rise to abnormal forms are always very interesting. There must be a very mysterious cause for certain species being particularly subject to abnormal growth, whilst it is scarcely ever found in the vast majority of species. It may be that some species are naturally fitted to survive, even with abnormal growth, in an environment where others would quickly perish, but what this particular endowment may be is at present a mystery.
Identifying diatoms necessitates having one or two books of reference. The Diatomaceae of Philadelphia and Vicinity by Boyer, and Bacillariophyta (Diatomeae) by Hustedt are to be particularly recommended. These two works will be found to embrace very many of the marine and freshwater species, and they have copious illustrations, that are generally true to type. The recognition of the genera will soon be mastered; their distinctive features will soon be realized; and though the same may truly be said of the species, these have, however, the quality of running in and out of each other, that is as troublesome as it is interesting to the recorder.
The beginner could not do better than stand on the margin of some pond, pour a tumblerful of the water into the pond from a height of a yard or so, and take up a tumblerful of the cloudy water that results.
This is almost sure to contain several genera and species of diatoms. In order to rid it of much of the unwanted matter, pour it into another glass through a filter, such as those that are used in cream separating machines. A pad of three or four of these cotton filters will allow most of the diatoms to pass, keeping back much of the mud.
In such condition the microscope would only be able to reveal the outward forms of a few species. In order to identify them, they will have to be cleaned.
When a sufficient quantity of the unwanted matter has been eradicated, the water, in which the diatoms are suspended, must be well acidified with sulphuric acid. Then add a few grains of potassium permanganate and stir well. It will simplify matters if a bottle of dissolved permanganate is kept available. If the pink diatom suspension should clear in the course of a minute or two, add a little more permanganate, and repeat if necessary until the sulution shows no further signs of clearing. Then bleach with a few grains of oxalic acid; add a few drops of strong ammonia; and proceed to separating and washing the diatoms. This is done by allowing the diatoms etc. to settle for a few minutes, pouring off the water, and adding fresh, repeating the process at least half a dozen times.
Great regard must be given to avoiding the possibility of diatoms from one gathering appearing in a,nother, which they are bound to do unless precautions are taken. That is why I use tumblers; they are easily cleaned. But the principal offender is the dipping rod, which must always be avoided. Instead, use an ordinary drinking straw, flattened between finger and thumb at the end that is to be dipped into the suspension. The straws may be cut in two in the interests of economy, and those of cellaphane are best. Of course every one is destroyed immediately after use.
By this means, as much of the suspension as needed may be spread on a cleaned cover glass, and quickly evaporated to dryness. The heat applied for this purpose should never be great, or the liquid will boil, and the diatoms will no longer remain spread, but rather massed together, and of course utterly unrecognisable. Have ready at hand a bottle of mounting fluid.
Now warm a glass slip and place in the middle of it a drop of mounting fluid, immediately putting on the cover-glass, diatoms downward. The mounting fluid Will quickly spread and occupy the whole space of the coverglass, and it must then be boiled vigorously for about half a minute, at the end of which, the cover-glass and slip may be clamped together with a very weak wire spring, and allowed to cool.
The mounting fluid will have set hard, and after scraping off any surplus gum, the slide will be ready for examination.
At this stage let me say that I always use Sirax as a mounting fluid, and well diluted with anhydrous toluine. This fluid is very convenient; it has high refractive index and is supposed to be fairly free from the crystallising habit. The slight pressure that the wire spring exerts whilst the mount is hot, almost completely eradicates the chances of air bubbles in the finished slide.
This is, of course, not the best method of cleaning diatoms; it will not separate them like boiling in acid, but it is certainly the most convenient, and a method that anyone can operate without the slightest objection. It is applicable in all cases of fresh gatherings, whether they are washed from stones by syringing into a suitable receptacle, or shaken from algae, liverwort, or mosses in a suitable sized bottle and a little water. In such cases the cotton-wool filtering pads will be serviceable.
It can be claimed for the modern classification of diatoms that it is a good deal more than phenomenal. That suggested by Hendey (Plankton Diatoms of the Southern Seas, 1937, pp. 202-5) claims to show the systematic arrangement of the genera. This arrangement shows the diatoms as a class of Algae, Bacillariophyceae, comprising one order, Bacillariales, which is divided into ten sub-orders.
The species are all according to certain types that have been figured and described by various authors from time to time; so that when any are referred to, it is necessary to state also the name of the author which will sometimes be enclosed in brackets, thereby signifying that a later and probably more correct description has been made by the author whose name will follow. This proceedure renders the nomenclature somewhat complicated, but it affords a wealth of interest to those who have the necessary literature.
Most diatoms have a semi-obscure organ, the raphe, which not only might serve the organism as the channel of nutrition, but may be also its means of locomotion; for only those that have a raphe have this latter faculty. In most cases also this organ, whether it is present or not, whether on both valves of the organism, or only on one, whether it is straight or curved, simple or convex, whether the cross-markings (striae) relative to it reach it or not, and its general appearance taken as a whole, all these are factors of use in determining to what genus or species a diatom may be said to belong. The raphe is not always centrally placed: In Cymbellae its position is always asymetrical, though sometimes scarcely noticeably so; in Nitzschiae it is in very close proximity to one edge of the organism, forming a sort of a kiel; in Cymatapleura and Surirella it traverses the whole organism in somewhat close proximity to its outer edge, and appears to have appendages travelling inwards. It is worth while making a close study of the raphe, as its nature, form and relative position are of so much importance in diatom identification.
Though, in general, diatoms keep shape to certain definite type forms, they are subject in more or less degree to varying shapes and proportions; and though the striae are generally of fixed frequency for all respective species and varieties, such frequencies cannot always be reckoned on. In diminishing sizes of frustule the striae are said to become closer together. The frequency is reckoned as so many striae in ten microns; but certain variations are always allowed, as the frequency nearly always varies in different parts, becoming very much closer as the apex of the diatom is being reached.
In closing these few general remarks, I would say how utterly impossible it would be in a paper of this kind to gave any useful advice for the identification of species in particular; but I hope that it will succeed in whetting the appetite of some microscopists for a study of the science, which is not only a pleasurable pursuit, but one also that is full of interest and surprises, so needful and deserving of a greater following.
(To be continued)