There remains to be considered the life history of the Horsetails. The spores are always produced on special processes, which are arranged in the form of a cone at the apex of the stem. The sporangia are borne on curious scales which are supported by stalks placed in the centre. These scales are arranged in whorls round the centre of the stem, and there may be twenty or more in each row. On the underside of these scales we shall find the sporangia—almost any number of them up to ten. Each sporangium produces a considerable number of spores, so that every cone is responsible for an enormous number. These spores are all of one kind, and they are so singular that they are worth a somewhat detailed description. The covering of the spore really consists of four layers, the outermost of which is split spirally in such a way that two long arms with flattened ends are produced. As long as the spore is damp these remain closely gathered round, but under dry conditions they are stretched out. The movements of these arms or elaters, as they are called, are readily watched under a microscope. By gently breathing on the spores we bring them under the influence of moist air, and this causes the elaters to curl up; after a while, however, when they become dry, the arms stretch out again. It is not certainly known what is the use of these elaters. A very little observation shows that the opening and closing of the arms keeps the spores on the move; this would aid dispersal at the time of the bursting of the sporangium. Another point which is worth consideration is that although the spores appear to be exactly the same, yet as a rule they develop on distinctive sex lines. It is obviously important that the male and the female prothallus should be together. The long arm-like processes on the spores often link the little bodies side by side, and this would be an advantage.