Many leaves are clothed with woolly hairs while in the bud, which afterward disappear. Thus, in the rhododendron, horse-chestnut, and other species, the young leaves are protected by a thick felt, which, when they expand, becomes detached and drops off. Many leaves are smooth on the upper side, while underneath they are clothed with a cottony, often whitish, felt. This probably serves as a protection for the stomata. In some cases the hairs probably tend to preserve the leaves from being eaten. In others, as Kerner has suggested, they serve to keep off insects-apparently with the special object of preventing 
Fig. 20 the flowers from being robbed of their honey by insects which are not adapted to fertilize them. Fritz Müller, to whom we are indebted for so many ingenious observations, gives an interesting case. The caterpillar of Eunomia eagrus, when about to turn into the chrysalis (Fig. 20), breaks off its hairs and fastens them to the twig which it has selected, so as to form on each side of itself about half a dozen stiff fences, to protect it during its helpless period of quiescence.
Vaucher long ago observed, though he gave no reason for the fact, that among the Malvaceæ (mallows) the species which produce honey are hairy, and those which do not are glabrous.
If we make a list of our English plants, marking out which species have honey and which have hairs, we shall find that we may lay it down as a general rule that honey and hairs go together. The exceptions, indeed, are very numerous, but when we come to examine them we shall find that they can generally be accounted for. I have made a rough list of the species in the English flora which have honey and yet are glabrous. It does not profess to be exactly correct, because there are some species with reference to which I was unable to ascertain by personal examination, or by reference to books, whether they produced honey or not. My list, however, comprised 110 species.
Now, in the first place, of these 110 species, in sixty the entrance to the honey is so narrow that even an ant could not force its way in; twenty are aquatic, and hence more or less protected from the visits of ants and other creeping insects; thus we shall frequently find that, if, in a generally hairy genus, one or more species are aquatic, they are also glabrous—as, for instance, Viola palustris, Veronica anagallis, V. beccabunga, and Ranunculus aquatilis. Polygonum amphibium is peculiarly interesting, because, as Kerner has pointed out, aquatic specimens are glabrous; while in those living on land the base of the leaf
