510 Mr. Brown on Lyellia, Leptostomum, and Buxbaumia. 
I have observed, never form complete septa, at least in the ripe 
fruit; though 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 rup- 
ture, the sides in several species remaining perfectly smooth, nor 
of any central cavity, this structure affords a powerful argu- 
ment in refutation of those hypotheses which assume the exist- 
ence 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 *, 
and in the other as pollen. 
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 assigued 
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. levigatum, where they are of still greater size, the plicæ of the 
inner membrane are probably also wanting. 
* Palisot Beauvois, Æihiog. p. 5. _ T Keith, Physiol. Bot. ii. p. 346. 
| Although 
