XLV] 



CHRISTOPTERIS 



215 



the petiole. The same is found in Neochciropteris. The course of the vascular 

 strands up the petiole to the blade is indicated by Fig. 723: it presents a 

 close analogy to what is seen in Platycerium (compare Studies V, Text- 

 fig. 10), and may be regarded as essentially similar to that of Cheiropleuria, 

 though complicated by more elaborate perforation (compare Fig. 714). In 

 the sections i-vi the gap between the two larger marginal strands remains 

 constantly open, its position being marked (x). Complications arise at the 



V// 



%iii 



Fig. 723. Christopteris tricuspis : a series of sections illustrating tlie vascular system of the sporophyll, 

 the adaxial system uppermost. ( x 4.) The mark ( x ) indicates the " heel" of the horseshoe, that is, 

 the adaxial side of the petiole. 



point of branching of the blade, and Fig. 723, vii-xvi, illustrate the course 

 upwards into a sporophyll. The gap (x) is maintained till the marginal 

 strands finally fuse in the mid-rib of the middle lobe (xvii). This last section 

 represents the structure of a single fertile lobe, with its enlarged mid-rib: 

 each lateral flap has its primary venation, while the lower surface on either 

 side is covered by the Acrostichoid sorus (Fig. 724). It thus appears that 

 the course of the strands is a modification of the primitive horseshoe, 

 complicated by segregation, and by various fusions: and that these are 



