34 § 
Bower. — A Theory of the 
which in other forms are separate. In the Anthoceroteae, 
however, a complete central columella is commonly found, as 
also in most of the true Mosses. In Dendroceros this is 
well seen, though it is stated by Leitgeb that it is absent in 
some small specimens of Notothylas. In writing on these 
plants, Leitgeb clearly recognized the probable origin of this 
columella by massing of sterile cells : he remarks, ‘ the sterile 
cells at first uniformly distributed through the spore-cavity, 
though connected together, first united at the axis to form 
a strand of cells.’ I think there is good reason for accepting 
this conclusion of Leitgeb that sterile tissue-masses, lying 
internally, may have originated in certain cases by coalition of 
isolated sterile cells : the actual spore-producing tissue was 
thus relegated to a more superficial position, which it occupies 
in all the higher Archegoniatae. In such plants with a columella, 
the sporogenous tissue is referable in origin to a dome-like 
layer of cells — the archesporium : this is the case in the Antho- 
ceroteae, in Andreaea , and Sphagnum. But in most Mosses 
the apex of the dome does not produce spores, the archesporium 
thus having the form of a cylinder open at both ends. If the 
ordinary Mosses were derived, as would seem probable, from 
types with dome-like archesporium, we should then see in the 
cylindrical archesporium the result of a partial sterilization 
involving the apex of the dome. 
It is further to be noted that the whole of the archesporium 
of the Anthoceroteae is not devoted to spore-production ; certain 
of the cells form sterile elaters in A nthoceros ; in Notothylas , 
however, the sterile cells form a network which holds the spore- 
mother-cells in its meshes. From such a condition as this to 
that of complete septation would appear to be but a slight step. 
But here we arrive at the Rubicon : for among the Bryophyta 
that apparently slight step is not taken, and it may even be 
a question whether complete septation ever was accomplished 
by plants really akin to our present Anthoceroteae. The 
distinction between the Bryophyta and Pteridophyta is most 
strongly defined by the facts that the sporophyte of the former 
has a concrete archesporium, and no appendicular organs, 
