The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) π
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The many large bladders on the plant are filled with air, as you will see if you split them open, and they help to buoy it up in the water. Notice, too, how flat the whole plant is; it is really a single sheet of tissue or βthallus,β which is much divided, but does not branch in many directions as a land-plant does. All these characters are those of the simple family of algΓ¦, to which all the seaweeds belong. Though in some cases they may form what look like very complicated structures, yet they are always built upon these simple lines.
Often you may find little plants growing on the bigger ones; sometimes a well-established weed may be almost covered by small seaweeds of many kinds, brown, green, or red. These attach themselves to the big plant in much the same way as they would to a rock, but only use it as a place of anchorage, and do not tap its food supply, as the parasitic mistletoe does to the land-plants. In the same way you may find numbers of seaweeds planted on shells or growing on the backs of crabs.
As the tide goes out it gradually exposes the rocks and pools with their innumerable inhabitants. Now in the case of those which are first uncovered, a long time must pass before the water returns, while those quite near the low water level are only uncovered for a little while. Follow the falling tide some day, and look for the effect which this difference (in the time for which they are exposed) has on the plants growing at different depths.
Fig. 149. The Laminarias, which are only exposed at quite low water.
As you go out towards the low water mark you will find first and commonest the bladderwracks, which get more luxuriant where they are a little removed from the region of the pounding waves at the actual shore. Then further out you will find that the bladderwrack gives up its place to another plant very like it, but with more jagged margins. Beyond this you will come to the big strap-shaped laminarias, which never grow where they are very long exposed without water (see fig. 149).
These different regions of seaweeds (some of which are only laid bare by the tides which go very far out) really depend on the fact that the different levels of the shore are left exposed for varying lengths of time according to their depth. If the shore is flat or gently sloping, then the tide has a very great distance to recede before the same depth is reached as would be attained much nearer in where the shore slopes steeply (see fig. 150). This explains how it is that in one place you may have to walk out a quarter of a mile till you come to the region of laminarias, while in another you need walk no distance, but merely clamber down the rather steep rocks to get to it. But as the actual time taken by the falling tide is the same in both cases, the plants at any level are left exposed for almost the same time whatever the kind of shore may be.
Fig. 150. A diagram to show how the slope of the shore influences the distance the tide goes out, and, therefore, the distance from high-water mark at which the different seaweeds grow. A, a gently-sloping shore; B, a steep shore. The line H indicates the high-tide level, and L the low-tide level.
One thing that may perhaps puzzle you about the seaweeds is their colour; some few of them are green, but most are blackish, brown, or even red. How then do they build their food? It is found that true chlorophyll is present as well as the other colours, and that though they hide the green tone from our eyes, they do not hinder its activity in the plant. You can see that the brown bladderwrack is really a green plant if you soak some of its tissues in hot water; the brown colour will be washed out and will leave the plant bright green. In almost all cases these simple algæ living in the sea are self-supporting plants, which have adapted themselves to the special conditions in the depths of the sea where no flowering plants can live, and there they reign supreme.
PLANTS OF LONG AGO
When we were on the moors we noticed that we may sometimes find plants being actually turned to stone under our eyes (see p. 156). These are plants which are living at the present time, but this same thing has also happened to plants which lived long ago, and which otherwise we could not see and study, because they are all dead. In those cases in which they did not decompose in the ordinary way after death, but were turned to stone, we are sometimes able to find out almost as much about them as we can about the plants living to-day.
Fig. 151. Plant which was living at the time coal was made, pressed in a stone and so preserved.
You must have seen in museums, or even found for yourself in stones, the remains of leaves and stems of plants which, too, are turned to stone, but which yet show the shape and form of the plant with great beauty. If you go to the north of England, where there are many coal-mines, you will have a good chance of finding pieces of stone which have been thrown out from the mines as refuse, and which have in them or on them most beautiful leaves of ferns and other plants. We know from geologists that these rocks are very old indeed, older than the valleys and downs of the south of England, yet we can see to-day what the plants which lived then looked like, because they have been turned into stone and kept for us in the rocks till the miners dig them out when digging the coal.
Fig. 152. Fern which was living at the time of the coal, pressed between sheets of stone.
But what is coal itself? You know that it is not at all like an ordinary rock, for it burns as well as wood, and has been found to be largely made of carbon. Even directly on top of the coal, and sometimes actually in the coal seams, we find plants preserved, and geologists and botanists have combined to prove that coal is really entirely composed of the crushed remains of ancient plants.
You will remember that we found that many of the plants in the peat bogs did not get decomposed entirely because of the preservative peaty acids present in the water and soil. Something of the same kind happened to the plants of the old forests which now form our coal. As they died they did not entirely decompose, but got pressed tightly together, all their living juices being squeezed away till little but the carbon in them remained. These masses of plants gradually sank beneath the sea, were covered by sandstones and limestones, and were preserved between the beds of rock, forming masses nearly as firm as the rocks themselves. These old plants, which to-day act as our fuel, are really βas old as the hills,β for they were growing in the country before the hills were made.
Fig. 153. The trunk, A, of a fossil tree turned into stone, still standing in the position in which it grew. It is surrounded and covered by the pressed masses of plants (coal) C, fine mud (shales), O, and sandstones, S. Its roots, R, are still in the clays, U, in which they grew, which are now hardened to rock.
As well as the many plants which were preserved in this way, and in which we can now see little but masses of carbon, there were others which were preserved in stone, sometimes pressed between the layers of stone as you press a flower between sheets of blotting-paper, in other cases turned directly into stone without crushing, so that they show their complete form, cell by cell. It is from these stone plants that we learn what the plants of the coal were like. Sometimes we find great trunks of trees standing petrified together in the positions in which they were growing, with their roots twining round one another, and entering the muddy soil on which they lived. Sometimes such tree-stumps stand up through the coal-beds and rocks which must have been deposited all round them (see fig. 153). We find also leaves and stems, cones and seeds, in the stones, till we can build up completely the form and life history of several of the plants which were then living. But in all the wealth of material which has been found, no flowers have ever been discovered. The seeds seem to have belonged to plants of the pine-tree family, so that these old forests were without any of the plants which are to-day the most important family of all, that is, the flowering plants. They lived so long ago that flowers had not come into existence by that time.
Another strange thing about these forests is, that although there were great trees in them, they were not like those of our present forests. To-day our trees are chiefly flowering plants, such as oaks, limes, and beeches; but the giants of these ancient forests were club-mosses and horsetails, plants belonging to the fern tribe. Their descendants, the club-mosses and horsetails growing now, have degenerated, and are humble plants not more than a few feet high at the most, and always of little real importance in the landscape.
The true ferns then living seem to have been more like those of the present, though perhaps a little larger and more important. In the family of ferns then living were some with strange histories, and among the ferns which you may find in the stones some leaves may have belonged to a plant which was truly a βmissing linkβ in the history of plants, and helps us to see the relationship between ferns and pines.
Many and strange are the tales the fossil plants can tell us of the life in the forests when the coal was made, and just as, in the moors, only those moss-plants which were turned to stone will still be there after centuries have gone by, so it was in the old coal-forests that only the plants which were turned to stone remain to tell us their story to-day. For this reason our knowledge of the forests of long ago is not complete; but even now it is enough to tell us something of the life of the plants which were then doing the food-building work of the world. Though the individual plants were so different, the βassociationsβ were in a general way the same as those now living. Great trees reared their heads into the air, and below them, or climbing round and over them, the smaller plants found place long ago as they do to-day.
PHYSICAL GEOGRAPHY AND PLANTS
If we examine the plants of any district, we find that a number of outside influences affect them very greatly.
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