XIl] 



ACROSTICHOID STATE 



!33 



its vascular supply, but its receptacle becameextended; itisnotacaseofthe sori 

 undergoing dissolution. The first step is shown by Cheiropleuria (Fig. 23 1, rt). 

 Here the receptacle with its vascular supply is enlarged and may even be 

 branched. Usually it is restricted to a single areola of the main venation. 



Fig. 231. a = part of a sporophyll of Cheii-oplenria, .seen as a transparency, showing the vascular 

 tissue. The steady Hnes are the venation : the irregular patches are the storage-xylem of the 

 soral receptacles. At the points ( x ) these have crossed the veins at a lower plane than that 

 in which the veins lie. ( x 8.) 



i5 — part of a sporophyll oi Platyceriuiii angolense, showing the origin of the receptacular xylem 

 from the ends of the blind veins. The receptacles are more elongated than in Cheiropleuria, 

 and pass to a lower level in the mesophyll, there extending frequently across the course of 

 the original venation, which is here represented by thin steady lines. ( x 4.) 



But occasionally, being extended in a plane close to the lower surface of the 

 leaf, the receptacle oversteps one of the veins, passing in a lower plane to 

 the next areola. This mode of extension, which is not a very marked feature 

 on Cheiropleuria, becomes very prominent in Platycerium (Fig. 231, b), and 

 still more so in Leptochiliis tri- 

 cuspis (Fig. 232). Thus there 

 came to be two parallel vascular 

 systems in the fertile region of 

 these Ferns, viz. the normal 

 venation, and the system of the 

 receptacle which ramifies in a 

 lower plane. This condition has 

 been called " diplodesmic'' (Fig. 

 233). An apparently parallel 

 condition is seen in some Pol}'- 

 botryas; but here it arises by 

 lapping of the extended sorus 

 backwards on to the upper sur- 

 face. The physiological result 

 is the same, but the method of its origin has been different (Fig. 234). 



Fig. 232. Leptochiliis tricuspis. Part of the soral region 

 of the sporophyll seen as a transparency. The heavier 

 lines represent the normal venation, which is nearer 

 the upper surface: the lighter lines are the recep- 

 tacular system extended in a plane nearer the lower 

 surface. ( x 6.) 



