Some ferns do not follow all the steps exactly as we have outlined, for all of them have not the structure of the typical one whose life history has been sketched above. In the adder’s-tongue fern, for instance there is a stalklike prolongation from the base of the only leaf the plant bears, on which all the spores are borne. In certain others, as in the ostrich fern, the spores are borne on leaflike growths that serve only this function. Most ferns, however, bear spores on otherwise unmodified foliage leaves and the great bulk of them on the under side of such leaves. There are several things about the life history of a fern that differ fundamentally from any flowering plant and perhaps the chief is what is known as the alternation of generations. A spore, for instance, can never produce a fern as a seed will always produce a flowering plant. In this respect they are like many insects that always have two or sometimes three different stages in their life history. Only by the complicated method of first a spore then the prothallus, from which archegonia and antheridia are produced, followed by the free swimming male cells fertilizing the female, can a fern reproduce itself. As we shall see in the chapter on the History of the Plant Kingdom, this alternation of generations, the absolute necessity of water in which to carry on the fertilization, and above all the ability of the male cells for free swimming in the water, are all landmarks in the development of plant life. In its simplest form fertilization in flowerless plants is characterized by one or all these processes, as it is in the ferns, while in the flowering plants, the act is accomplished by processes, discussed previously, which, in the development of the plant kingdom, mark a period only comparable, in the history of man, to such tremendous achievements as the acquirement of speech or the ability to make a fire. LIFE HISTORY OF A MOSS Ever since the war, the peat-forming mosses, known as sphagnum, have become more widely known to the general public than any of the ten or twelve thousand mosses known to grow on the earth. Its power of absorption, greater than linen bandages, made it extensively used to pad surgical dressings. Hundreds of thousands of these sphagnum [Illustration: A SMALL CLOUD OF WIND-BLOWN POLLEN OF THE JAPANESE RED PINE (_Pinus densiflora_). (_Photo by C. Stuart Gager. Courtesy of Brooklyn Botanic Garden._)] [Illustration: A COCONUT GROVE IN THE PHILIPPINE ISLANDS. The people of tropical regions have more uses for this plant than there are days of the year. Its fruits will float in the sea for months without injury and it is thought to have been distributed all over the tropical world by ocean currents. Its true wild home is not certainly known, but is probably tropical America. See chapter V for an account of the tree. (_Courtesy of Brooklyn Botanic Garden._)]