409 
scribed it to be, should be one of the last convolute leaves of the axis which 
the theca terminates, bearing the same relation to the latter as the convolute 
bractea to the flower of Magnolia, or, to speak more precisely still, as the ca- 
lyptrifonn bracteae to the flower of Pileanthus ? If the calyptra be anatomi- 
cally examined, especially in such genera as Tortula and Dicranum, no 
difi’erence in its tissue and that of the leaves wdll be observable ; and that very 
common tendency to dehisce on one side only as the diameter of the theca 
increases, which characterises the dimidiate calyptra, may not unreasonably be 
understood to be the separation at the line where the margins of the supposed leaf 
united; in the mitriform calyptra this separation at a given line does not take place, 
and the consequence is an irregular laceration of its base. The analogy of the ca- 
lyptra being of this nature, the next inference would naturally be, that the part 
it contains corresponds with a flower-bud. Upon this supposition, the external 
series of parts belonging to this supposed bud would be the operculum ; the ad- 
hesion of this to the theca, which would answer to the apex of the axis, or to 
the tube of the calyx of flowering plants, would be analogous to that which ob- 
tains in Eucalyptus, or perhaps more exactly to that of Eschscholtzia ; but it 
would remain to determine of how many parts, in a state of cohesion, it was 
made up. In the paragraph above quoted, it is stated to be one only; but I 
confess I have no better reason to offer for this than the absence of any trace 
of division upon its surface or in the substance of its tissue, and also perhaps 
the apparent identity of nature between it and the calyptra when both are 
young, in the Tortula and Dicranum genera already cited. With regard to 
the peristomium, I would beg attention to the following particulars : — The 
teeth, as they are called, occupy one or more whorls ; they are evidently not 
mere lacerations of a membrane, because they are in a constant and regular 
number in each genus, and that number is universally some multiple of 4, 
as the floral leaves of flowering plants are ordinarily of 3, 4, or 5 ; they have 
the power of contracting an adhesion with each other by their contiguous 
margins, as the floral leaves of flowering plants ; they alter their position from 
being inflexed with their points to the axis, to being recurved with their points 
turned outwards, — exactly what happens in flowering plants ; the teeth of 
the inner peristomium often alternate with those of the outer, thus conform- 
ing to the law of alternation prevalent in the floral leaves of flowering plants ; 
and, finally, if we compare the various states of the leaves of Buxbaumia 
aphyUa with the teeth of Mosses, it is impossible not to be struck v/ith the 
great similarity in the anatomical structure of the two. These are the con- 
siderations which have led me to the conclusion, that the calyptra, the oper- 
culum, and the teeth of Mosses, are all modified leaves ; and hence that the 
theca is to be considered more analogous to a flower than to a seed-vessel. 
With regard to the membrane, or epiphragma, which occasionally closes up 
the orifice of the theca, it may be considered as formed by the absolute cohe- 
sion of the leaves of the peristome, just as the operculum of Eudesmia is 
formed by the cohesion of the petals ; and this is confirmed, first, by Calym- 
peres, in which the membrane ^timately separates into teeth, and by the fact 
that the horizontal membrane exists most perfectly in such genera as Polytri-. 
chum and Lyellia, in which there is no distinct peristome. It now remains 
to explain the internal structure of the theca consistently with the theory that 
has been advanced of the peristomium, operculum, and calyptra. I consider 
the theca to be merely the thickened apex of the axis, the sporules to be a par- 
tial dissolution of its cellular tissue, and the columella to be the unconverted 
centre. That the end of the axis or growing point of plants frequently be- 
comes much more incrassated than the theca of Mosses, requires no illustration 
for those who are acquainted with the spongy receptacle of Nelumbium, Rubus, 
and Fragraria, the dilated disk of Ochna, the curious genus Eschscholtzia, or 
Rosa, or Calycanthus, or, finally, the spadix of Arums. That the tissue is 
