I/O 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



details indicate not a primitive cylinder, such as that of Botryopteris, but 

 actual condensation from a curved source. A good example is found in 

 sections of the leaf-base of 

 Gleichenia dicarpa, one of the 

 section Eti- Gleichenia (Fig. 

 164). The meristele appears 

 almost circular : the xylem 

 shows the usual horse-shoe 

 curve, but the two curved 

 margins are fused together so 

 as to form a complete ring. 

 The condensation which has 

 produced this state is prob- 

 ably related to the straggling 

 habit of this Fern of the 

 savannahs. It widens out 

 into a dilated curve in upper 

 regions of the leaf, thus indicating 

 adaptive. 



Fig. 163. Portion of the vascular system of the stem of 

 Cibotiiim {Dicksoiiia) Baroinetz, seen from within, and 

 showing the departure of three leaf-traces, which become 

 disintegrated as they pass into the petiole. (After Gwynne- 

 Vaughan.) 



that the contraction is local and 



Fig. 164. Transverse section of the base of the petiole of Gleichenia 

 diiarpa, showing the pseudo-stelar structure resulting from contraction 

 of the horse-shoe-like xylem, till its margins fuse. (Photograph by 

 Dr Kidston from section by Gwynne-Vaughan. ) 



