If you would only be content to see bubbles rise and believe that it is oxygen, tear a few leaves from a growing waterweed plant under water. From time to time many and large bubbles of oxygen rise from it.
What do the plants produce that oxygen from? You know that water consists of 2 parts hydrogen and 1 part oxygen. Would our aquatic plants now use the hydrogen from the water and thus release the oxygen? That seems possible, but you remember that the evolution of gas ended at some point, although there[ 136 ]there was still enough water in the bottle. Take the test again—and it will happen again. But when the dead point in the gas development has been reached again, empty the deep plate as far as possible and then refill it with sparkling water. It goes without saying that this aerated water spreads through the entire water mass and thus also penetrates into the bottle with the plants. And what happens after a few moments? Soon the gasworks will be up and running again—and whenever the work strikes, you can bring forth new development by adding more aerated water. So for the formation of oxygen, something is needed which is present in the aerated water—you know that this is also a gas, namely carbon dioxide. You immediately conclude, of course, that ordinary fresh water must also contain carbon dioxide, which is quite right; you can convince yourself with the lime water test.
Also carbon dioxide is dissolved in the water with normal atmospheric air and can be driven out by heating. Let us cool such boiled-off water again closed, and then carefully put in some aquatic plants, then we will not notice any gas evolution and the plants die. You already know that animals cannot live in such gaseous water either.
So our plants turn carbon dioxide into oxygen by a simple subtraction; they swallow the carbon dioxide with small bites, prepare the cabbage into a nutrient and return the oxygen as surplus. The oxygen is deposited in extremely fine vesicles on the surface of the leaves, these vesicles unite into bubbles, which we see rise upwards when they are large enough. However, not all the oxygen reaches the surface, much of it remains dissolved in the water itself and that in turn makes that[ 137 ]the animals, which have to breathe oxygen from the water, can live in it. The animals then exhale carbon dioxide again—which the plants in turn use the carbon to return pure oxygen in return. Thus plants and animals make life possible for each other.
Before I knew this, I often wondered why the water always had to be changed in aquariums and goldfish bowls, and why, at great expense, the spillage of fountains and drain pipes for the ventilation had to be installed in an aquarium, while in still pools the richest life could develop. Did the rain bring fresh air with it? But what if it doesn't rain in months? It is obvious that living beings, plants and animals remain alive in stagnant water through mutual cooperation. It is also quite possible to keep an aquarium with live plants and animals for years without changing the water once.
In parks, which are well (too well sometimes) maintained, there are ponds in which no water plants are tolerated. I know a great city, where there is a very beautiful park, and in that park are gigantic ponds, in which not a water plant floats, and along the banks none of those beautiful shore plants—lisch, swanflower, reed, dullen—is allowed, It is a great pity that it is so, but the administration of the Park seems to want it no other way and maintains a real Broek-in-waterland cleanliness in its ponds. This does not alter the fact that at certain times of the year the water surface is covered with a dirty seaweed foam, but the most diligent park authorities can do nothing about that, because those seaweed germs come, so to speak, but suddenly fall from the sky.
But I didn't want to talk about that. the ordinary[ 138 ]So the situation in those dull ponds is that no aquatic plants grow in them. Yet fish live in it, beautiful large bream in abundance, and so large that the anglers who pass the park can only with watery mouth resist the desire to take a chance in the forbidden fishing water.
How do those fish survive without water plants, don't they need oxygen? Sure, and they get enough of it too.
Have you ever had spikes in a bottle without water plants? After a while you see that they are all swimming close to the surface of the water and gasping for air there. But they do not gasp in the air, but only in the top layer of water, which, by its direct contact with the air itself, absorbs, absorbs a sufficient quantity of gases, as we learned in physics class.
Now the water in the pond is really only a very thin layer and compared to its depth there is such a large surface in contact with the air that this alone provides sufficient ventilation for the water, especially if the wind blows from time to time. time helps by mixing the curly waves with air.
It is clear that this ventilation ceases immediately when the air can no longer come into contact with the water, eg in winter, when a thick layer of ice covers the ponds. Then the fish lack oxygen; and the fishermen know this well, who bite into the ice, where then within a few moments the fish jostle one another to get a single breath of air.
