The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕
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Fig. 144. A water channel grown over by floating plants and the advancing reeds and rushes.
In this way the shore slowly advances, the floating plants first building up some mud, and the reeds following and bringing shore plants in their train, till in the end the edges of the pond all meet in the middle, and the pond, as such, no longer exists. Only a marsh remains, till this may be gradually grown over by the ever-increasing land-plants, and an oak-tree may grow where once the water-lilies bloomed. If the advancing reeds at the edge had been kept cut back, as they often are, then the land-plants could not have taken such hold, and the pond would have remained a pond with all its “water-weeds.”
PLATE VI.
LOOKING DOWN ON A SANDY SHORE AND RIVER MOUTH
The long spit of sand-dune A protects the marshy land B from the strength of the waves, and here many salt-marsh plants grow. C is the open sea, which at full tide beats on the sandy shore so that no seaweeds or marsh-plants can grow on this side of the dune.
ALONG THE SHORE
Sandy shores with dunes are so common round Britain that you will probably have opportunities of studying them. Did you ever notice with any care what kind of plants grow on the sand next the sea? As you walk inland from the sea, you will find first little hummocks of sand with a few low, bent grasses, scattered and often far apart. Then as you go a little further inland, the sand mounds are higher, and a stronger grass grows first in tufts and then thickly over them; this grass is the useful sand-binder, or marram grass, and grows on the shifting sand, quite near the sea (see fig. 145). Try to pull up a plant of this grass, and you will probably find out some of the things which help it to hold its position in the moving sand. It is not at all easy to pull up, and you will have to dig rather carefully if you are to get it out at all complete.
You will find that what you thought was a simple tuft of grass is really connected, by an underground stem, with other tufts. If you follow this along, you will find that the underground stem runs for a long distance, burrowing in the sand and sending up tufts of leaves at intervals. The tip of the stem always remains under the sand, prepared to grow in whatever direction is best, and unless it is buried to a very great depth it will always continue growing. Coming off from the stem there are very many long roots, and at the places where the leaf tufts arise there are generally one or two much longer and stronger than the others, which run a very great distance into the sand, and if you wish to get them out without breaking them, you may have to dig for several hours. It is by means of these branching underground stems and long roots that the marram grass gets its hold on the sand. When once this grass holds the sand it is soon helped by a number of other plants, which come on behind it and cover the surface, and so prevent the wind from scattering the sand-grains, and blowing them about in clouds.
Fig. 145. A Sand-dune by the sea with the Marram grass in tufts, and the Carex tufts coming up in straight lines from their underground stems.
One of the first plants to follow the marram is the sea-star grass, or carex. You have probably seen its little tufts following in lines across bare banks of sand (see fig. 145). This appearance is due to the underground stem, which runs very great distances in nearly straight lines, sending up groups of leaves at short intervals as well as side-stems, which form lines crossing the main line. Often a bank may be covered with lines of this plant. A little piece of the plant is shown in fig. 146, where you can see that the structures are on very much the same plan as those described for the marram grass. There are many other plants with this kind of habit, which enables them to live on the sandy shores and dunes. Look at all the plants you can find on the sand-hills, and you will see that in some way they have their parts adapted to suit the conditions. Very long roots and a running stem are the commonest characters, and these you will find on almost every plant you try to dig up.
Fig. 146. A small piece of the underground stem of Carex, with tufts of leaves coming above the level of the sand; (s) stem, (r) roots (cut off) with small side roots, (sc.) scale leaves underground.
Sometimes the stem can grow up and up, even though it is continually buried by the shifting sand, as you can see very well in the case of the sea holly. You may dig for more than a dozen feet before you come to the end of the vertical stem of what seemed to be quite a small plant (see fig. 147).
Fig. 147. Sea Holly, showing the plant at the surface, and the long stem below the level of the sand (s).
