mass of colorless cells, which may not exceed one twenty-fifth of an inch in diameter. This group forms another step toward a greaterFig. 8. simplicity of the sexual generation in plants. The spores of the Ophioglossaceæ are developed less superficially on the fronds than in the lower orders of ferns. This is a morphological point which is worthy of mention here. The whole structure of the asexual generation is more highly developed than in other ferns, while the sexual generation is much reduced and simplified.

The Equisetaceæ, or horse-tails, form a small group of flowerless plants, with hollow, jointed stems and cone-like spore-heads (Fig. 9). The scouring-rush, with its rough, grooved stem, is a leading member of this family. The prothallia are small and irregularly branched, and in most species the male and female parts are on separate plants (diœcious). The antheridia-bearing prothallia are much smaller than the female, the latter being sometimes half an inch in length. The structure of the male and female organs is

much the same as in ferns. The antherozoids are larger, and the archegonia are more deeply situated in the prothallus. The conspicu-