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. [Illustration: Fig. 122. Flowerhead of the Cornflower; (~a~) a single flower from the side with big petals.] PART IV. 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.