Page:Popular Science Monthly Volume 25.djvu/176

166 of them bearing the small males pores, and the third, a, the large female spores. In the next higher group, as arranged by botanists, we find the club-mosses; these are common plants with trailing or upright very leafy stems. Fig. 13, A, shows the tip of a spore-bearing branch, natural size, and B a longitudinal section much enlarged. The large spores are borne in sporangia on one side, while the small ones are on the left. The differentiation has now reached the place where there is a definite arrangement of the sporangia on the plant bearing them. In the development of the male spores the cells in which the antherozoids form are not produced directly from the spore-contents. This is a valuable link in the chain of relationship which binds this group with higher plants—in fact, helps to bridge what gulf there may have been thought existing between the flowerless and flowering plants. The ripe male spore-contents are changed into a few cells, one of which remains sterile and is considered the prothallus, while from the other cells—which taken as a whole constitute the antheridium—the cells which afterward bear the antherozoids are formed. In the genus Selaginella the female spore produces a small prothallus, as shown at 1, Fig. 14. The portion above d d, in this cross-section of the spore, is the prothallium, and at e e are two embryo plants. At 2 is a young archegonium not opened; 3 shows one further advanced, with the fertilized germ-cell divided. A is a male spore, showing the cell-division; D is a later state of the same, with the large antheridium filled with sperm-cells. The rudimentary prothallus is at V. The female is still more simple in Isoetes, shown in Fig. 15: 1 is the longitudinal section of the female spore, with an archegonium, a r, at the top; 2 shows the early differentiation of cells into archegonia, a r, a r, with their germ-cells, g g; 3, 4, and 5 show successive stages in the development of the germ-cell.