The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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When we examined flowers in general, we found that the insects do a very important work in carrying the pollen from flower to flower, and that their structures are arranged to attract insects and to make it easy for them to get covered with the pollen of one flower and leave it on the next. If we look at the details in some of the flowers, we shall see how elaborate their structures may be, and how carefully they are planned to make sure that the bee gets the pollen on its body and carries it with it to the neighbouring flowers.
Fig. 116. Circular flower of Rose, with many stamens in the centre.
In the simple circular flowers, such as roses, poppies, and lilies, the bee can enter freely from any side that it chooses, and it generally goes straight to the centre. Many of these simple flowers, therefore, have large numbers of stamens which stand up in a crown in the middle, so that the bee must touch and stir some of them as he dives in the centre for the honey.
Fig. 117. Slightly two-sided flower of the Foxglove, with the petal tube cut open to show the four stamens bending to the front.
In others which are nearly circular, there is a little difference between the back and front of the flower, and the stamens are so placed that the visiting insect must touch them. For example, look into the bell of a foxglove, where you will find only four stamens, but they are bent so that the anthers together form a kind of platform in the front of the flower, over which the bees must pass as they enter (see fig. 117). Frequently the stamens bend in this way towards the front of the flower, and in many cases the whole flower becomes quite definitely two-sided, with a front and back, and a special place for the entrance of the bee. This is the case with the violet, pea, monkshood, and many others (see fig. 118).
Fig. 118. Two-sided flowers: A, Monkshood; B, Violet.
When flowers have this form, you frequently find that the number of stamens is quite small, seldom more than ten, and often less.
A plant of this kind very interesting to watch is the yellow gorse. If you can get up and sit by a flowering bush from about half-past five to seven one sunny morning, you will be able to learn a great deal about the doings of the bees and flowers.
Fig. 119. (a) Flower of the Gorse after the insect’s visit, showing the inner parts exposed; (b) young flower nearly ready to be visited.
First examine a flower so that you know how it is arranged. At the back lies the big petal, or “standard,” as in the pea; there are two side wings, and in the front the two petals close together forming the “keel.” The two-sidedness of this flower is very well marked. Inside the keel you will find ten stamens, all joined to form a tube except the back one, which is free, and inside them lies the carpel with its curved style. When the stamens are ripe they are so fitted that they lie inside the keel of the petals in a bent form, and when they are pressed from above they fly out with a little explosion and scatter the pollen dust about. Now watch a bee alighting on the flowers; he presses the two front petals with his legs to open them to get at the honey, and the stamen explosion covers him all over with pollen. Then he goes to the other flowers, but perhaps the next one he visits has already exploded and the ripe stigma is exposed in the front of the flower, and as he settles he touches it with his furry body all covered with pollen, and leaves some on it. If you watch the bees doing this yourself, you will find out a number of things which I have not told you, while you may notice how some of the bees are lazy and enter the wrong side of the flower, others are stupid and go to flowers which have already been visited several times, and therefore are of no use, while other bees which come late may open up buds which were not ready for them and steal the honey before the stamens are ripe enough to smother them with pollen. I have watched them opening buds which were still so tightly closed that it took them all their strength to get in. But we must not stop too long with one flower, for almost every flower has some special arrangement of its own, and all are worth study.
Fig. 120. The two kinds of Primrose flowers, A, with long style and stamens low in the petal tube; B, short style, with stamens at the mouth of the petal tube.
The primroses and cowslips are interesting, as they have two kinds of flowers. It you gather a bunch of primroses and look into them you will find that in some you can see the little central green ball of the stigma, and in others at the top of the tube are the five small anthers. These two kinds of flowers make an arrangement which ensures that the pollen from the one kind of flower reaches the stigma of the other. A big fly like the wasp-fly, and several others, visit these flowers most frequently, and carry the pollen from flower A (see fig. 120) to the stigma of B, and the pollen of B to the stigma of A.
Fig. 121. A, Flowerhead of the Daisy; (b) a single little flower from the side with big petals fused together; (c) a single little flower from the middle with very small petals.
