The author trusts that those of his readers who are interested will pursue the study in the admirable textbooks which are now available. The Horsetails, like the Club Mosses, have had a very important past. Although they are few in number, as far as the species are concerned, they still retain many striking characteristics. Without a doubt the Palæozoic Horsetails grew into giant plants, sending out branches and developing trunks which in some ways are comparable to those possessed by our trees at the present time. These great stems seem to have arisen from rhizomes which travelled about in the mud of the coal jungles. It is usual to refer to these Palæozoic Horsetails as Calamites, owing to the fact that they were originally supposed to bear a resemblance to a reed (_Calamus_). In the later rocks, such as those which belong to the Jurassic and Triassic periods, occur the Equisetites, plants which were still of great size, but already in some respects showing signs of that decline which has culminated at the present day in the little group of plants which, were it not for a certain robustness of growth, would find it hard to maintain their position at all. To complete our brief survey of the Vascular Cryptogams it is now necessary that we should review the position of these plants at the present time. Of course in number the Ferns are enormously in advance of all the other plants put together. In the whole world, there are not far short of seventy distinct genera, which include anything between three and four thousand species. The Ferns of the United Kingdom number not far short of fifty, and there are certain variations from the type which some folk are tempted to include as species. For some reasons which we cannot well understand, the Ferns alone amongst the Vascular Cryptogams have been able to hold their own in the world. It is probable that there are quite as many species, and that these are as varied, to-day as has ever been the case. The size of Ferns, as we have already seen, varies enormously. In the tropics and in Australasia there are Tree Ferns eighty feet in height, whilst with many of the Filmy Ferns the size is scarcely larger than that of Mosses. Owing to the fact that it is so necessary in the scheme of reproduction, the majority of Ferns are lovers of moisture. None the less, a few specimens have adapted themselves marvellously to drier conditions. Thus the Bracken will grow on the exposed hillside or cliff-top even where its rhizomes cannot carry the roots to a great amount of moisture. Some of the most interesting species of Ferns are those which grow on walls and rocks, where there is little dampness, during the summer at any rate. Many of these have adopted special devices to cope with drought, such as are to be seen in the Scaly Spleenwort. Here the underside of the frond is covered with hairy scales, and in dry weather the leaves roll up so that the well-protected underside is alone exposed to the sun.