If you now have few plants in an aquarium , it is not good to cover it up, because this makes a significant amount of air exchange impossible.[ 139 ]Yet covering an aquarium is very necessary, partly to prevent the flying insects from escaping, partly to keep out the harmful dust.
Just make sure to always provide it with plants, and carefully remove the dead leaves and twigs. Most plants, even those that grow in water, are dependent on the light; good light is a vital condition for them. Now your aquarium can never count on being as well lit as a ditch in a flat meadow, where the sun can reflect itself all day long and where only a narrow margin of shadow remains a refuge for the usually light-shy animals.
Hence your plants seldom remain healthy and vigorous all the time, especially if they have to be without "skylight,"—after a few weeks they begin to wither—they do not die, but they become slender and pale and limp; they get anemia. He who takes good care of his aquarium, replaces such weaklings in time with new fresh plants.
This immediately gives the opportunity to go out for an afternoon with a landing net and fork, and thus the chance to make remarkable finds. It doesn't always have to be just Waterweed or Hornwort or Yarrow. We can also use floating aquatic plants. Our fish like shade and cannot easily find it in the thin or threadlike leaflets of our three principal oxygenators. There must be a green roof on top of the water palace, only then is it pleasant to wander in the cool space between the green pillars and garlands! What would a ditch be without duckweed! Where would the little silver stone carp have to spend their lives, if there were no duckweed, duckweed, among and under which it swarms and swarms with all manner of small aquatic creatures, but between which all too often[ 140 ]the sharp hook and seductive worm of the troublesome angler comes to put an end to their carefree existence.
It would be a long list if we were to list all the creatures that hunt for loot in and under the duckweed, from the little hydras, to anglers and natural history buffs included. The sluggish ducks also do their part and slobber in at the same time, à la whale, water, duckweed and small animals, daphnias and hydras. And green, glassy frog eyes lurk among the duckweed for the little flies that run to and fro upon it; little flies never seem to have anything to do but run back and forth. And the duckweed itself basks in the sun and eats and grows. And blooms—that rhymes so beautifully and that is so part of 'growing'. But have you ever seen duckweed flowers? I very rarely; I have only found them once and I have no doubt that you will see them again, for they are not so rare now, not nearly as the water nut. Go out in July, but bring your loupe and patiently examine a few duckweed plants. You may not find flowers or blossoms, but your efforts are not in vain because of that—who knows how many hydras you may find. You also discover that all duckweed is not yet the same.
First of all you find the common duckweed; green above, white below: round discs, floating flat on the water, usually connected to the edges by three, four or five, each provided with one white, stringy root, which ends in a cap at its tip—most of the carrots end in such a cap, called a root cap, and that serves to protect the delicate, growing tip of the root itself.
When the slices are thick, sometimes at the top[ 141 ]a little hollow, and spherical at the bottom—then you've found humped duckweed—which otherwise looks quite similar to the first—also each slice with one root. There is also many-rooted duckweed, which has four or five rootlets on the underside, springing from the center of the slide in a tuft.
These three duckweed species float on the water, their green surface is in contact with the air, so they are air scoopers—not really water plants anymore. If a ditch dries out, they can continue to live very well on the moist soil and await better times. The oxygen they give off therefore does not benefit the water—they only help with ditch management by providing shade and coolness and a hiding place for small animals. For that they may in return enjoy the nourishing ditch water—for, if a man can live on bread alone, so little can a plant survive on air alone—a meager diet, by the way.
A plant can only obtain carbon from the air, and now you know very well that the plant body is still built of many other materials; or if you did not know it yet, you can easily convince yourself of it by burning some straw in a hot fire. If that straw consisted only of coal, the whole thing would disappear in the fire; there remains, however, a white ashes which, however hot the fire may be, does not want to be burned.
