340 ON LYEI.LIA, LEPTOSTOMUM, 



in those species having quadrangular capsules they nearly 

 reach the parietes opposite to the external angles, towards 

 which they are directed. 



As the columella of Polytrichum retains its regular form 

 in the ripe capsule, its real structure may even then be in a 

 great measure determined. In this stage I find its wings, 

 or compressed sides, to consist of a double membrane with 

 an intermediate spongy substance, in which there is no 

 appearance of granules ; and the internal denser substance 

 of the axis is equally free from granular matter. But as 

 there is no indication of lateral rupture, the sides in several 

 species remaining perfectly smooth, nor of any central cavity, 

 this structure affords a powerful argument in refutation of 

 those hypotheses which assume the existence of two kinds 

 of granules in the capsules of mosses ; the one produced in 

 the cavity formed by the internal membrane, the other in the 

 substance or supposed cavity of the columella itself; the 

 latter being considered as seeds in one of the hypotheses, 1 

 and in the other as pollen. 2 



This argument, however, is not here advanced to disprove 

 the existence of two kinds of granules in the capsules of 

 mosses, but merely against their production in the distinct 

 cavities assigned to them in the hypotheses referred to. 



In the greater number of Polytricha as well as in Lyellia 

 and Dawsonia the seeds are extremely minute ; a fact with 

 which the increased surface for their production is probably 

 connected : for in P. undulatum, where the seeds are larger 

 than in most other species of the genus, this increased 

 surface does not exist : and in P. Jcevigatum, where they are 

 of still greater size, the plicae of the inner membrane are 

 probably also wanting. 



5711 Although there is but little resemblance in the structure 

 of the peristomium among the different genera of Poly- 

 trichoidecB, they may still be said essentially to agree in the 

 function of this part : for in all of them the complete 

 separation of the seeds is ensured by the smallness of the 

 apertures for their discharge. It may be remarked, that the 



1 Palisot Beauvois, ^Etheog. p. 5. 2 Keith, Physiol. Bot. ii. p. 346. 



