The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
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Simple seeds which have wings are rather rare, but we find them on pine seeds (see fig. 125), and the seeds of the willow herb are covered with a number of silky hairs, which make them so light that they fly in the wind. It you watch a spray of willow herb ripening, you will find that the old carpels, or fruits, split up into four parts and let out a number of fluffy white seeds. These are true flying seeds (see fig. 84).
Fig. 84. Fruit of the Willow herb, opening and allowing the flying seeds to escape.
Other seeds get scattered by the wind although they do not fly. For example, in the poppy the fruit is the hardened ripe carpels which have become quite dry, and together look like a little round box, within which there are many tiny dry seeds. When the box or capsule is quite ripe openings come in it, just below the projecting top, and then, when the weather is dry and they are open, a strong wind may bend the stalk of the fruit and shake the capsule strongly. The seeds come scattering out like pepper from a pepper-pot, and may get carried some distance from their parent plant (see Plate II. and fig. 85).
Fig. 85. Ripe Poppy capsule, showing the little pores at the top which let out the ripe seeds when the capsule is shaken.
Some fruits are covered with spines and hooks, which catch on to the wool of animals, and so get carried quite a distance before they are dropped. This gives the seedlings a good chance of reaching a new spot where they can grow away from the parent, and so not be too crowded. Well-known fruits of this kind are the burs, which stick tightly to one another with their dozens of little hooks, the “bur” being really a cluster of many fruits together. Simple fruits of the same kind are the bidens, each with its two long spines, and the small fruits of the goose grass, which are covered with the finest hooks.
Fig. 86. A Bur, which is a cluster of hooked fruits.
Fig. 87. Simple fruits: (a) of the Goose grass with its hooks; (b) the Bidens with its harpoon-like spines.
Fig. 88. Strawberry. Each of the little “seeds” is a whole fruit, and the “flesh” the swollen receptacle.
Quite a special kind of fruit is the strawberry, which, as you know, has a thick fleshy pulp covered with a number of small, yellow “seeds.” In reality, each of these “seeds” is a whole fruit, and the thick flesh which we eat is the swollen end of the flower stalk which we call the “receptacle.” Therefore a strawberry really consists of a large number of fruits and a piece of stalk which is altered to form the fleshy, attractive mass which induces birds and people to eat the whole, and so scatter the little dry fruits.
There are very many other kinds of fruits which all have special devices to make sure that their seeds are scattered, and all proper fruits have seeds in them. But, just as we found that some garden flowers are grown only for their beauty, and do not set any seed, so we find that some fruits are grown specially without any seeds, such as bananas and some oranges. Such fruits are the result of our liking to eat the soft, sweet pulp without the trouble of the seeds, but such fruits are of no use to the plant.
Now let us look at the structure of the ripe seeds themselves, and see how they are fitted to go out alone into the world prepared to make a new plant. Seeds are all very much alike in the important points of their structure, although they vary much in the shape, size, and colour of their parts. We already know what beans are like from our careful study of them at the beginning of our work (see Chapter III.), and beans show us particularly well all the important parts of a true seed, so that we may take them as being typical of one large family of flowering plants. The maize embryo (see p. 10) is typical of the rest of the flowering plants. In the ripe seeds of both of these groups (you should examine them again if you have forgotten any of the facts) we find that the important thing is the baby plant, which is supplied with food and protected by two seed coats, till it is time for it to grow out and form a new plant like its parent.
Fig. 89. A, outside of Bean; (h) black scar showing where the bean was attached to the pod; (r) ridge made by young root; B, bean split open; (n) nurse leaves; (p) embryo; (a) scar where the embryo was separated from the nurse leaf on that side.
Fig. 90. A, outside of Maize, showing the embryo (e) on one side; B, sprouting, showing the root (r) and shoot (s); C, the same further grown.
THE TISSUES BUILDING UP THE PLANT BODY
Fig. 91. Two cells from plant tissue. (c) Living contents; (n) cell nucleus; (v) spaces filled with sap; (w) wall of cell (much magnified).
