exactly known, it is suggested that this may be malic acid, seeing that experiments have shown that the spermatozoids are attracted to this product when it is artificially introduced. Of course the matter is a difficult one to prove, in that the viscid drop is so minute that it is impossible to determine the nature of the substance which it contains. Malic acid has, however, been discovered to be present in the prothallus as a whole. Although there is quite a competition amongst the spermatozoids as to which shall enter the archegonium, it is likely that only one actually succeeds in entering the egg cell. As a rule, too, in the prothallus only one of the archegonia shows any further signs of development.

After the fertilization is completed the first happening is the formation of a cell-wall round the ovum. Passing through various stages of growth and subdivision it finally forms the embryo of the young plant. For a while the newly-born Fern relies upon the prothallus for sustenance, but eventually starts an independent existence. The baby sends down roots into the ground and leaves up into the air, and from thenceforward its development into a mature plant will only be a matter of time.

The life histories of the Club Mosses have certain points which make them of special interest. In the first place, the manner of reproduction to be observed in the Lycopodiums may be outlined. Sometimes the sporangia are borne on fertile leaves which exactly resemble the ordinary ones; a good instance of this is seen in the case of the Fir Club Moss (Lycopodium selago). On the other hand, the fertile leaves may be gathered together into cones such as are to be observed in the case of the Common Club Moss (L. clavatum). The sporangium is quite a large affair, easily discerned with the naked eye. The number of spores produced is enormous—so much so that these can be shaken out in a thick powder. In some of the Lycopodiums the spores have a remarkable habit of resting before the development of the prothallus. It is said that in the case of the Common Club Moss the prothalli do not appear until the end of the sixth year. Even then, several more years elapse before the prothallus is sufficiently mature to bear the organs of sex. In the case of L. inundatum the length of time which elapses between the coming of the prothallus is nothing like so great. With all the Lycopodiums, however, the prothallus, which varies a good deal in the different species, bears both kinds of sexual organs. In most of the cases where the development of the spore is such a long business the prothallus is produced underground. With L. inundatum, however, the prothallus is green and leaf-like. The underground prothalli have no chlorophyll, and hence cannot live the life of an independent green plant. It has lately been demonstrated that these are always found in conjunction with a certain fungus; probably the benefits of the association are mutual. Making allowance for certain differences which are not of great importance to the general student, the fertilization of the egg cell in the prothallus of the Lycopodium is carried out on very similar lines to those which have been described in the case of the Fern.

The life histories of the Selaginellas evidence some important differences which call for special comment. Here throughout the whole family the spore-bearing part of the plant is in the form of very definite cones. As in the case of Lycopodium there is only one sporangium to each leaf, but they are of two kinds. One, on account of the fact that it is responsible for the production of small spores, is called the microsporangium; the other, the megasporangium. The two kinds of sporangia are usually present on the same cone, although the microsporangia are as a rule higher up the stem than the megasporangia. The number of microspores produced is very large, but only four megaspores are borne in each megasporangium. The megaspores are, of course, very much larger than the microspores. The germination of the megaspores is started in the sporangium; at a certain point in their development they are shed. In the case of the microspores germination commences after the spores have fallen on to moist soil. The prothallus is exceedingly small, being little more than a group of cells forming an antheridium. Should there be sufficient moisture about, the spermatozoids which are produced by the antheridium swim towards the archegonia in any female prothallus which may be near. Fertilization then takes place, and the final outcome is the young plant. Now and again in certain species it is seen that the megaspores develop to such an extent within the sporangium that fertilization takes place, and even an embryo or young plant may be formed.

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.

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