In this chapter we shall confine ourselves to the true water-dwelling plants, as distinct from those, such as the water lilies, which though never found growing on dry land, appear undecided whether they will be water plants or land plants. Looking at the matter from a more scientific point of view, all our pond plants will be much lower in the scale of development than the water lilies and other flowering plants.
Pond life is rich in subjects for the microscopist. Any stagnant pool may contain organisms which will delight the naturalist who has always depended upon his unaided vision. Curiously enough, amongst the most wonderful of all these pond-dwelling plants are the Diatoms, which consist of but a single cell. They are so numerous, they exist in so many different forms and in so many different situations that were we able to describe them all, we should require the whole of a large volume, much larger than this. In colour, Diatoms are usually brown or brownish, although they contain chlorophyll, the green colouring matter of higher plants. In shape they may be rod-shaped, crescent-shaped, circular, wedge-shaped, oblong or oval. Some float about freely in the water, some are attached to supports by means of stalks. Some lead a solitary life and others dwell together in colonies. One feature they have in common, a curious flinty cell wall, and this is their most interesting point to the microscopist. This natural armour is in two parts which fit one within the other like the two halves of a Japanese basket. All manner of beautiful sculpturing marks these beautiful frustules as they are called; in some cases they are perforated and the living matter from within passes through the pores and forms a jelly-like covering for the little plant.
We must make a point of collecting all the Diatoms we can find, for they are always interesting; moreover, they are easily preserved and made into permanent slides, for the little plants may be boiled in acid to destroy their living parts and the frustules will survive the boiling undamaged.
One might wonder how such humble plants, surrounded as they are with flinty walls, could increase. They frequently do so in a simple manner. The living matter of which the plant is composed pushes the frustules apart and divides across the middle. The result of this event is the formation of two plants, each with a single frustule. In a very short time each plant grows a new frustule, but it is always much smaller than the one with which it started.
The movements of some of the free swimming, that is to say non-attached Diatoms, are worthy of study. The scientific name of one kind, translated into everyday language, means little boats, and indeed they are well named for their beautiful aquatic manœuvres rival those of any ship.
Somewhat similar in habit to the brown Diatoms are the green Desmids, but, whereas, the former also occur in the sea, the latter are all confined to fresh water. Sometimes Desmids are so numerous that they make the pond water as green as green-pea soup. It would be as impossible to describe all these plants as was the case with the Diatoms, but generally they may be recognised by the fact that they are composed of two similar halves, separated by more or less of a waist. Although some of the Desmids exhibit a certain amount of movement they are not active like the Diatoms.
Late spring and autumn are the best seasons to hunt the ponds for our next object, which rejoices in the name Chlamydomonas Angulosa, a good example of the extraordinary fact that some of the smallest animals and plants have the longest names. This little plant is interesting in itself and doubly so, because it was for long thought to be an animal; it is wonderfully animal-like in its movements. Chlamydomonas is very minute, so we must use our highest magnification when we examine it. It is an oval, one-celled plant enclosed in a clear membrane. The green colouring matter is arranged in the form of a cup, within the hollow of which is a mass of granular living substance. At the forward end of the plant, there is a clear space and near the tip a brownish dot, known as the eye spot; which, though incapable of seeing as we understand it, is sensitive to light, as shown by the fact that the little plant will swim towards moderately intense light and away from strong light. If we stain the plant with iodine we can plainly see a pair of little whips arising from the clear portion at the forward end; it is by the lashing of these that the plant is enabled to swim.
In the mud of our pond we may find a little colony of plants which might forgivably be mistaken for a collection of the individuals we have just studied. Each member of the colony is very much smaller than Chlamydomonas to be sure, but each one has the outer membrane, the brownish eye spot and the pair of little whips. On the other hand the chlorophyll fills the whole of each cell and is not arranged in the form of a cup. Sixteen cells form a colony, and the whole mass is a flat plate; the little whips move in unison and the whole colony revolves after the manner of a wheel.