In our study of plants up to the present, we have only looked at their structures from the outside. We have examined the form, uses, and life of the parts of their bodies without looking for the details which might answer the question—“How are they built up?” Just as a house as a whole has a definite form, with rooms, and doors, and windows, each with their definite form and use, and at the same time every one of these things is built up of small separate bricks, tiles, pipes, and pieces of wood: so we find that the whole plant is composed of a number of definite parts, which are themselves built up of tiny individual parts, which we may take to correspond with the bricks of a house. Of course, they do not do this completely, for a plant is a living thing, and is far more complicated than a house, and each of the tiny individual parts is also a living, growing thing. These little building structures are called cells both in plants and animals, and they are so very small that you cannot study them fully without a microscope, and that is a very complicated and expensive thing, so we will leave it alone and only study what we can see of the structure of plants without it. Try, however, just to see a few cells under the microscope, so as to know what they are like. A typical cell has a wall, within which is the actual living substance, a clear, jelly-like mass, which contains many granules of food and stored material. Within this living substance is a more solid mass of still more actively living substance, which is called the nucleus. Cells like these, or cells which were like these when they were young, and which have become modified for special work, build up the whole plant body (see fig. 91).
Fig. 92. Piece of thin section across Water-lily stem, showing mesh-work tissue seen with a magnifying glass.
You can get a good hand magnifying-glass for several shillings, and with this and a very sharp knife, you can find out something of the structure of the insides of plants, even though most of the cells are so small as to be out of sight except when looked at through a microscope.
Let us first cut as thin a slice as possible across a water-lily stem, and put it on a small piece of glass and hold it up to the light. Examine it with the magnifying glass, and you will see that it is not a solid mass of tissue, but that it is built up of a fine network like lace, with quite large spaces between the threads (see fig. 92). These spaces are air spaces, and the fine lace-work threads are meshes built up of single rows of cells, which you may be able to see if your glass is a good one. Cells may be packed loosely like this, or they may be in more compact form something like a honeycomb, as you may see in the pith of an elder twig and many other stems. You can crush these soft cells between your fingers, and we cannot imagine that they could build up the hard, firm branches of trees.
Now examine the stem of a seedling sunflower, by cutting a very thin slice across it; you will see in it a ring of strands where the cells are smaller than those of the soft tissue and also much more closely packed (see fig. 93). Then cut a thin slice longways down the stem, and you will see that these more solid strands are the cut ends of long strings of such tissue which run through the stem. The cells which build up these strings are not quite ordinary cells, but are exceptionally long, like water-pipes, and they have thickened walls. These cells do the carrying of water and liquid food up and down the plant (see fig. 94).
Fig. 93. Piece of the stem of a seedling Sunflower cut across, showing strands of “water-pipe” cells.
Fig. 94. Piece of stem cut across and then split lengthways, showing the strands of thicker “water-pipe” walls.
You can see that the water travels up these cells if you cut across a stem near the root and place it in a little red ink. After a few hours if you cut a section several inches from the bottom of the stem you will find that these strands are coloured red by the ink which has passed through them, while the rest of the stem is very little coloured, or quite colourless. This shows us that these strands are the special water-pipes of the plant.
Fig. 95. Cross section through a Lime twig three years old seen with a magnifying glass.
Large numbers of such cells closely packed together, and with some other hard cells between them, make up the wood in woody stems. Cut across a small twig of lime or oak and examine it with your lens. Outside is the brown bark, then within that some green cells and a little soft tissue, while most of the stem is made up of a mass of hard wood cells, among which you can see some of the larger water vessels distinct from the rest. All this hard tissue really corresponds to the joined up separate strands which we saw in the sunflower stem (see fig. 95). Trees like the lime and oak, which live for a long time, grow for a certain amount every year, and each year they add a ring of wood to their stems. In old stems you can see clearly the rings of wood which have been formed by each year’s growth. This
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