Scientists and agronomists know exactly what that is, and how much of each—perhaps we will talk about it later on—but the matter of which it consists, and still more which has disappeared with the coal in the flame, the grain plant had to have. from substances which she had to absorb from the ground, where they were dissolved in water.[ 142 ]
Ditch water also contains all these substances, originating from the soil or from perished animals or plants, and everything that grows in the ditch is a wonderful guest. Each duckweed plant pumps itself through its root and through its underside full of nourishing water, takes out the food, evaporates the unused portion at the top through small openings, to absorb new at the bottom. It flourishes and grows and grows thick and broad for a duckweed plant—bone out at its edge, so that three, four, five new plants arise, which, after being attached to the old plant for some time, detach and grow and bud again on its own initiative to infinity—or rather—until winter comes.
There is also a kind of duckweed—in all ditches you can find it, which looks little like its brothers, for it is not round and does not float on the water, but usually floats a little below the surface, so a real water plant, who also has to get his air out of the water. It floats in clumps together, and when you take it out of the water, a whole lot comes with it at once. Just all scraps of a cutting work, triangular, rounded at the wide end and glued criss-cross over each other with the narrow end. There is also a carrot at the narrow end. You should put this triangular duckweed in your aquarium, first of all because it is an oxygenator, just like Elodea and Ceratophyllum and Myriophyllum, and then, the spines like to hide in it, they are always lurking in it.
These are all our duckweed varieties; do you now find very small duckweed (as big as a pinhead) without roots; then write to our Magazine, The Living Nature , that you have found there and there and then and then, rootless duckweed. This occurs in central Germany and may have been brought here by waterfowl.[ 143 ]
Now we have found enough duckweed, but not the flowers yet, they are also so small and colorless besides. Plants are not lacking, many ditches have been floored as with them, in three spades you have a basket full, and the pigs can feast ! They love duckweed!
Lidsteng (Hippurus vulgaris) (bottom left: a flower enlarged.)
Lidsteng (Hippurus vulgaris) (bottom left: a flower enlarged.)
We search our scoop and find now a larva, now a small snail, a nematode, a hydra, with which we gradually fill our jar, but the duckweed flowers do not appear. Let's take a look at other flowers; in and near the water there are many that do not catch the eye because they are colorless. That way we can practice a bit.
There are a multitude of plants in a shallow corner, which have a bit of pine twigs, they sting 2 to 3 dM. above the water, beautiful, fine dark green, a little stiff. You see no flower, and yet they are in full bloom; each plant bears hundreds of flowers! But there are also flowers for it, search[ 144 ]but once in the corners between stem and leaf: a pistil and one stamen—that's all, the whole floral splendor of the Lidsteng (Hippuris vulgaris).
Take a hornwort, which is in bloom now too—all aquatic plants bloom at about the same time at the beginning of July. You have to look for the flowers again and you will of course find them in the corners where the leaves come out of the stem, It is that we are interested in them now, but otherwise you could live to be a hundred years old and be in the middle of the hornwort without ever seeing a flower of it. If you look closely, you will always find twenty-five stamens or thereabouts together, surrounded by a serrated membrane; in other places the flower consists of a single pistil with a thread-like style and stigma, also encased in a membranous pouch. And all that underwater.
Have I told you that water is death to stamens or to pollen? Not? But you have certainly noticed with Elodea and Vallisneria, even with the Lidsteng, how they bring their flowers to the surface; and if we find some more flowers, we have ample opportunity to notice that they all try to protect their pollen from rain and dampness. But hornwort does not do that, and neither does a kind of featherweed, they bloom comforted under water and let their pollen carry through the stream to the stigmas of the pistil flowers. Such plants are very few, and it is curious that two of the most common aquatic plants in our country have this rare mode of pollen transport.
I spoke a moment ago of a kind of featherweed. For in your searches for aquatic plants you will find two kinds of them equally abundant, perhaps even three, [ 145 ]but the third, as your identification book will teach you, is a rarity. One species of featherwort, then, has its ceratophyllum-like flowers in the axils of the leaves, small stamens with 4-8 stamens, and pistils with one ovary with four stigmas—mostly submerged. Nevertheless, signs of improvement can already be noticed with this plant, because when it grows in shallow water or in ditches that are half silted up, the ends of the branches, which bear flowers, stick out above the water, the flowers open in the free air and the wind must ensure the transfer of the pollen.
