Common Objects of the Microscope

CHAPTER VI

Algæ and their Growth—Desmidiaceæ, where found—Diatoms, their Flinty Deposit—Volvox—Mould, Blight, and Mildew—Mosses and Ferns—Mare’s-Tail and the Spores—Common Sea-weeds and their Growth.

On Plate IV. will be seen many examples of the curious vegetables called respectively algæ and fungi, which exhibit some of the lowest forms of vegetable life, and are remarkable for their almost universal presence in all parts of this globe, and also almost all conditions of cold, heat, or climate. Many of them are well known under the popular name of sea-weeds, others are equally familiar under the titles of “mould,” “blight,” or “mildew,” while many of the minuter kinds exhibit such capability of motion, and such apparent symptoms of volition, that they have long been described as microscopic animalcules, and thought to belong to the animal rather than to the vegetable kingdoms.

Fig. 1 represents one of the very lowest forms of vegetable life, being known to the man of science as the Palmella, and to the general public as “gory dew.” It may be seen on almost any damp wall, extending in red patches of various sizes, looking just as if some blood had been dashed on the wall, and allowed to dry there. With a tolerably powerful lens this substance can be resolved into the exceedingly minute cells depicted in the figure. Generally, these cells are single, but in many instances they are double, owing to the process of subdivision by which the plant grows, if such a term may be used.

Fig. 2 affords an example of another very low form of vegetable, the Palmoglæa, that green slimy substance which is so common on damp stones. When placed under the microscope, this plant is resolvable into a multitude of green cells, each being surrounded with a kind of gelatinous substance. The mode of growth of this plant is very simple. A line appears across one of the cells, and after a while it assumes a kind of hour-glass aspect, as if a string had been tied tightly round its middle. By degrees the cell fairly divides into two parts, and then each part becomes surrounded with its own layer of gelatine, so as to form two separate cells, placed end to end.

One of the figures, that on the right hand, represents the various processes of “conjugation,” i.e. the union and fusion together of two cells. Each cell throws out a little projection; these meet together, and then uniting, form a sort of isthmus connecting the two main bodies. This rapidly widens, until the two cells become fused into one large body. The whole subject of conjugation is very interesting, and is treated at great length in the Micrographic Dictionary of Messrs. Griffith and Henfrey, a work to which the reader is referred for further information on many of the subjects that, in this small work, can receive but a very hasty treatment.

Few persons would suppose that the slug-like object on Fig. 3, the little rounded globules with a pair of hair-like appendages, and the round disc with a dark centre, are only different forms of the same organism. Such, however, is the case, and these are three of the modifications which the Protococcus undergoes. This vegetable may be seen floating like green froth on the surface of rain-water.

On collecting some of this froth and putting it under the microscope, it is seen to consist of a vast number of little green bodies, moving briskly about in all directions, and guiding their course with such apparent exercise of volition that they might very readily be taken for animals. It may be noticed that the colour of the plant is sometimes red, and in that state it has been called the Hæmatococcus.

The “still” state of this plant is shown in the round disc. After a while the interior substance splits into two portions; these again subdivide, and the process is repeated until sixteen or thirty-two cells become developed out of the single parent-cell. These little ones then escape, and, being furnished with two long “cilia” or thread-like appendages, whirl themselves merrily through the water. When they have spent some time in this state, growing all the while, they lose their cilia, become clothed with a strong envelope, and pass into the still stage from which they had previously emerged. This curious process is repeated in endless succession, and causes a very rapid growth of the plant. The moving bodies are technically called zoospores, or living spores, and are found in many other plants besides those of the lowest order.

IV.

IV.

On Fig. 13 is delineated a very minute plant, called from its colour Chlorococcus. It may be found upon tree-trunks, walls, etc., in the form of green dust, and has recently been found to take part in forming the first stage of lichens.

A large and interesting family of the “confervoid algæ,” as these low forms of vegetable life are termed, is the Desmidiaceæ, called in more common parlance desmids. A few examples of this family are given in Plate IV.

