30 



COENOPTERIDACEAE 



[CH. 



of which bore terminal sporangia" (Seward). The mode of origin of 

 those branches is shown in Fig. 331. The branching is evidently com- 

 parable to that of the Dineuroideae. Structurally their vascular tract 

 becomes simpler in the upper branchlets (Fig. 331, H, /), the number of 

 protoxylems being reduced in them to three, two, or lastly to one single 

 central strand. Aphlebiae («, Fig. 331) are borne right and left by the 

 successive pairs of pinnae and pinnules. So long as the base of the plant 

 remains unknown it is impossible to say what relation this strange structure 

 bears to other parts. It may perhaps represent the chief organ of a plant 

 in which the distinction of leaf and axis as constituents of the shoot was 

 not existent. 



Fig- 331- Vascular supply to the ramifications of Staiiropieris, after P. Ber- 

 trand. The protoxylem is represented by black dots. A, rachis of the first 

 order, showing a mean condition of the four xylem -tracts ; B, C, D, stages 

 in the departure, on the right, of the supply to two pinnae of the first order; 

 a, aphlebia-traces ; E, rachis of the second order, showing undivided xylem, 

 from which pinna-traces of the second order have been detached, with their 

 aphlebiae {a) ; F, G, stages in the departure of triangular traces for pinnae 

 from the triangular xylem-tract of a rachis of the third order; H, triangular 

 trace of the fourth order ; /, the same, giving off pinnules and aphlebiae [a). 

 Compare Fig. / with Figs. D, E, G, which together illustrate the pro- 

 gressive simplification of the smaller branchlets. It is on the ends of these 

 that the sporangia are borne. Compare Fig. 332. 



Sporangia 



Sporangia are found associated with the fronds of various Zygopterids : 

 but it is only in a few instances that their connection has been proved with 

 the parts that bore them. Three good examples are, however, fully 

 established in their connection with the parent plants, viz. Stmiropteris 



