Bower . — Studies in the Phytogeny of the Filicales . 399 
itself, and has been more or less clearly recognized as leading to such 
conditions as are seen in W oodwardia, and in Doodia. It is not right to 
assume that such disintegration will necessarily follow, in converse, the lines 
of the original construction of the fusion-sorus. Before any opinion can be 
formulated on such a question, it will be necessary to examine the facts, 
and so to ascertain how far there may be correspondence between the soral 
fragments resulting from the disintegration of the Blechnoid fusion-sorus 
and the original sori from which, according to the views stated above, the 
fusion-sorus was constructed. 
An essential question at the outset will be the relation of the com- 
missure to the original venation. This has already been considered by 
Mettenius (Fil. Hort. Lips., p. 60). He there describes the origin of the 
commissure as a phenomenon of branching of veins, which link up together 
as a consequence of part of their course running parallel to the midrib. So 
long as the course of the strands and not their constitution is taken into 
account, this explanation may serve, as it apparently would do quite well 
in the case of a normal pinna-apex of Blechnum fraxineum , Willd. = longi- 
folium , H. B. K. (Plate XXIX, Fig. 20, d). But further examination shows 
that there are two types of xylem involved — that with long narrow conducting 
tracheides, which are characteristic of the veins ; and that with shorter, broader, 
almost brick-shaped tracheides, which are of the nature of storage tracheides, 
and are characteristic of the receptacle, and especially of the commissure. 
The distinction of these two is shown somewhat conventionally in B. Fraseri 
(Fig. 21). Here it was possible to see plainly that the continuous commis- 
sure differs in structure from the veins, which run past the commissures in 
a plane nearer to the upper surface of the leaf, but intimately related to 
them. Having recognized the existence of these two types of xylem, we 
may now consider the question of the disintegration of the ‘ fusion-sorus 5 , 
and their relation to the process. 
A very simple example of disintegration has been seen and delineated 
by Mettenius (Fil. Hort. Lips., PI. IV, Fig. 21) in Blechnum capense , in the 
case of a leaf which was only partially fertile. At the transitional region 
each vein was found to bear its own sorus, and often its own indusial flap. 
Fortunately, I collected in Jamaica material of this same species in like 
condition, and Fig. 22 has been drawn to show the relations of the sori at 
the transitional region. Below, the normal Blechnoid state is seen, with 
continuous sorus and commissure on either side of the midrib, and with the 
curled-over margin of the sterile flanges. These gradually uncurl higher up, 
till the pinna becomes flattened like a sterile pinna. Following the fusion- 
sori from below upwards, the indusial flap is seen to be interrupted, at first 
irregularly ; but finally it appears as small isolated flaps upon the several 
veins. The vascular commissure, at first continuous, becomes also inter- 
rupted, and often appears as mere streamers of storage xylem to right and 