They may be found in water, always preferring the cleanest and the brightest pools, mostly congregating in masses of green film at the bottom of the water, or investing the stems of plants. Their removal is not very easy, but is best accomplished by very carefully taking up this green slippery substance in a spoon, and straining the water away through fine muslin. They may also be separated by allowing a ring, covered with muslin, to float upon the surface of the water collected in a jar, for, being great lovers of light, they assemble where it is most abundant. An opaque jar should be used. For preservation, glycerine-gelatine seems to be the best fluid. A very full and accurate description of these plants may be found in Ralfs’ British Desmidieæ.

Fig. 4 represents one of the species of Closterium, more than twenty of which are known. These beautiful objects can be obtained from the bottom of almost every clear pool, and are of some interest on account of the circulating currents that may be seen within the living plants. A high power is required to see this phenomenon clearly. The Closteria are reproduced in various ways. Mostly they divide across the centre, being joined for a while by two half-cells. Sometimes they reproduce by means of conjugation, the process being almost entirely conducted on the convex sides. Fig. 5 represents the end of a Closterium, much magnified in order to show the actively moving bodies contained within it.

Fig. 16 is a supposed desmid, called Ankistrodesmus, and presumed to be an earlier stage of Closterium.

Fig. 6 is a very pretty desmid called the Pediastrum, and valuable to the microscopist as exhibiting a curious mode of reproduction. The figure shows a perfect plant composed of a number of cells arranged systematically in a star-like shape; Fig. 15 is the same species without the colouring matter, in order to show the shape of the cells. The Pediastrum reproduces by continual subdivision of the contents of each cell into a number of smaller cells, termed “gonidia” on account of their function on the perpetuation of the species. When a sufficient number has been formed, they burst through the envelope of the original cell, taking with them a portion of its internal layer, so as to form a vesicle, in which they move actively. In a few minutes they arrange themselves in a circle, and after a while they gradually assume the perfect form, the whole process occupying about two days. Fig. 18 exhibits an example of the genus Desmidium. In this genus the cells are either square or triangular in their form, having two teeth at their angles, and twisted regularly throughout their length, causing the wavy or oblique lines which distinguish them. The plants of this genus are common, and may be found almost in any water. I may as well mention that I have obtained nearly all the preceding species, together with many others, from a little pond on Blackheath.

Fig. 7 is another desmid called Scenedesmus, in which the cells are arranged in rows of from two to ten in number, the cell at each extremity being often furnished with a pair of bristle-like appendages. Fig. 14 is another species of the same plant, and both may be found in the water supplied for drinking in London, as well as in any pond.

A common species of desmid is seen at Fig. 12, called Sphærozosma, looking much like a row of stomata set chainwise together. It multiplies by self-division.

Fig. 17 is a specimen of desmid named Cosmarium, plentifully found in ponds on heaths and commons, and having a very pretty appearance in the microscope, with its glittering green centre and beautifully transparent envelope. The manner in which the Cosmarium conjugates is very remarkable, and is shown at Fig. 19.

The two conjugating cells become very deeply cleft, and by degrees separate, suffering the contents to pour out freely, and, as at present appears, without any envelope to protect them. The mass, however, soon acquires an envelope of its own, and by degrees assumes a dark reddish-brown tint. It is now termed a sporangium, and is covered with a vast number of projections, which in this genus are forked at their tip, but in others, which also form sporangia, are simply pointed. The Closteria conjugate after a somewhat similar manner, and it is not unfrequent to find a pair in this condition, but in their case the sporangium is quite smooth on its surface.

Another very remarkable family of confervoid algæ is that which is known under the name of Oscillatoriæ, from the oscillating movement of the plant. They are always long and filamentous in character, and may be seen moving up and down with a curious irregularity of motion. Their growth is extremely rapid, and may be watched under a tolerably powerful lens, thus giving many valuable hints as to the mode by which these plants are reproduced. One of the commonest species is represented at Fig. 8.

Figs. 9, 10, and 11 are examples of another family, called technically the Zygnemaceæ, because they are so constantly yoked together by conjugation. They all consist of a series of cylindrical cells, set end to end, and having their green contents arranged in similar patterns. Two of the most common and typical species are here given.

Fig. 9 is the Spirogyra, so called from the spiral arrangement of the chlorophyll; and Fig. 10 is the Tyndaridea, or Zygnema, as it is called by some writers. A casual inspection will show how easy it is to distinguish the one from the other. Fig. 11 represents a portion of the Tyndaridea during the process of conjugation, showing the tube of connection between the cells and one of the spores.

