Lecture X Bees And Flowers Continued…
In all these flowers the top petal stands up like a flag to catch the eye of the insect, and for this reason botanists call it the “standard”. Below it are two side-petals called the “wings,” and if you pick these off you will find that the remaining two petals are joined together at the tip in a shape like the keel of a boat. For this reason they are called the “keel”. Notice as we pass that these two last petals have in them a curious little hollow or depression, and if you look inside the “wings” you will notice a little knob that fits into this hollow, and so locks the two together. We shall see by-and-by that this is important.
Next let us look at the half-flower when it is cut open, and see what there is inside. There are ten stamens in all, enclosed with the stigma in the keel; nine are joined together and one is by itself. The anthers of five of these stamens burst open while the flower is still a bud, but the other stamens go on growing, and push the pollen-dust, which is very moist and sticky, right up into the tip of the keel. Here you see it lies right round the stigma, but as we saw before in the geranium, the stigma is not ripe and sticky yet, and so it does not use the pollen grains.
Now suppose that a bee comes to the flower. The honey she has to fetch lies inside the tube, and the one stamen being loose she is able to get her proboscis in. but if she is to be of any use to the flower she must uncover the pollen-dust. See how cunningly the flower has contrived this. In order to put her head into the tube the bee must stand upon the wings, and her weight bends them down. but they are locked to the keel by the knob fitting in the hole, and so the keel is pushed down too, and the sticky pollen- dust is uncovered and comes right against the stomach of the bee and sticks there! As soon as she has done feeding and flies away, up go the wings and the keel with them, covering up any pollen that remains ready for next time. Then when the bee goes to another flower, as she touches the stigma as well as the pollen, she leaves some of the foreign dust upon it, and the flower uses that rather than its own, because it is better for its seeds. If however no bee happens to come to one of these flowers, after a time the stigma becomes sticky and it uses its own pollen: and this is perhaps one reason why the bird’s-foot trefoil is so very common, because it can do its own work if the bee does not help it.
Now we come lastly to the Orchis flower. Mr. Darwin has written a whole book on the many curious and wonderful ways in which orchids tempt bees and other insects to fertilize them. We can only take the simplest, but I think you will say that even this blossom is more like a conjuror’s box than you would have supposed it possible that a flower could be.
Let us examine it closely. It has sic deep-red covering leaves, Fig. 62, three
belonging to the calyx or outer cup, and three belonging to the corolla or crown of the flower; but all six are coloured alike, except that the large on in front, called the “lip”, has spots and lines upon it which will suggest to you at once that they point to the honey.
But where are the anthers, and where is the stigma? Look just under the arch made by those three bending flower-leaves, and there you will see two small slits, and in these some little club-shaped bodies, which you can pick out with the point of a needle. One of these enlarged is shown. It is composed of sticky grains of pollen held together by fine threads on the top of a thin stalk; and at the bottom of the stalk there is a little round body. This is all that you will find to represent the stamens of the flower. When these masses of pollen, or pollinia as they are called, are within the flower, the knob at the bottom is covered by a little lid, shutting them in like the lid of a box, and just below this lid you will see two yellowish lumps, which are very sticky. These are the top of the stigma, and they are just above the seed-vessel, which you can see in the lowest flower in the picture.
Now let us see how this flower gives up its pollen. When a bee comes to look for honey in the orchis, she alights on the lip, and guided by the lines makes straight for the opening just in front of the stigmas. Putting her head into this opening she pushes down into the spur, where by biting the inside skin she gets some juicy sap. Notice that she has to bite, which takes time.
You will see at once that she must touch the stigmas in going in, and so give them any pollen she has on her head. but she also touches the little lid and it flies instantly open, bringing the glands at the end of the pollen-masses against her head. These glands are moist and sticky, and while she is gnawing the inside of the spur they dry a little and cling to her head and she brings them out with her. Darwin once caught a bee with as many as sixteen of these pollen-masses clinging to her head.
But if the bee went into the next flower with these pollinia sticking upright, she would simply put them into the same slits in the next flower, she would not touch them against the stigma. Nature, however, has provided against this. As the bee flies along, the glands sticking to its head dry more and more, and as they dry they curl up and drag the pollen-masses down, so that instead of standing upright, as in 1, Fig. 63, they point forwards, as in 2.
And now, when the bee goes into the next flower, she will thrust them right against the sticky stigmas, and as they cling there the fine threads which hold the grains together break away, and the flower is fertilized.
If you will gather some of these orchids during your next spring walk in the woods, and will put a pencil down the tube to represent the head of the bee you may see the little box open, and the two pollen-masses cling to the pencil. Then if you draw it out you may see them gradually bend forwards, and by thrusting your pencil into the next flower you may see the grains of pollen bread away, and you will have followed out the work of a bee.
Go to Lecture 10-Part 5 here.