126 MANUAL OF BOTANY 



it takes place by means of gemmae, or by branches which be- 

 come detached. Similar reproduction of the sporophyte is 

 brought about by the development of adventitious buds, arising 

 on either the petiole or the lamina of the leaf in many ferns. 

 In some of the equisetums some of the underground slioots 

 form tubers, which may remain dormant for some time and 

 ultimately produce new stems. A similar formation is found 

 in some of the club-mosses. 



The phenomenon of apospory is not infrequent. A specially 

 interesting form of it occurs in Athyrium Filix-fcemina, where 

 sporangia produced in the normal position grow out into pro- 

 thallia instead of forming spores. In other cases the prothallia 

 spring from the tips of the pinnae of the leaves. 



The converse process, the production of a sporophyte from 

 the prothallium without the formation of an oospore, is occasion- 

 ally seen. The prothaUium in this case sometimes produces 

 sterile archegonia ; sometimes none at all. The sporophyte in 

 either case arises as an adventitious bud upon it. This phe- 

 nomenon is known as apoga/my. 



The Pteridophyta are usually arranged in three groups or 

 series, being thought to have arisen by separate lines of develop- 

 ment from a common Bryophytic ancestor. These three classes 

 are the FUicinse, the Equisetinte, and the Lycopodinae. 