The other species of featherweed never blooms underwater; that is Myriophyllum spicatum (the featherweed, which bears a spike): I think it is somewhat more common than the previous species, which is called Myriophyllum verticillatum, (featherweed, which has its flowers in whorls).
Now that Myriophyllum spicatum, the blossoming ends of its branches always protrude above the water, and as if it wished now to lighten those ends, the finely divided green leaves have been replaced by very small leaflets, in whose axils the flowers stand, above the pollen flowers, at the bottom of the pistils.
That is starting to look a lot like blooming and the ear is beautiful too; the anthers sway on very thin threads; they are red, as long as they are not yet open, then bright yellow. Before they open, they are protected by four beautiful carmine-red leaves, but they fall off very quickly.
Once we have found the flower spikes of the featherweed, we see even more spikelets sticking out of the water. Bring a few to you with the fork, there is something to look at again, we still have time—the summer days are long.—You must yank, the plant is sitting[ 146 ]rooted in the soil.—Handsome! there we have a branch. Dark green, brownish leaves are at different heights on the stem. Those leaves are not flat, as usual, but wavy all over their surface, mostly at the margins. The inflorescence emerges from one of the upper leaf axils, and contains a multitude of flowers close together. This plant is called Potamogeton crispus (curly pondweed). Many other fountain herbs can be found in our ditches; you can distinguish the different types using the table at the end of this book. The most common is the floating pondweed (P. natans), which grows very luxuriantly, so that sometimes ditches and ponds are completely covered with it. Then the ditch is full of 'tea leaves', the boys say.
Flower of Potamogeton crispus, first period of flowering.
Flower of Potamogeton crispus, first epoch of flowering .
At first glance it seems as if the upper pistil flowers. The lower ones have stamens, four, which have abundant pollen in their thick anthers, but in the middle is also a pistil, or rather four pistils, each with a thick outward stigma. But now look again at the top flowers. The four pistils are well encased in four brown-green leaves. Only the stiles and stamps stick out. However, if you remove those four petals—the cover scales—you will find four, thick, yellow, shiny stamens, pressed against the pistil. Actually I had to say sacks of flour, because there is hardly anything to be seen of a helmet wire; they are all anther. Each anther consists of two equal parts, the anthers, which are connected to each other by the helmet binding.[ 147 ]not ripe, they are not open yet. However, you can already notice a longitudinal stripe on every anther. In a while, when the pollen is ripe, the anther along that stripe bursts open and the pollen falls out. This never happens while it's raining.
When the weather is dry, windy, the pollen is carried by the wind, like the dust on the roads. Ten, twenty meters further it blows against another pondweed bloomer and sticks to the stigmas. Of course a lot falls into the water, but that's not the point. If only a few grains of pollen are placed on each stigma of our pondweed, it is enough—so if a few dusts from each anther reach their destination, seeds can be formed—and each cell contains thousands and thousands of those dusts.
If the helmet lofts spring open when there is no wind, which does happen, the pollen falls out, not into the water, but onto the lower cover scale, which forms a saucer, as it were. There it remains until a strong gust of wind chases it out. If it rains before then, that's okay, because then the flower closes and the pollen stays dry.
In this way it is ensured that the pollen from the anthers never ends up on stigmas of the same flower, but almost always on stigmas of another plant. This is more succinctly expressed by saying that the pondweeds are cross-pollinated, and it is known that seeds produced by cross-pollination produce far more vigorous plants than those grown from seeds formed by pollen landing on the stigmas of the flower from which it originated.
Most flowers have facilities that allow cross-pollination[ 148 ]promote. With Elodea, Vallisneria, Ceratophyllum, and Myriophyllum, it goes without saying that cross-pollination must always take place, because their flowers have either only pistils or only stamens. The same is actually the case with the fountain herbs; stamens and pistils are in the same flower, but they are never ripe at the same time; if the stigmas are suitable for receiving pollen, the stamens in the same flower are still closed and hidden, and they open only when pollen has already been supplied to the stigmas from elsewhere. Then those stigmas are already half withered, as you can clearly see from the lower flowers of a spike.