We now arrive at the diatoms, so called because of their method of reproduction, in which it appears as if a cut were made right along the original cell. The commonest of these plants is the Diatóma vulgáre, seen in Fig. 21 as it appears while growing. The reproduction of this plant is effected by splitting down the centre, each half increasing to the full size of the original cell; and in almost every specimen of water taken from a pond, examples of this diatom undergoing the process of division will be distinguished. It also grows by conjugation. The diatoms are remarkable for the delicate shell or flinty matter which forms the cell skeleton, and which will retain its shape even after intense heat and the action of nitric acid. While the diatoms are alive, swimming through the water, their beautiful markings are clearly distinct, glittering as if the form were spun from crystalline glass. Just above the figure, and to the right hand, are two outlines of single cells of this diatom, the one showing the front view and the other the profile.

Fig. 20 is an example of a diatom—Cocconéma lanceolátum—furnished with a stalk. The left-hand branch sustains a “frustule” exhibiting the front view, while the other is seen sideways.

Another common diatom is shown in Fig. 23, and is known by the name of Synedra. This constitutes a very large genus, containing about seventy known species. In this genus the frustules are at first arranged upon a sort of cushion, but in course of time they mostly break away from their attachment. In some species they radiate in every direction from the cushion, like the spikes of the ancient cavalier’s mace.

Fig. 24 is another stalked diatom called Gomphonéma acuminátum, found commonly in ponds and ditches. There are nearly forty species belonging to this genus. A pair of frustules are also shown which exhibit the beautiful flinty outline without the coloured contents (technically called endochrome).

Fig. 27 is a side view of a beautiful diatom, called Eunótia diadéma from its diadem-like form. There are many species of this genus. When seen upon the upper surface, it looks at first sight like a mere row of cells with a band running along them; but by careful arrangement of the light its true form may easily be made out.

Fig. 28 represents a very common fresh-water diatom, named Melosíra várians. The plants of this genus look like a cylindrical rod composed of a variable number of segments, mostly cylindrical, but sometimes disc-shaped or rounded. An end view of one of the frustules is seen at the left hand, still coloured with its dots of “endochrome,” and showing the cylindrical shape. Immediately above is a figure of another frustule seen under both aspects with the endochrome removed.

A rather curious species of diatom, called Cocconeïs pedículus, is seen at Fig. 29 as it appears on the surface of common water-cress. Sometimes the frustules, which in all cases are single, are crowded very closely upon each other and almost wholly hide the substance on which they repose. Fig. 30 is another diatom of a flag-like shape, named Achnanthes, having a long slender filament attached to one end of the lower frustule, representing the flag-staff. There are many wonderful species of such diatoms, some running almost end to end like a bundle of sticks, and therefore called Bacillária; others spreading out like a number of fans, such as the genus Licmophora; while some assume a beautiful wheel-like aspect, of which the genus Meridion affords an excellent example.

A very remarkable, and not uncommon, fresh-water diatom is the Bacillária paradóxa. It looks, when at rest, like a broad brown ribbon of varying length. The diatoms lie across the ribbon, on edge, and slide upon each other exactly like the ladders of a fire-escape, so that the broad ribbon is converted into a fine long thread, which speedily closes up again into the original ribbon, and so da capo. The reason for this movement, and how it is effected, is absolutely unknown; indeed, nothing certain is known as to the way in which diatoms move, nor has ever a probable guess yet been made.

The last of the diatoms which we shall be able to mention in this work is that represented on Fig. 31. The members of this genus have the name of Navícula, on account of their boat-like shape and their habit of gliding through the water in a canoe-like fashion. There are many species of this genus, all of which are notable for the graceful and varied courses formed by their outlines, and the extreme delicacy of their markings. In many species the markings are so extremely minute that they can only be made out with the highest powers of the microscope and the most careful illumination, so that they serve as test objects whereby the performance of a microscope can be judged by a practical man.

The large spherical figure in the centre of Plate IV. represents an example of a family belonging to the confervoid algæ, and known by the name of Volvox globator. There seems to be but one species known.

