The spores of the Horsetails are not long in developing after they have settled in a damp situation. The actual forms which the prothalli take are often very irregular. That of the male prothallus is usually rather small; on the other hand, the female prothallus is sometimes large, and may have complicated branchings. As in the case of the other Vascular Cryptogams which have been considered, spermatozoids are produced in the antheridia. These are very active, and travel through the agency of water to the archegonia on the female prothallus. The spermatozoids unite with the various egg cells, and in this way an embryo is formed which finally develops into the mature plant. Owing to the fact that the prothalli of the Horsetails have proved to be excessively difficult to cultivate, the life history has not been so completely worked out as in the case of the Ferns and Club Mosses. One interesting point in connection with the cultivation of Horsetail spores has been brought to light. Whenever the spores are growing on poor soil, by far the larger number of them produce antheridia. On the other hand, where there is plenty of nourishment the tendency is all the other way. The matter is of interest, as it appears to show that the amount of available nutriment is a definite factor in the determination of sex. CHAPTER III YESTERDAY AND TO-DAY Although to most people the study of fossil botany may appear to be an uninviting pursuit, there can be no question as to the importance of the science. It is only in this way that we are able to appreciate the changes which have led up to the existing types of plants. Now the question of the past history of the Vascular Cryptogams is of very special interest in more ways than one. It is, of course, most fascinating to be able to discover what kinds of Ferns flourished, for instance, at the period when the coal deposits were being formed. But, even in a cursory description, it will be quite impossible to allow the matter to rest there. The story of the past, in the case of the Vascular Cryptogams, is closely interwoven with some of the most absorbing phases in the evolution of the Flowering Plants which are such a dominant feature on the earth at the present time.