The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Fig. 129. A Prothallium (p), with a young Fern (f) growing out from it. (Magnified).
Some ferns take a short cut, and bear little ones directly on their leaves without any prothallium. You see this in the “Hundreds and Thousands” fern, where the old plant is sometimes covered over with little ones, which will grow if they are taken off and planted carefully.
Sometimes people are deceived by what is called the “flowering fern,” and expect that it will have flowers. In this fern we find that all the spore-cases grow together on a special leaf, which is so covered by them that it looks quite different from a usual one, and is called the flower, though it is not one. In all other ways the story of the spore building and growth is like that of usual ferns.
In our study of ferns, you see that they have many characters which are exceedingly different from either the flowering plants or pine-trees. In fact, they are so different that we require to add some new points to our list of characters for family divisions, which are:—
7. Instead of flowers there are little spore-cases, which contain a number of simple one-celled spores. These are generally found on leaves which are otherwise like the rest of the leaves of the plant.
8. Each spore grows out to form a small green structure, which differs from the parent, and which we call the prothallium.
9. The new fern-plant grows at first attached to the prothallium, but soon grows out beyond it, and is quite independent.
What we call “ferns” are not the only plants which belong to this big family, for the club-mosses and also the horsetails have almost the same arrangement for their building of new plants. Our character-points (7) (8) and (9) apply to them, even though the rest of their structures appear to be so different from the ferns. They are, therefore, put in the same big family with the ferns, though they have smaller classes for themselves apart from the true ferns.
Neither the ferns nor their near relatives are very important in the vegetation of to-day, but very long ago they were among the chief plants in the world, and grew to be as big as forest trees. Even then, however, they had almost the same way of forming spores that they have to-day, a fact which still marks them out as a family different from all the other families of plants.
MOSSES AND THEIR RELATIVES
Mosses form another big family, the members of which are generally easy to recognise, even when you know little about them, because they all have a very strong family likeness. If you look for mosses in a shady wood, or on stones and tree stumps near a waterfall, you will often find large numbers of them growing together, sometimes forming sheets of soft green, covering the stones and earth and tree stumps. These luxuriant mosses grow, as a rule, in moist and shady places, but there are others which grow on dry walls or between the cobbles of little-used paths, and generally form brilliant green patches of tiny plants, like masses of velvet. If you pick out a separate plant from among these and look at it through a magnifying-glass, you will see that it is very like the bigger ones of the wood.
Fig. 130. A clump of Mosses, showing the flower-like appearance of the tips of their branches.
For our study it is perhaps better to choose one of the bigger ones, because all its parts show so clearly.
1. If you take a single plant, you will find that it appears to be marked out into root, stem, and leaves, though all these parts are small and simple.
2. The stem is delicate, and you will not be able to see any “water-pipe” cells when you examine it with your magnifying-glass.
3. The leaves are always very simple and small, generally narrow, pointed, and clustered thickly round the stem with no special leaf stalks.
4. At the ends of the stems, you will often find little structures, sometimes rather pink in colour, which look something like flowers (see fig. 130), but they are really quite different in their nature from true flowers.
Fig. 131. (a) The part of the Moss corresponding to the prothallium; (b) with the spore-capsule attached; (c) enlarged capsule, showing the covering; (d) naked capsule, showing the lid which falls off at (l).
5 and 6. There are no seeds and no seedlings.
7. At the top of some of those plants which seem to have flowers you will find later that a long slender stalk grows out with a little capsule or box at the end of it (see fig. 131 (b)). This single box or capsule really corresponds to the numbers of small spore-cases on the backs of fern-leaves, for it is in this capsule that we find the spores, which are simple and single-celled like those of the fern.
8. When these spores grow, however, they do not form a prothallium as they do in the ferns, but they grow out into the leafy moss-plant.
It is very difficult really to see how this can be the case, unless you study mosses very carefully with a microscope, but all the same it is true that the leafy moss-plant corresponds to the prothallium of the fern.