Another little colony we may encounter also, consists likewise of sixteen cells very like the ones we have described, but somewhat wedge-shaped instead of oval. A jelly-like mantle encloses the colony and, in outline, it is spherical, so that when the little whips, which project through the mantle, lash the water the whole colony revolves.
The most remarkable of these colonies of cells is known as Volvox Globator, it may be recognised by its perfectly spherical shape and its characteristic movements in water. Volvox is about 1/25 inch in diameter, and although to the uninitiated it appears to be a single minute plant, in reality it is a colony of upwards of twenty thousand cells. The colony may be considered as being made up of thousands upon thousands of cells, very similar to those of Chlamydomonas, and each one arranged with its pair of little whips directed outwards.
Within the Volvox sphere we may observe a number, usually one to eight, of smaller spheres. These are so-called daughter colonies which have arisen from the continued division of special cells. They develop, fairly rapidly, into young Volvox colonies, then they burst through the cells of the parent colony, swim out into the water and quickly grow to the size of the Volvox from which they were formed.
There is another kind of Volvox of a yellowish colour and much smaller than Globator, the big one is the one we must procure; it is much more easily studied.
Certain of the plants we shall find in our pond are so animal-like in their movements, that the microscopist who sees them for the first time may wonder whether we are not mistaken in calling them plants. We have already described the common Chlamydomonas, with its curious jerky method of propelling itself through the water. There is, however, an equally common one-celled, pond-frequenting plant which has puzzled naturalists even more, for it certainly possesses many very animal-like characteristics. Its name is Euglena Viridis and we require our highest magnification to examine it for it does not exceed one-two hundred and fiftieth part of an inch in length. Usually, Euglena is cigar-shaped but, as it possesses the very unplant-like characteristic of not having a firm cell wall it can change its shape to a considerable extent and it often assumes curious forms. At the forward end of this minute plant there is a single whip-like thread, by means of which it swims; a little below the base of the whip, there is a red eye spot. Elsewhere we have described how the protean animalcule feeds by flowing round its food-material, Euglena feeds in a similar manner, but it also feeds after the manner of a plant. When this active little plant is about to increase it either divides into two lengthways or becomes surrounded with a firm wall within which it breaks up into a number of young forms which are released later by the bursting of the wall.
Photos by Flatters & Garnett
1. Bladderwort
A British water plant which entraps small animals in its bladders and digests its captives.
2. Spores of Horse Tail
These spores with their thread-like outgrowths vary in appearance according to the moisture in the air.
3. Hairs on a Potato Leaf
The star-shaped hairs on a potato leaf make beautiful objects for the microscope. Leaves of many plants are clothed with curious hairs.
4. Spirogyra
A green thread-like water weed. Two threads are shown fusing together; from each part of these fused threads a new plant will arise.
Small wonder that scientists were in doubt concerning the true nature of Euglena, its non-walled cell and peculiar mode of feeding are indeed puzzling. There are many similar cases amongst these lowly organisms, in fact there are some creatures which may best be described as partly plant and partly animal. Those of our readers who are interested in such problems may read an excellent account given much more fully than we could give it here, in Professor F. W. Keeble’s Plant Animals, one of the excellent Cambridge Manuals of Science. The fact is that the demarcation between plants and animals, low in the scale of development, is not nearly so pronounced as it is amongst the higher forms of life. Amongst the plants of our pond we shall find that, like the land plants, most of them are green, and many of them are thread-like, so that the task of distinguishing one from another may appear difficult. Examined with the naked eye, many of them appear remarkably similar to one another; under the microscope the differences are obvious.
One of the most remarkable of the commoner pond plants is known as Oscillatoria; it does not boast of a popular name but its scientific name is not very difficult to remember after we have witnessed its oscillations. Oscillatoria is a plant with particularly animal-like movements. It is merely a thread, usually green-blue in colour, but sometimes red or violet. The threads are never branched and, except when in motion, are straight. Under a moderately high magnification we can see that this thread-like plant is not composed of a single cell but that it consists of a number of cells, placed end to end. Sometimes a few of these cells will break away and start life on their own account.
