XI THE HOMOSPOROUS LEPTOSPORANGIAT.E 369 



former genus, however, show an intermediate condition, with the 

 margin of the indusium deeply two-Hpped. 



The first sporangia arise at the top of the placenta (Fig. 191), 

 but the apex itself does not usually develop into a sporangium. 

 Afterthe first sporangia have formed, new ones continue to develop. 

 Near the base of the placenta a zone of meristem is formed, which 

 constantly contributes to its growth, and the young sporangia arise 

 from the surface cells formed from this meristem. The mother cell 

 is very easily distinguished by its larger size and denser contents. 

 About every third cell seems to develop a sporangium, but this 

 probably is not absolutely uniform. The first wall is usually 

 nearly vertical, and cuts off a narrow segment from one side of the 

 mother cell (Fig. 191, A). This in most cases examined was 

 next followed by a wall almost at right angles, forming a small 

 basal cell. After these preliminary divisions, which form the 

 very short stalk, the next divisions are exactly as in the Pol}-- 

 podiacea;, and give rise to the central tetrahedral cell with the 

 four peripheral ones. Prantl ^ states that the first divisions of the 

 cap cell are also spirally arranged. In T. cyrtotJieca (Fig. 191) 

 the tapetum is massive, and composed throughout of two layers. 

 The archesporium divides into eight cells, whose further history 

 is the same as in other Ferns. The annulus in the Hymeno- 

 phyllacese is large, and situated much as in Gleicheiiia. 

 According to Prantl " it arises in part from the cap cell and 

 partly from numbers one and three of the primary peripheral 

 cells. Where the young sporangium is cut longitudinally 

 (Fig. 191), the annulus cells are at once recognised by their 

 larger size, especially upon the dorsal side. Their radial and 

 inner walls become very thick, and a horizontal section (Fig. 191) 

 shows that the annulus is not complete, but is interrupted on the 

 inner side where the stomium is formed. 



Apoganiy and Apospory 



Both of these phenomena have been discovered by Bower ^ 

 to occur not infrequently in Trichoviajies, and probably further 

 investigations will reveal other instances. Apogamy was com- 

 mon in T. alatuin, in which species archegonia were not seen at 

 all, and the origin of the young sporophyte was unmistakably 

 non-sexual. Prothallia, arising directly from the leaf, or from 



1 Prantl (1), p. 39. - Prantl, I.e. p. 40. ^ Bower (8). 



2 B 



