132 EVOLUTION OF PLANTS 



ferns may have an erect stem ten or fifteen metres in 

 height, with leaves five or six metres long. These gi- 

 gantic sporophytes offer a strong contrast to the insig- 

 nificant sporophyte of the mosses, and corresponding to 

 this is the late appearance of the sporogenous tissue, 

 which may not be formed until after many years. This 

 extreme subordination of the sporogenous tissue is a 

 wide departure from the condition existing in such low 

 liverworts as Riccia, where practically the whole sporo- 

 phyte is composed of sporogenous cells. 



In all of the Pteridophytes the sporogenous tissue is 

 restricted to certain definite areas, these being confined 

 to more or less distinct organs, sporangia. The latter 

 are possibly foreshadowed by the imperfect segregation 

 of the sporogenous tissue in the Anthocerotacese, the 

 highest of the liverworts. Among the ferns, the forms 

 which approach nearest the condition existing A in the 

 Anthocerotaceae are the species of Ophioglossum or 

 adder-tongues, where the limits of the sporangia are 

 scarcely indicated at all upon the surface (Fig. 34, 

 A, B, C). In these ferns the sporogenous tissue occurs 

 in masses of considerable size, but is not clearly sepa- 

 rated from the surrounding tissue. The archesporial 

 cells are separated from the epidermis of the leaf 

 (sporophyll) by several layers of cells, and the spores 

 finally escape through a cleft which opens at the sur- 

 face of the sporophyll. The archesporium is at first 

 of sub-epidermal origin, as in Anthoceros, the latter 

 being in this particular more like the ferns than like 

 the typical mosses, where the sporogenous cells are 

 originally derived from the central part of the sporo- 

 phyte. Even in Anthoceros, however, the separate 



