270 



DICKSONIACEAE 



[CH. 



seen presently that there is a progression in soral characters in the Denn- 

 staecltiinae which runs parallel with that in their vascular system. 



The origin of the sorus in all of these Ferns is marginal. This has been 

 shown very clearly by Conard for D. punctilobula, together with the basipetal 

 sequence of the sporangia, of which the first arises from a marginal cell. 

 The same was found to hold for D. apiifolia, in which the sequence is regu- 

 larly sustained (Fig. 539, A). As in other Ferns of this affinity the upper 

 lobe of the indusium, which is here again of superficial origin, is larger and 



Fig. 538. A series of transverse sections arranged in ascending sequence, sliowing the relations of 

 the polycyclic axial system to the leaf-traces in the upright stock of Saccoloina clcgans Klf. The 

 numbers relate to the actual sections cut. (Natural size.) 



thicker than the lower, and as development proceeds the whole sorus is 

 deflected strongly downwards. In fact, in the species named it is constructed 

 on the same plan as in Hymeiiophylhnn, Loxsoma, Thyrsopteris, and Dicksonia, 

 though smaller and more strongly deflected than in the latter. But the 

 sporangia themselves are smaller: the stalk is elongated as in Dicksonia, 

 but it consists of only three rows of cells, while the head is flattened on 

 either side of the almost vertical annulus (Fig. 539, C). The ring of cells 

 runs round the margin of the sporangium almost in a vertical plane, but at 

 the stalk it is slightly diverted to one side: usually the sequence of its cells 



