Have you ever wondered why cup-shaped flowers—the harebell, the snowdrop, and many others—droop their heads? It is because they would become filled with rain or dew if they did not do so, and thus their nectar would be spoiled, and insects would no longer visit them. For the same reason daisies will close their petals when dark clouds come up, and will remain closed until the sun shines again. Have you ever seen a flower of the white dead nettle? It actually protects its nectaries with one of its petals, which overhangs the others, and acts like a little umbrella.
The ordinary nasturtiums (Plate XXXVII.) have the edge of the three lower petals cut into fine strips. These keep the rain from the nectar, which is situated at the end of the long spur. You will notice that hive bees are not often seen on nasturtiums, for their tongues are not long enough to reach the nectar; so these flowers depend more on humble-bees for fertilisation. The nasturtium is a flower which illustrates very well what was said about “honey-guides” just now, for all the lines on the petals point to where the nectar is to be found.
Some flowers have to protect their honey from certain insects, who wish to take it without fertilising the flower in return. Ants, for instance, are very fond of honey; and, as you can easily imagine, they are so small that they can creep right down to the nectaries without dusting themselves with pollen, or fertilising the flower. So certain flowers—like the primrose—have their stalks covered with multitudes of tiny hairs. These serve as a barricade to the ant, and prevent it from climbing to the flower above. The cross-leaved heather has its stalk and calyx covered with sticky hairs, so that not only are the little thieves prevented from getting to the flower, but they are actually held prisoners as well.
CHAPTER XLIV
HOW FLOWERS ARE FERTILISED
WE have now seen something of the contrivances of flowers to aid in their fertilisation, and in this chapter we shall consider the ingenious arrangement some flowers possess to assist their fertilisation.
(a)(b)
Let us first look at the primrose. Have you ever noticed that there are two kinds of primrose flowers? From the outside perhaps they look very similar, but if you look closely, or better still, cut them open, you will find where they differ. Let us look at these sketches and we shall see that the one kind (a) has a long style, which reaches nearly to the top of the corolla. The other kind (b) has quite a short style, so that instead of the stigma, or knob, being at the top of the corolla, it is really half-way down. We notice, too, that the anthers, or pollen bags, in the first kind (a) are placed half-way down the corolla, and in the other flower (b) they are at the top. We might think that Nature had made some mistake here, for it seems that if the pollen bags belonging to flower (a) were placed in flower (b), or vice versa, things would be more natural.
Let us suppose that a bee visits flower (a) and dips her tongue down the corolla to collect the nectar. Half-way down the flower the tongue has to pass the pollen bags, and in doing so gets dusted over with pollen grains. The bee, having collected the nectar, flies to another plant, which we will suppose bears flowers of the other kind. She dips down her tongue, which touches the stigma just at the place where it had been covered with pollen by the first flower. By this means, therefore, the flower (b) is fertilised. But, you will ask, what about flower (a)? While the fertilisation of flower (b) has been going on, the pollen bags of (b) at the top of the corolla have dusted the root of the bee’s tongue, so that when she goes to a flower of the (a) type, the pollen dust at the root of her tongue touches the stigma, and the flower is thus fertilised.
What a wonderful arrangement this is, for you will see that it is almost impossible for the flowers of one primrose plant to fertilise each other; the pollen must come from the flowers of a different plant.