THE FERNS 125 



especially well developed upon the leaves, where it is 

 furnished with stomata communicating with the green 

 tissue of the leaf, and also is often provided with hairs 

 and scales of characteristic form. The remaining tissue, 

 usually known as the "ground-tissue," shows a much 

 greater diversity of structure than is met with in any of 

 the lower plants, and closely approaches in this respect 

 the higher flowering plants. 



While in the mosses the existence of the sporophyte 

 usually ends with the dispersal of the spores, in the 

 ferns spore-formation is subordinated to the vegetative 

 existence of the sporophyte. The spores themselves, 

 instead of arising from a large, continuous archesporium, 

 are here restricted to certain definite structures of the 

 sporophyte called sporangia (Figs. 34, 35). A faint 

 indication of this segregation of the sporogenous tissue 

 is seen in the Anthocerotacese, among the liverworts, 

 where there is an imperfect separation of small sporo- 

 genous areas by means of sterile tissue between them. 



In the ferns, as a rule, the development of spores 

 usually takes place only after the sporophyte has 

 reached an advanced stage of development, and this 

 is often not accomplished for many years in some of 

 the large ferns, although in a few cases the sporophyte 

 lives but a single season. 



A study of the development of an individual case 

 illustrates very clearly the homologies which exist be- 

 tween ferns and the lower mosses. It is well known 

 to botanists that the germinating fern-spore does not 

 at once produce the leafy sporophyte, but there is first 

 formed a much simpler plant, the gametophyte (Fig. 32). 

 On first germinating, the unicellular spore usually pro- 