This singular plant has been greatly bandied about between the vegetable and animal kingdoms, but seems now to be satisfactorily settled among the vegetables. In the summer it may be found in pools of water, sufficiently large to be visible to the naked eye, like a little green speck proceeding slowly through the water. When a moderate power is used, it appears as shown in the figure, and always contains within its body a number of smaller individuals, which after a while burst through the envelope of the parent and start upon an independent existence. On a closer examination, a further generation may be discovered even within the bodies of the children. The whole surface is profusely covered with little green bodies, each being furnished with a pair of movable cilia, by means of which the whole organism is moved through the water. These bodies are analogous to the zoospores already mentioned, and are connected with each other by a network of filaments. Reproduction also takes place by conjugation as in other algæ. A more magnified representation of one of the green bodies is shown immediately above the larger figure. The volvox is apt to die soon when confined in a bottle.

Fig. 25 is the common yeast-plant, consisting simply of a chain of cells, which increase by budding, and only form spores when they have exhausted the nutriment in the fluid in which they live. Fig. 26 is a curious object, whose scientific name is Sárcina ventrículi. It is found in the human stomach. Similar forms are often to be found in the air; for instance, a piece of cocoa-nut will exhibit this, and many other kinds of Bacteria and moulds, after a few days’ exposure to the air, preferably in a dark cupboard.

We now come upon a few of the blights and mildews. A very interesting series of forms is first to be alluded to. Upon the bramble-leaf may often be found spots, at first red, then orange, then reddish black. These are known as Œcidium berberidis. Fig. 32 shows the “red-rust” of wheat, the Urédo; and Fig. 33 is the mildew of corn, known as Puccinia. The interest lies in the fact that these three forms are successive stages in the life-history of the same plant. Another species of Urédo, together with a Phragmídium, once thought to be another kind of fungus, is seen on a rose-leaf on Plate V. Fig. 1. On Fig. 10, however, of the same Plate, the Phragmídium may be seen proceeding from Urédo, thus proving them to be but two states of the same plant. There is room for any amount of observation and work in connection with the life-histories of many of these fungi.

Another species of Puccinia, found on the thistle, is shown on Plate V. Fig. 7. Fig. 34 is the mould found upon decaying grapes, and called therefrom, or from the clustered spores, Botrýtis. Some of the detached spores are seen by its side. Fig. 35 is another species of the same genus, termed Botrýtis parasítica, and is the cause of the well-known “potato-disease.”

The mosses and ferns afford an endless variety of interesting objects to the microscopist; but as their numbers are so vast, and the details of their structure so elaborate, they can only be casually noticed in the present work. Fig. 38 represents a spore-case of the Polypodium, one of the ferns, as it appears while in the act of bursting and scattering the contents around. One of the spores is seen more magnified below. The spore-cases of many ferns may be seen bursting under the microscope, and have a very curious appearance, writhing and twisting like worms, and then suddenly filling the field with a cloud of spores. Fig. 9, Plate V., is a piece of the brown, chaff-like, scaly structure found at the base of the stalk of male fern cells, showing the manner in which a flat membrane is formed. Fig. 39 is a capsule of the Hypnum, one of the mosses, showing the beautiful double fringe with which its edge is crowned. Fig. 2, Plate V., is the capsule of another moss, Polytríchum, to show the toothed rim; on the right hand is one of the teeth much more magnified.

Fig. 3, Plate V., is the capsule of the Jungermannia, one of the liverworts, showing the “elaters” bursting out on every side, and scattering the spores. Fig. 4 is a single elater much magnified, showing it to be a spirally coiled filament, that, by sudden expansion, shoots out the spores just as a child’s toy-gun discharges the arrow. Fig. 5 is a part of the leaf of the Sphagnum moss, common in fresh water, showing the curious spiral arrangement of secondary fibre which is found in the cells, as well as the circular pores which are found in each cell at a certain stage of growth. Just below, and to the left hand, is a single cell greatly magnified, in order to show these peculiarities more strongly. Fig. 8 is part of a leaf of Jungermannia, showing the dotted cells.

Fig. 6, Plate V., is a part of a rootlet of moss, showing how it is formed of cells elongated and joined end to end.