Whatever interest the structure of Oscillatoria may have for us, we cannot help being struck with its movements, and we must make a point of observing them. The movement is peculiar and not easy to describe nor, so far as we know, has it ever been explained. A thread will be seen to glide backwards and forwards, becoming somewhat curved and, at the same time, revolving on its axis. Eel-like is perhaps a good description of the movement. It is possible to distinguish this plant from other pond dwellers by its slimy “feel” which arises from the fact that each thread is enclosed in a jelly-like sheath.
The silk weeds, Cladophora Glomerata are somewhat similar in appearance to the plant we have just described, but they are denizens of running streams rather than of ponds. They are the green thread-like plants we so often see attached by one end to rocks and stones beneath running water. Each plant consists of a long, cylindrical structure composed of several cells. We mention the silk weeds here, because they are best of all plants for showing cell growth. This growth takes place at or near the unattached end of the plant and is easily observed. The end cell may be watched for the process. Its green contents will be observed to contract in the middle so that it assumes an hour-glass shape. Then, where the contraction has taken place, we can watch the formation of a wall right across the cell, so that when the process is completed we have two cells where formerly there was one. More rarely, the contents of a cell will be observed to bulge out a side wall, then a new wall is formed to divide it off from the main cell and thus the beginning of a branch is formed. The silk weeds exhibit no movements, except such as are imparted to them by the running water.
A very beautiful little pond plant is known to science as Draparnalda Glomerata. We shall probably forget its name, but we can never forget the plant itself when once we have been fortunate enough to see it. A single row of large, transparent cells, containing very little green colouring matter, forms the main part of the plant. At regular intervals from these transparent cells, there arise rings of deep green branches, each one tipped with an extraordinarily long, colourless hair. Draparnalda is indeed a plant worth looking for.
Two common little green plants grow so near to the edges of ponds that they may well be included amongst our pond plants. The simpler of the two, known as Vaucheria Sessilis thrives on almost any damp soil and may even form a covering on soil in pots. In structure, Vaucheria is very simple for it consists of a single, frequently branched, tubular cell. The little attaching organ, by means of which the plant fixes itself to some firm support, is colourless, so too is the tip of the cell where growth takes place.
We must examine this little plant when we come across it and we must not fail to notice that it is composed of but one cell. The only time at which we can find any cross walls, is when Vaucheria is about to increase. Then the tip of the cell swells, like a little club, and a cross wall separates it from the rest of the plant. The contents of this cell rounds itself off, becomes fringed with innumerable little lashing whips and escapes from a pore at the tip of the cell in which it was formed. This little organism swims about for a time in the water, for Vaucheria only increases in this manner when it is under water, at length it comes to rest and forms a new plant.
Closely related to Vaucheria, but not quite so common, is the curious little plant known as Botrydium Granulatum. Like our previous example, it is a one-celled plant and herein lies its interest. When find a specimen growing by a pond, we shall notice a green bladderlike portion, not more than 1/6 inch in diameter, which projects above the ground. Below ground, if we pick the soil away carefully we may observe a number of colourless, branched structures which do duty for roots. The bladder and underground portions are hollow, being lined in the case of the former, with a network of chlorophyll beautiful to observe under the microscope.
Botrydium usually increases by buds which form on the green bladder, break away and grow into new plants. Should the level of the water in the pond rise, and the little plant become submerged, it splits up into a number of small bodies, each provided with a minute whip-like organ, with which they swim rapidly ashore where they develop into new plants. In dry weather the plant forms a number of little cells each one surrounded by a firm cell wall. When moisture comes again these little cells give rise to structures which will grow into new plants.
