242 Mr. VALENTINE on the existence of Stomata in Mosses. 
influence of the air, the fewer are the granules they contain. The size of the 
cavity between the sporular and thecal membranes varies in different species, 
and in the same species at different periods of its growth, whilst in some, as 
Orthotrichum diaphanum, these membranes are in contact; in others, of which 
Funaria hygrometrica and Bartramia pomiformis are the most marked exam- 
ples, they are widely distant; this distance, however, is constantly diminish- - 
ing by the growth of the columella and the gradual development of the spo- 
rules. 
I shall venture to offer a conjecture as to the use of the stomata in mosses ; 
itis but this moment formed, therefore you cannot expect it to be very matured. 
In my paper published in the last volume of the Linnean Transactiens*, I 
have endeavoured to prove that the sporules are, in point of fact, pollen, dif- 
fering from ordinary pollen merely in the greater firmness of its coats, a pro- 
vision rendered necessary by its immediate 'exposure to the soil without the 
intervention of a peculiar apparatus (the ovulum) to prepare it for germination. 
Now my conjecture is, that the stomata, by admitting the access of the air, do, 
if not fully cause, at least promote, the hardening of the coats of the sporules. 
There are some facts which favour this conjecture: the Phascums, with but 
one exception, have stomata, and as the operculum is persistent, it would 
appear that there is no other way by which the action of the atmosphere on 
the sporules can take place; again, they are not found on the Polytrichums 
nor on. Hymenostomum microstomum, which are so constructed, the first with 
a tympanum and closely approximating teeth; the last, in the first instance, 
with a perfect tympanum, which finally becomes perforated with a minute 
hole; that after the fall of the operculum, in either instance, the dispersion of 
the sporules is prevented until the hardening process has taken place. Opposed 
to these last are the Gymnostomums, in which the sporules would be almost 
instantly dispersed after the fall of the operculum, and therefore the harden- 
ing must be effected before that occurs; and as far as I have yet seen, stomata 
exist on all the species. The exception in the Phascums, and some few others 
amongst the peristomed mosses, are at present a stumbling-block ; but it is 
not impossible that future observations may discover either a peculiarity in 
the hygrometric nature of the peristome, or that the operculum after its de- 
* Linn. Trans. vol. xvii. p. 465. et seqq. 