Flower of Potamogeton crispus, second epoch of flowering.
Flower of Potamogeton crispus, second epoch of flowering.
Still no duckweed flowers, but what is that on the edge of that reed pool? Approach it carefully, we are in a dangerous spot here, the bottom vibrates below us and waves before us; here and there, among the plump sphagnum moss, green slimy pools gleam. Step on the alder stumps and try every step, but we must have that little plant there—the find is unique—I'm happier with it than with any duckweed flowers. Rare? Oh, don't always and only ask for the rare—the not rare, the general is often the most interesting. But this sermon does not suit me, for although the plant—for which we now risk our lives—is by no means rare, it cannot be called general either. So go ahead. Towards the edge of the pool the bottom becomes a little firmer—you get that more in the swamps—and with our fork we can haul the loot to us.[ 149 ]I always have an iron fork that I can attach to the pole of the landing net.
See, that's inside. Now we can sit on a willow stove and tell me; what would this be?
"Yellow water snapdragons with hair blades, and there are bugs among them too!"
Well answered.
Those flowers really look like snapdragons, there are about five of them on top of a stem, sticking upright an inch or two above the water. They are beautiful yellow with darker spots and stripes where the lower lip and upper lip are firmly pressed together, almost as with the well-known yellow flaxbill, which can be found everywhere on dikes, ridges and roads in August and September. It has, however, four stamens, while our aquatic plant has only two, which you find in many cases so bent that their anthers are pressed against the stile—including the stigma, so that the pollen from the anthers cannot possibly fall on the stigma. Pollen must therefore be brought in from other flowers, as with the fountain-weeds, but the wind cannot help us here.
Shall we have another look at the yellow flowers that are still there in the water?—Perhaps we will find the solution to the riddle.
It's such beautiful, quiet warm weather; the birches, which fill our swamp, give off their delicious spicy scent as of… sour pepper—as one of my young friends once said. Over the thousands of colorful flowers a buzz of insects of all kinds—shooting blue bolts[ 150 ]through and over the water; those are swallows on the prowl; blue swallows. Others, too, dull gray with white—these are sand martins ; they have their nest somewhere nearby, deep burrows in a dike or steep ditch.
It is mainly they who hunt over the water surface—because there too there are insects in abundance—including among our yellow flowers. There a little bumblebee with yellow and brown on its abdomen clumps against a flower—close to it. The lower part of the flower bends down under its weight, its head disappears into the opening up to the shoulders. He stays like this for three or four seconds, then he has sucked up the honey from the track and looks for another flower. In this way he successively finishes all those yellow flowers and then flies off, but not without having done her a favor in return.
Flower of Utricularia in section, at the moment when a bumblebee's tongue is inserted.
Flower of Utricularia in section, at the moment when a bumblebee's tongue is inserted.
The stamens are so arranged that the bumblebee must inevitably run along them with its tongue if it wants to suck up the honey from the spur. Of course some of the sticky pollen remains attached to that tongue, and then when he puts it in another flower, he touches the stigma there, to which the pollen grains adhere in turn, so that the cross-pollination is complete.
Yes, but, you say, does not the tongue already lose the pollen in the first flower when it retreats past the stigma?
That's so bad not noticed—let's see.
Take a blade of grass and imitate the movement of a bumblebee's tongue in a living flower. You open the corolla, put in the blade of grass; it passes by[ 151 ]the lower division of the stigma, between the stamens, into the spur.
But what is that? The touched part of the stamp moves, slowly but steadily the tab curls up, until it comes to rest flat against the top part. If you withdraw your blade of grass, it will no longer touch the mark; it got out of the way.
At home, in your window or in the garden you may have a little plant showing the same curious arrangement—I mean the yellow Mimulus. Its flower is also snapdragon-like, but open, the stigma is like that of the Bladderwort (that's what our yellow water flower is called) 'two-lobed' and the lower lobe curls up at the slightest touch. It is an astonishing sight to see such sudden movements occurring in plants—which are generally regarded as motionless.