Surpassing even Drapernalda Glomerata in point of beauty, is the “water-net,” Hydrodictyon Reticulatum, but it is not nearly so common, being confined to ponds in the South and Midlands. When full grown it hardly comes under the heading of a microscopic object, for it may measure as much as six inches in length. This remarkable pond plant consists of an open network of green filaments. Apart from its striking appearance the most remarkable thing about the “water net” is its rapid growth. Carpenter, a noted microscopist says:—“The original cells of which the net is composed measure one-two thousand five hundredth part of an inch in length but in a few hours they grow to one-twelfth of an inch or 1/3 of an inch in length. We often hear people remark that they can see plants grow, but their statements are not literally true; in the case of the ‘water net’ it is actually possible to see the growth.”
By the side of our pond we shall probably observe some masses of a bluish-green jelly-like substance. It is uninteresting-looking material for the microscope, but we must not pass it by. The blue-green jelly encloses a plant called Nostoc, which resembles nothing so much as a necklace of beads; this we can plainly see under a low magnification. We shall observe that the plant is twisted spirally within its covering and also that most of the cells, which we have compared to beads, are similar to one another in size. At intervals there are larger cells and as the plant increases in length they are emptied periodically, so they are evidently used as food stores. From time to time portions of the plant break away, worm their way out of the jelly and move about, rather after the manner of a worm. Eventually, these wanderers come to rest and become surrounded with more of the blue-green jelly, thus forming a new Nostoc colony.
These plants sometimes appear, apparently from nowhere, on garden paths, walls and similar situations, during damp autumn weather. On this account the plants have been called “fallen stars.” Their appearance is not so mysterious as it might seem for the Nostoc colony has probably been where it is found, all through the summer, in a dried up, contracted state. Only when rain comes, does the jelly envelope absorb water, swell up and assume its normal appearance. The blue-green scum which floats on stagnant water is a closely related plant.
Another plant which looks like scum on the water is known as Spirogyra and a very beautiful object it makes for the microscope. It is bright green, without a bluish tinge, so it need not be confused with the plant we have just mentioned. There are seventy or so different kinds of Spirogyra, therefore our description must be the one that will apply to all. The plant is thread-like and, even in the larger kinds, the threads are not more than one-hundredth part of an inch in diameter. Like Oscillatoria the plants are not attached to any support. Each thread is composed of many cells, arranged end to end; we can distinguish the cell walls clearly, but what will chiefly attract our attention are the beautiful bands of green colouring matter, running spirally, round each cell. If we are fortunate, we shall see the plant in the act of increasing; this is not the simple operation we witnessed in the silk weeds. From two threads lying parallel to one another, we shall see swellings arise on the adjacent cell walls; the beautiful spiral bands will begin to break up at the same time and to collect in a mass towards the centre of each cell. The swellings of adjacent cells touch, their end walls break down so that the two cells become connected, our figure shows the fusion taking place, then the contents of one cell passes into the adjacent cell and fuses with the contents of the latter. The new cell, which now contains not only its original contents but that from a cell in another plant, becomes detached from the thread, assumes an oval shape and sinks to the bottom of the pond, where it rests awhile. At a later period the cell wall bursts and a new thread of Spirogyra develops from it.
When we are examining Spirogyra we may notice some very minute brick-red, spherical bodies adhering to the green threads. These are little pond animals, known by the name of Vampyrella Spirogyræ. This little creature passes through an interesting and easily observed series of changes. Its life is very uneventful, consisting of a good meal of Spirogyra, a period of rest followed by an increase, and this is repeated over and over again. If we watch our little sphere through the microscope, we may be lucky enough to see the contents divide into four parts. Now we must watch carefully, for very interesting events are about to take place. Each of the four parts into which the contents of the sphere has split, escapes into the water and swims about for a time; it then becomes spherical, but instead of having a smooth outer surface, as it had when we first observed it, we can see that it is now studded all over with very fine threads. It then wanders along a Spirogyra plant, attaches itself to one of the cells, perforates its wall and sucks out the contents. This performance it repeats several times then, evidently satiated, it loses its threads and resumes the appearance it had when first observed and thus it rests, till division of its contents into four parts takes place once more and the little comedy is repeated.
Now we must tear ourselves away from our pond; there are very many interesting objects for our microscope on the sea shore and if we delay too long here, we shall not be able to give them the attention they deserve.