The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
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Fig. 76. Buttercup flower laid open, showing that there are many seed-boxes (s) in the centre of the flower. R, the receptacle is the swollen end of the flower stalk.
Fig. 77. Carpels of the Tulip cut open to show that there are three spaces with seeds, each division representing a single carpel.
In the pea, tulip, buttercup, and many others, the carpels are in the centre of the flower, above the petals, and attached to the swollen end of the flower stalk, which is called the receptacle, as in fig. 76 R. Other flowers have the receptacle hollowed out like a cup or goblet, and the carpels sunk right in it. When this is the case, we generally find that the sepals and petals lie above the carpels and not below them, as in fig. 78. This is also the case in the rose, where in fig. 70 flower B shows clearly the swollen part below the bud, which is the hollowed receptacle containing the carpels, and the same is true of the harebell (see fig. 69) and many flowers.
Fig. 78. Flower of Cherry cut open to show the hollowed receptacle, R, below the level of the petals, and containing the carpel, C.
What can be the use of the sticky tip that we found on the carpels? Examine the tip of the carpel of a lily which is well open, and you will very likely find some of the yellow pollen sticking to its surface. It is a curious fact that the little structures within the carpels which will become seeds cannot ripen into true seeds unless they are waked up to growth by the pollen grains. The sticky tip of the carpels (or stigma, as it is called) catches the pollen grains and holds them; then they grow down into the carpels and carry with them the nuclei (see p. 92) that enter the undeveloped seeds. These stir the cells to divide, and after many divisions the embryos are formed and the seeds ripen. Sometimes the stigma has a long stalk which places it in a good position to catch the pollen grains. This stalk is called the style, and is to be found in many flowers (see fig. 72).
The pollen dust is fine and light, and may be carried by the wind on to the stigma, as it sometimes is in poppies, and always is in pine-trees; but this is rather a wasteful way, because the wind blows so irregularly that very much pollen is lost and never reaches the stigma. In order to save some of this loss, and to make the pollinating more certain, flowers have arranged their parts so as to make use of the help of insects. You know that very many flowers have sweet honey in them which the bees like, and come to collect, going from flower to flower to do so. When the bee settles on the flower it gets covered with the pollen dust, and then when it goes to the next flower and walks over it, it is almost sure to leave behind it some of the pollen sticking on the stigma. Of course, in this way also some pollen is lost, but insects are far more reliable than the wind. We now see the use of the bright coloured petals; they help to attract the bees to the flower. The flowers have made the bees and other insects their special carriers of pollen, and they pay the insect with honey, and some of the surplus pollen. Bees generally go from flower to flower of the same kind on any one day’s journey, so that the flowers get pollen from others of their own kind. This is important, for “foreign pollen” (as the pollen from quite different kinds of flowers is called) does not help the young seeds at all.
We have now found a use for all the parts of the flower.
Fig. 79. A, Violet, a two-sides flower. B, Primrose, a circular flower.
There are many special things about flowers which we must leave till later on, but we may just notice now how some are regular, like the primrose, rose, poppy, and so on, which are after the pattern of a circle, and appear the same from whichever side you look. Others, like the violet, larkspur, or sweet pea, are not regular, but have only two sides alike. This difference is very often due to some special structure of the flower in relation to the insects which visit it, and if you examine and compare the two-sided with the circular flowers you will generally find that the two-sided flowers are the more complicated. Some of them become very complicated indeed, like the orchids, which have such strange flowers, and in which the relation between the insects and the flower has become very special.
We must leave these more complicated cases till Chapter XXII., and come back to the simple important facts about the work which all flowers have to do. They must make sure that in some way or other, seeds are formed for the plant. If the flower does not do this, then it is not doing the work for which it was made.
You will find a number of flowers in gardens which do not do their work properly, and very often have no seeds at all, but they are specially cultivated by gardeners to do other things. For the study of the true structures of flowers, it is generally better to examine wild flowers instead of garden ones, which are often much altered by the rather unnatural conditions in which they are made to live.
FRUITS AND SEEDS
Fig. 80. A, Pea-flower. B, the same beginning to fade, with the ripening carpel breaking through the keel. C, the same carpel much enlarged, the petals and stamens quite faded.
Within the flowers we saw, protected and shut in, the carpels or seed-box, within which are the very young structures which will become seeds. Now let us watch them develop. In such flowers as the sweet pea, for example, in summer-time, this will not take very long. Mark a special flower, and watch it each day; you will find the little green pod will gradually grow bigger, till it splits away the petals which are beginning to wither, and pushes out between them. As the pod gets larger you can see the seeds within growing too, if you look at the pod carefully against the light. The stigma does not grow any further, as its work was finished when it had caught the pollen grains. After a time the petals and stamens drop right away, and only the calyx remains; it does not grow very much, but it keeps fresh and green for some time, as it has still to act as a cup to hold the pod. It only takes a few days for these things to happen, then till the pea pod is quite ripe may take a week or two more. The pod continues to grow and turns yellowish brown and dry, then one day when the sun is warm you may see and hear it split open suddenly down its central ridge, and shoot out the brown, dry seeds. Then the work of the flower is quite over, and the seeds have started to make their own way in the world.
Fig. 81. A, ripe carpel of pea or “pea pod.” B, pod suddenly split and twisted up, scattering the seeds.
Let us pick a nearly ripe pea-pod and examine it; it is the ripe carpel, with several ripe seeds in it, and together they form what is called a fruit. In the case of the pod the “fruit” itself is a dry pea-pod husk, but in other plants the fruits may be very different. Examine a marrow, for example. Watch it in the course of its growth, if possible, and you will find that the marrow flower is one of those with its seed-box below and outside the calyx and petals. As the marrow ripens this swells with the food stored in it, and the many growing seeds, till the flowers are only small shrivelled structures at one end. If you then cut the ripe, or nearly ripe, marrow fruit across, you will find that its wall is very thick and fleshy, and that the many seeds are buried in a soft pulp. The melon shows us just the same thing. Such fruits seldom split suddenly to shoot out their seeds (though some foreign ones do); they depend more on animals which may eat them and so scatter the seeds about.
In all cases it is better for the plant to have the seeds scattered so that they do not sprout too near together, but have room enough to grow without crowding each other out.
In the pea and marrow there are many seeds, but there are large numbers of fruits in which we find only one. For example, in plums, cherries, and peaches we have a fleshy outer fruit-case with a stony lining covering over one large seed. Such fruits do not open, for there is only one seed within, and so the fruit is scattered whole. These fruits nearly always get scattered by animals, for the flesh is very sweet and attractive to eat, and then, as a rule, they get rid of the stone (which contains the seed) at some distance from the parent.
Fig. 82. Cherry fruit cut open, showing the flesh (f), stone (s), and seed S.
Sometimes we find a number of fruits just like the cherry clustered together, only instead of each of them being large, they are all very small, so that the whole cluster of fruits may be the size of a single cherry. This is the case in the common blackberry and the raspberry, where each of the little fruitlets really corresponds to a cherry.
Then there are many fruits which belong to quite a different class, and arrange to scatter themselves by the help of the wind, such as the fruits of the dandelion, thistle, and many others, which have light “parachutes,” and therefore blow away with the least puff of wind when they are ripe and dry (see fig. 83).
Fig. 83. A, head of Dandelion fruits, with most of them scattered. B, single Dandelion fruit.
Other fruits like the sycamore
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