Flower of Utricularia vulgaris, when retrieving the bumblebee tongue.
Flower of Utricularia vulgaris, when retrieving the bumblebee tongue.
Now I want to tell you something about the flower of our bladderwort . It sometimes happens that the insects do not visit a flower; in unfavorable, rainy weather, for example, they do not like to venture above the dangerous element. How does the pollen get to the stigma? Yes, then the flower has to make do. The lower stigmatic lobe then slowly curls (hesitantly, I almost wrote) downwards, until it touches the open stamens. Pollen from the same flower has then arrived on the stigma, seeds now also ripen, although they are not as good as those produced by cross-pollination.[ 152 ]
Nice decor, isn't it? But we did not have to risk our lives for this (the daredevil was not so great otherwise), there are enough flowers, which are arranged even more 'nicely', to get strong seeds by cross-pollination.
But there's more! Put the flower aside and look at the leaves—or rather the green, because those branched threads can hardly be called anything else.
Soon you'll see little creatures in between, won't you? They're still there—we've been sitting in our willow chair for fifteen minutes, talking about the flowers, and they haven't crawled out among the damp sphagnum moss to begin their twists and turns again in their element. Now that you want to grab one, you notice that it is attached to the plant, that it has grown on it: your creatures are not animals, but leaves or something resembling them.
There you are now.
If we were now living three hundred years earlier, we would throw the plant back into the ditch and go home to find a few folios with miracle stories, preferably in Latin. We then dug into some places that seemed appropriate to our discovery and then took a newly cut quill pen to start a voluminous book about the precious ditch flower, clarified with quotations from Aristotle and Pliny and Dioscorides, etc. Yes, you can. laugh; but that's what they used to do, and I'll have occasion later to show you pictures of brent geese growing on the trees, or live sheep coming to amble from under a fern, where they have first performed the service of roots. . In all seriousness such things were in books printed with royal privilege.[ 153 ]
We, however, pack our bladderwort among some damp moss in our plant container and place it at home in our aquarium—or, if we are not already rich, in a deep dish of water. Let us also take in our jar some water with branched-winged daphnia and such one-eyed four-legged (Daphnia and Cyclops) and more of that little thing, to pour it all at home in the same dish with the bladderwort. Later we go out again, to look for duckweed flowers.
The planting of our bladderwort does not cause much trouble, for it has no roots: we have only to put it upright in the water, then it sinks so far, until it floats (Physics, chapter so much) and at the same time the branches spread with the threadlike leaves out. The vesicles then form, as it were, a herd of peaceful daphnias immersed in ruminating reflections. I don't just use that word "ruminating" here, because the word "herd" reminds me of cows or sheep. That will soon become clear to you.
Our true Daphnids must have special thoughts from those strange, immobile tribesmen. They themselves shoot through the water in their familiar shocking way, hunting one another or even smaller creatures. Sometimes they approach our vesicles curiously , sometimes very close, yes they sometimes crawl in.
After a few days you remark that your living daphnias have wreaked havoc among themselves, or that an epidemic has broken out among them—for their numbers are considerably reduced. However, there are no daphnia corpses to be seen anywhere, not even empty bowls.
That seems mysterious and you've already guessed that the vesicles are secretly involved. Cut off a twig with blisters from your plant and place it on a piece of white filter paper, [ 154 ]then the water that clings to it will not hinder your investigation. Now you see that each of the ovoid vesicles has four branched "sprits" at its pointed tip; start tearing open the blister there with two needles—then you will see what's in it.
Nothing? All right, then take another blister, if necessary a third. There you have it: a dead daphnia. Open another blister.—A moribund Cyclops. Another! Do you understand now?
The bladders of the bladderwort are traps for small animals, the bladderwort itself is a carnivorous plant! The water flea, which has crawled into such a vesicle out of curiosity or in its fear of escaping some persecutor, can never get out. In darkness and seclusion, after repeated fruitless attempts to escape, the animal must die of starvation and suffocation. The trap is closed with a flap, which can be pushed open so easily inwards, but which, by any pressure outwards, only closes the exit more firmly.
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