BRYOPHYTA 



43 



Fig. II. — Diagram to show the increase in 

 prominence of the sporophyte stage of plant 

 life from the algae to the higher seed-plants. 

 Among the thallophytes, both the sexual and 

 asexual methods of reproduction are repre- 

 sented. A illustrates the asexual, wherein 

 certain cells of the plant divide into smaller 

 cells, — the zoospores, which, without union 

 with other cells, develop directly into new 

 plants. B-E' illustrate the sexual method 

 effected through an alternation of genera- 

 tions, wherein a vegetative stage — the 

 sporophyte — alternates with a reproductive 

 stage — the gametophyte. The gameto- 

 phyte stage of the various forms is below 

 the line a-b, the sporophyte stage above it. 

 The size of the plants in the diagram bears 

 no relation to their size in nature. This diagram need not imply that the seed-plant 

 has been evolved from the algae successively through the mosses, etc. ; the fern may have 

 evolved directly from the algae. Any direct fossil evidence in favor of either line of evolu- 

 tion is wholly lacking. A, the marine alga, Laminana saccharina, the devil's apron. 

 B, the very common liverwort, Marchantia polymorpha. B', same, with the tip of one of 

 the umbrella-like lobes magnified. C, the hair-cap moss, Polytrichum commune. D, a 

 very young sensitive fern, Onoclea sensihilis. E, the elm tree, Ulmus americanus. 

 E', same, with the gametophyte stage enlarged (see p. 77). Among the br>'ophytes, B 

 and C, the gametophyte is the prominent stage. This is the common liverwort or moss 

 plant which bears the sporophyte ; this latter remains attached to and dependent upon 

 the gametophyte throughout its whole existence. In the pteridophytes, D, the sporo- 

 phyte is the prominent, common, leafy fern plant which has become independent of 

 the gametophyte. The latter is the true two-lobed prothallus which, though independ- 

 ent of the sporophyte, is tiny and short-lived. In. the spermatophytes, E, the gamet- 

 ophyte stage has become so reduced that its whole existence is passed invisibly dependent 

 upon and within the sporophyte. ■ p, pollen grain with its single included nucleus; 

 p', the same at a later stage with three nuclei ; p" , the same lodged upon the stigma, 

 with the tube cell already within the ovule; t.c. tube cell. 