Along the shore are other plants of quite a different kind, which have also special characters to help them to conquer a region which seems to be very inaccessible to land plants. Many curious plants live in the mud-flats that are frequently covered by the tides, and which can therefore only get salt water. You remember that salt kills ordinary land-plants, so that these must be specially built to be able to stand it. Most of them have very thick, fleshy leaves, and rather bushy stems, while others have leathery leaves covered with a kind of wax, or with hairs, which make them look grey. Look at the sea-daisy, and you will see that the leaves are very thick and juicy; so are those of the sea-blite and salt spurry. The boldest of all these plants, the marsh samphire, which goes furthest out to sea, and may grow on bare mud covered by every tide, has not leaves at all, but very thick, fleshy stems, which are green and do the work of leaves (see fig. 148).
Fig. 148. Marsh Samphire or Glasswort, a plant with swollen green stems which do the work of leaves.
All these forms must remind you of the plants which were characteristic of dry regions; how is it that these plants, often actually growing in the water, should yet be specialised in the same way? It is because all the water they get is salt, and it is very difficult for them to live in it. They can only use a relatively small quantity, otherwise they would be forced to take in too much salt, so they must prevent their leaves from transpiring much and using the water up. In this way they are really in the same kind of position and so require to have the same kind of leaves as a plant growing where very little water of any kind is to be had. They are in the same difficulty as the Ancient Mariner, with “Water, water everywhere, nor any drop to drink.”
Pull up a marsh samphire, and you will see that it has a very much branched, spreading root, which gives the plant a firm grip on the sand or mud, but it has not long roots like the sand-dune plants, for all the water which it can use is to be had quite easily and is near at hand.
You may notice, too, on these mud flats the mingling of plants from land and sea. When the marsh samphire and sea-daisy invade the flats which are covered every day by the tide, they are entering the region of the sea-plants, and you may find them growing side by side with the true seaweeds, and even in some cases we may notice the bladderwrack seaweed further in toward the shore than the samphire, which has ventured far out to sea.
As you will find in everything in nature, it is always difficult to draw a fixed line and say that on one side lies one type of thing, and on the other side something different; so, in dealing with different “plant associations,” we find that they have their special regions, but that they tend to cross over any limiting lines set between them. In deep water and on high, dry land, we find quite different kinds of plants which never mix with each other, but on the border land between such regions the boundary is not strictly kept, and we sometimes find plants growing where we might expect the conditions to be unsuited to them.
PLATE VII.
BLADDERWRACK GROWING ON THE ROCKS EXPOSED AT LOW TIDE.
IN THE SEA
All the plants which grow in the sea are hastily grouped together by most people under the name “seaweeds.” We know that there are many kinds of seaweeds, and yet even to one who has not studied them, they do not seem to differ so much from each other as to deserve special classes. And this general view is quite a correct one, for with very few exceptions, all the plants which actually live in the sea are algæ, and so belong to the simplest family of plants (see Chapter XXVII.). Yet they are not without interest and individuality. In the sea these simple plants have everything to themselves; and it is there that we get them developed in a very special way.
You must have noticed that you never find seaweeds actually rooted in the sand (except in protected marshes, where the sea samphire and some flowering plants may grow), because sand is always shifting and being churned up by the waves, so that they cannot get a firm hold. This is almost the same on the pebbly shores where the stones are rolled over by the waves, and so would batter any unfortunate plant growing on them. If you go along a rocky coast at low water, however, you will find countless true seaweeds, growing so thickly that the rocks are covered by their slimy masses, while in the rock pools are beautiful tufts of more delicate seaweeds of all colours (see Plate VII.).
Examine a single plant of bladderwrack or fucus, and pull it up if you can. You will find that it is very slimy and slips out of your fingers, and then, that when you have got a firm hold on it, it sticks so fast to the rocks that it is difficult to get it off without breaking it. Does this mean that it has roots which go right into the rock as the roots of land-plants go into the soil? Find a plant growing on a small stone, if possible, and look closely at it; the “root” does not go into the stone at all, but is much divided and clasps round it, bending into every little crevice and sticking tight. Note, too, that there are
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