As we noticed before, the chief duty of the petals is to act as flags to attract the visiting insects by their bright colours. Now we find that some flowers club together, and grow clustering closely on one head, so that it is sufficient for a few of them to have the flag petals which attract the insect to the group, as it goes from one to the other when once it is there. When a few of the flowers do this, the rest can economise in petals and have quite small ones, and yet all the same they have a good chance of insect visits. Such an arrangement as this is found in the daisy (see fig. 121). A single daisy is not one flower, but a whole bunch of flowers, in which some of the outer flowers of the bunch (see fig. 121 (b)) form big petals, while all the inner ones (fig. 121 (c)) are quite small and inconspicuous, and by themselves would hardly attract any visitors. Just the same thing happens in the cornflower, sunflower, and very many members of the daisy family. The big outer petals attract the insect, and once on the head of flowers it walks about over them, and they all get the benefit.
In such cases we get a division of labour among the flowers of a head, and this represents what is perhaps the highest state of development that flowers have reached.
Fig. 122. Flowerhead of the Cornflower; (a) a single flower from the side with big petals.
THE FIVE GREAT CLASSES OF PLANTS INTRODUCTORY
If you go out into the garden, or fields and woods in summer, and look around you at the plants, you will find that nearly all of them are flowering, or have flower-buds, or have the proof of having had flowers in the shape of fruits and seeds. Even among the few which do not show any of these things, many will probably be plants which you know to be the same as others of their kind which you have seen flowering.
Generally flowers (such as roses and daisies) are easy to see, but in some plants they are less showy, as in the oak, for example, where the little green tails or catkins which come out early in the spring are the flowers. On the whole, however, if you look carefully, you will have no difficulty in seeing proof that nearly all of the conspicuous plants of our gardens and woods bear flowers.
All the same, there are very many other plants, some of them quite easy to see, and others very small and inclined to hide, which do not have flowers at all, and which are so different from the flowering plants that even before you have studied them, you instinctively separate them. The seaweeds or mosses, for example, are at once recognized by any one as being of a different family from roses and lilies.
When you have studied all the plants carefully, you will see how true is this instinctive separation of the chief families, and how nature seems to have made five principal big families, so that both scientists and quite unlearned people see more or less clearly the limits she has set to each.
The family which is most highly advanced is that of the flowering plants, but the others, too, are well worth study, and we will now notice some of the points about their structure which are characteristic of each of the families.
FLOWERING PLANTS
All the plants which have flowers are put into one big family, about which you already know a good deal, because nearly all the plants we have studied up to the present have been plants which have flowers. Let us now go systematically over the chief points about their structure, so that we may have a clear idea of their characters, and be able to compare other families with them.
1. We find that the plant body is clearly marked out into root, stem, leaves, and flowers. The stem may be green and delicate, or it may be thick and strong like an oak tree, and on the stem or its branches we find the leaves.
2. The stem and root have definite strands of “water-pipe” cells, and very often the stems have many rings of wood, one of which is added every year.
3. The leaves are very various in the different plants, but they are generally thin and big, though they are seldom much more compound than those of the sensitive plant.
4. The flowers are easily recognized, as a rule, and consist of a number of parts, some of which are often brilliantly coloured. The stamens and carpels are generally in the same flower.
5. The seeds are always enclosed within the carpels, and have generally two seed-coats.
6. Within the seed are always either two cotyledons, as in the bean, or one cotyledon, as in the grasses. Thus when the seedling grows out of the seed it may have two first leaves or one only.
These are the chief characters of the whole big family of the flowering plants, but this big family is separated into two smaller groups according to the number of cotyledons in the seed. Those that have two form the group of Dicotyledons, those with one the group of Monocotyledons. This may not seem a very important point to form the ground for separating plants with flowers so alike as tulips and roses, but we find that, as well as the number of cotyledons, many other differences distinguish the two groups when we separate them in this way. For example, the Dicotyledons have the veins of their leaves so arranged as to form a network, as in the lime, while the Monocotyledons have them parallel, as we noticed in the grasses and lilies.
We also find that it
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