On the common mare’s-tail, or Equisétum, may be seen a very remarkable arrangement for scattering the spores. On the last joint of the stem is a process called a fruit-spike, being a pointed head around which are set a number of little bodies just like garden-tables, with their tops outward. One of these bodies is seen in Fig. 40. From the top of the table depend a number of tiny pouches, which are called sporangia; these lie closely against each other, and contain the spores. At the proper moment these pouches burst from the inside, and fling out the spores, which then look like round balls with irregular surfaces, as shown in Fig. 40, c. This irregularity is caused by four elastic filaments, knobbed at the end, which are originally coiled tightly round the body of the spore, but by rapidly untwisting themselves cause the spore to leap about, and so aid in the distribution. A spore with uncoiled filaments is seen at Fig. 40, b. By breathing on them they may be made to repeat this process at will.

Fig. 36 is a common little sea-weed, called Ectocarpus siliculósus, that is found parasitically adhering to large plants, and is figured in order to show the manner in which the extremities of the branches are developed into sporangia. Fig. 37 is a piece of the common green laver, Ulva latíssima, showing the green masses that are ultimately converted into zoospores, and by their extraordinary fertility cause the plant to grow with such rapid luxuriance wherever the conditions are favourable. Every possessor of a marine aquarium knows how rapidly the glass sides become covered with growing masses of this plant. The smaller figure above is a section of the same plant, showing that it is composed of a double plate of cellular tissue.

Fig. 41 is a piece of purple laver or “sloke,” Porphýra laciniáta, to show the manner in which the cells are arranged in groups of four, technically named “tetraspores.” This plant has only one layer of cells.

On Plate V. may be seen a number of curious details of the higher algæ.

Fig. 11 is the Sphacelária, so called from the curious capsule cells found at the end of the branches, and termed sphacelæ. This portion of the plant is shown more magnified in Fig. 12. Another sea-weed is represented in Fig. 13, in order to show the manner in which the fruit is arranged; and a portion of the same plant is given on a larger scale at Fig. 14.

A very pretty little sea-weed called Cerámium is shown at Fig. 15; and a portion showing the fruit much more magnified is drawn at Fig. 22. Fig. 23 is a little alga called Myrionéma, growing parasitically on the preceding plant.

Fig. 16 is a section of a capsule belonging to the Hálydris siliquósa, showing the manner in which the fruit is arranged; and Fig. 17 shows one of the spores more magnified.

Fig. 18 shows the Polysiphónia parasítica, a rather common species of a very extensive genus of sea-weeds, containing nearly three hundred species. Fig. 19 is a portion of the stem of the same plant, cut across in order to show the curious mode in which it is built up of a number of longitudinal cells, surrounding a central cell of large dimensions, so that a section of this plant has the aspect of a rosette when placed under the microscope. A capsule or “ceramídium” of the same plant is shown at Fig. 20, for the purpose of exhibiting the pear-shaped spores, and the mode of their escape from the parent-cell previous to their own development into fresh plants. The same plant has another form of reproduction, shown in Fig. 21, where the “tetraspores” are seen imbedded in the substance of the branches. There is yet a third mode of reproduction by means of “antheridia,” or elongated white tufts at the extremities of the branches. The cells produced by these tufts fertilise the rudimentary capsules, and so fulfil the function of the pollen in flowering plants.

Fig. 25 is the Cladóphora, a green alga, figured to illustrate its mode of growth; and Fig. 26 represents one of the red sea-weeds, Ptilóta élegans, beautifully feathered, and with a small portion shown also on a larger scale, in order to show its structure more fully. A good contrast to this species is seen on Fig. 27, and the mode in which the long, slender, filamentary fronds are built up of many-sided cells is seen just to the left hand of the upper frond. Fig. 24 is a portion of the lovely Delesséria sanguínea, given in order to show the formation of the cells, as also the arrangement by which the indistinct nervures are formed.

V.

V.

The figure on the bottom left-hand corner of Plate V. is a portion of the pretty Nitophyllum lacerátum, a plant belonging to the same family as the preceding one. The specimen here represented has a gathering of spores upon the frond, in which state the frond is said to be “in fruit.”

Fig. 27 represents a portion of the common sea-grass (Enteromorpha), so common on rocks and stones between the range of high and low water. On the left hand of the figure, and near the top, is a small piece of the same plant much more magnified, in order to show the form of its cells.