9. On the leafy moss-plant you find the simple stalk and capsule which gives rise to the spores; this spore forming part of the plant always remains attached to the leafy plant, so that we find the two portions of the plant in contact all their lives, and not separated as they are in the fern.
Fig. 132. A piece of Liverwort, showing the flat, creeping body, not divided into root, stem, and leaves.
The only other plants which are built on anything like this plan are the liverworts, though you might hardly believe it, because most of them are not marked out into leaf and stem at all, but are only flat, creeping, green structures, which do not look in the least like the mosses. It is true that they are not very near relatives, but because they have spore-cases rather like those of the mosses in some very important ways, the scientists have put them together in the big moss family. The true mosses have a special smaller family to themselves within this, a family which is quite easy for you to recognise when you go out on your rambles into the woods.
ALGÆ AND FUNGI
The last big family of plants is that containing the simplest plants of all. They are often very small and apparently unimportant, sometimes so small that we cannot study them at all without magnifying them very much with the microscope. In other cases they are quite large and easy to see; for example, the big red and brown seaweeds, and the many toadstools in the autumn woods. Sometimes they may even be very huge indeed, as are some of the seaweeds which grow in tropical seas. All the same, though we examine one which is as big as can be, it is really more simple in its detail than the mosses.
In very many of the algæ and fungi, the whole plant body consists only of one single cell. When this is the case, the plant lives floating or swimming about in water, or in very damp places. In rain-water which has stood for a long time you may find numbers of these tiny algæ. If you put some of the water in a glass tube and hold it against the light you may just see them, with a magnifying-glass, as specks of green, often swimming actively about.
The fine green “scum” which floats on many ponds and slow-moving streams consists of masses of these simple plants, in this case generally of forms in which the single cells keep attached together in long rows or chains, forming hair-like plants. Colourless plants of this kind are the fungi, which are often built on the same plan as the hair-like green algæ, only they do no food-building work for themselves, but live as parasites on other things. This is the case in many moulds and the plants which form potato-disease, and, in fact, the greatest number of plant-diseases are caused by such simple parasites.
All these plants are very small and simple, and as you can see at once, are not at all to be compared even with the mosses, but there are others which seem to be more complicated, as are the big seaweeds and the toadstools. Let us see how it is they are put in the same family as the simplest plants of all.
You can see, even with your magnifying-glass, that they have no special “water-pipes” in what you may call their “stem,” (for want of a better name), but that their whole body is built up of numbers of soft cells all very much alike, which twine in and out, and build a kind of soft weft; they have no really marked out stem and leaves. Look at a toadstool, for example, there is just a stalk and a cap spreading out above ground, while under the ground there are many twining thread-like strands (see fig. 133).
Fig. 133. A Toadstool, showing the “cap” and “stalk.” Under the cap are the radiating gills, on which the spores are formed. Thread-like strands under the soil.
Even in the seaweeds, which may seem to have stems, you will find that such is not really the case. They have generally a flat body, which is thin at the edges, with a stronger mid-rib, and the flat edges get worn away in the older parts of the plant, and so leave the mid-rib looking like a stem, though it is not so really (see fig. 134).
Fig. 134. A Seaweed, showing the branched body, which is not divided into stem and leaves.
When we come to look for flowers or even spore capsules, we see still more clearly how simple these plants are; they have not nearly such a complicated history as the moss. For example, in the toadstools we find that there are many spores formed directly on its lower surface, on the “gills,” and these grow out to form new toadstool plants. You can see the spores if you cut off a toadstool or mushroom head which looks full grown and is quite expanded, and then lay it on a sheet of gummed paper over-night, with the gills downwards, and another beside it with the gills up. Next day you will find that the paper under the one where the gills were downwards is covered with radiating lines of spores, just as they fell from the gills, and repeating their pattern.
Fig. 135. Part of a Bladder-wrack, showing the floats (f) and special swollen tips (s).
The seaweeds have the most complicated way of forming
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