226 
Arber—On the Past History of the Ferns . 
It may be pointed out in this connexion that we have in the fructifica- 
tion of most of the modern Eusporangiate Ferns, as compared with the male 
organs of both the Palaeozoic Pteridosperms and the Mesozoic Bennettiteae, 
an interesting case of homoeomorphy. In each group, the sporangia, though 
dissimilar physiologically, are all exannulate, and more or less united into 
synangia, and they were all borne on fronds. Opinions will no doubt differ 
as to the precise significance attaching to this fact. Some may be inclined 
to regard it as indicating some degree of affinity between the Fern-line of 
descent and that of the Pteridosperms and Cycadophyta. Though affinity 
between these two life - lines at some remote geological period can 
hardly be doubted, I am not at all sure that this case of homoeomorphy 
implies a connexion between the Cycadean line of descent and that of the 
Eusporangiate Ferns in particular. These characteristics in common, 
though in dissimilar organs, may be simply due to parallelism of develop- 
ment, a frequent phenomenon in both the vegetable and animal kingdoms. 
Even in the case of organs of similar function in different groups, such 
similarities may be of little or no value as clues to affinities. For instance, 
in Palaeozoic times both the Pteridosperms and the Cordaitales, as well 
as some Lycopods, bore seeds ; yet one would not, on this ground, claim 
that these groups must be nearly related. Exannulate synangia may 
well be an ancient type of fructification ; but it does not follow that the 
life-line of the Eusporangiatae goes back to any distant geological period, 
because present members of the group also possess this feature. Thus it 
appears to me that the argument, based on the similarity of the male 
fructification of the Pteridosperm to the homosporous sporangium of 
modern Marattiaceae, can have little weight as a proof of the existence of 
the Eusporangiatae in Palaeozoic times, using the term in the sense applied 
to the modern Ferns. 
But even if we are inclined to dismiss a large number, but not perhaps 
all, of the exannulate Palaeozoic sporangia from the Eusporangiatae, there 
is yet other proof of this line of descent in Carboniferous and Permian 
times, and one founded on the surer ground of structural anatomy. The 
well-known and very Fern-like stems included in the genus Psaronius are 
in the details, as well as in the general plan of their anatomy, strikingly 
like the modern Marattiaceae. A detailed comparison of their structure, as 
compared with the recent Ferns, on the part of Rudolph 1 , has recently 
emphasized this similarity even more clearly than before. Yet a puzzling 
feature connected with this type of stem is that certain Psaronii are known 
to have borne Pecopterid fronds, which in turn possessed an Aster otheccc- 
like fructification However, though it may appear difficult to reconcile 
this fact with conclusions already arrived at with regard to the fructifica- 
tions discussed above, it may be readily admitted that, in the anatomy of 
1 Rudolph (’05). 
