. Evolution of plants . borne in four rows;their margin is fringed with hairs, and on theupper surface is a remarkably well-developedligule. The structure of the stem is much likethat of one of the simpler species of Selaginella. The fructifications are grouped in a loose cone;on the upper surface of each sporophyll is asporangium, containing a single megaspore oflarge size, which occupies the whole cavity of thesporangium, like the embryo-sac in a seed (seefig. 19). The prothallus in the megaspore issometimes preserved. The megasporangium isenclosed in an envelope or coat, by which it iscompl
. Evolution of plants . borne in four rows;their margin is fringed with hairs, and on theupper surface is a remarkably well-developedligule. The structure of the stem is much likethat of one of the simpler species of Selaginella. The fructifications are grouped in a loose cone;on the upper surface of each sporophyll is asporangium, containing a single megaspore oflarge size, which occupies the whole cavity of thesporangium, like the embryo-sac in a seed (seefig. 19). The prothallus in the megaspore issometimes preserved. The megasporangium isenclosed in an envelope or coat, by which it iscompletely covered; only a narrow opening, which THE CLUB-MOSSES 171 we may call the micropyle, is left, at the far endof the sporangium. The envelope bears a num-ber of long tentacles, which project beyond themicropyle and may have served to detain themicrospores. At the base of the sporangiumthe sporophyll bears a ligule like that on thevegetative leaf. The sporophyll has a broadblade extending far beyond the sporangium on. Fig. 19.—Miadesmia membranacea. Longitudinal sectionof seed; (t-6) vascular strand of sporophyll; (/) blade ofsporophyll; (Ig) ligule; (/) tentacles; (v) integument ofseed; (sm) sporangium-wall; (m) megaspore. FromScott, Studies. X 30. either side, so that when the whole was shedthe organ was much like a winged seed. It has been said above that the PalaeozoicLycopods attempted to rival the Seed-plantson their own ground. The Selaginella-like plantMiadesmia is one of the Club-mosses which camenearest to the formation of a true seed. Theenvelope or integument, and the single megaspore, 172 THE EVOLUTION OF PLANTS retained permanently within the sporangium, areessential characters of a seed. The fertile leaf,however, formed part of the seed-like organ,and the whole was shed together, like the fruit of aComposite. It is curious to find so great an advance inreproductive methods, so nearly approachingthe highest types, in a little inconspicuous Lyco-pod of Carboni
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