156 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



West (Ann. of Bot. 191 7, p. 361). But in referring the structure of the Marat- 

 tiaceae to this source it is to be remembered that after the first ontogenetic 

 stages are past there is no regular endodermis surrounding the vascular 

 tissue ; and the same appears to have been the case with the Psaronieae. In 

 this respect these Eusporangiate Ferns differ from all the Leptosporangiates. 



Accessory Strands 

 The structure seen in the dendroid Cyatheaceae (excl. Dicksonieae) is 

 fundamentally dictyostelic, with very broad meristeles, each enclosed within 

 equally broad plates of strengthening sclerenchyma. The leaf-traces come 

 off from the lower margins of each leaf-gap, as a number of strands which 

 take an oblique course outwards through the cortex (Fig. 150). In addition 

 to this vascular system other strands 

 are found in the pith, and even in the 

 cortex in the larger stems, such as C. 

 Imrayana, investigated by De Bary. 

 The medullary system consists of 

 numerous thin strands, each sur- 

 rounded by endodermis, and often 

 accompanied by sclerenchyma, scat- 

 tered through the pith. They anasto- 

 mose freely, and show blind endings 

 downwards. Some of them may pass 

 outwards with the strands of the leaf- 

 trace into the petiole, contributing in 

 varying number to its central region. 

 Accessory cortical strands are also 

 seen in C. Imrayana, but not in ajl 

 species. They are connected with the 

 leaf-trace-bundles as strands contin- 

 uous downwards, sometimes anasto- 

 mosing, sometimes ending blindly, 

 sometimes joining up with the traces of 

 lower leaves. It would seem probable 

 from the irregularity of their course 

 and occurrence that all these cortical and medullary strands are accessory, 

 in the sense that they have originated in the enlarged parenchymatous tracts 

 independently of the general vascular system, not as a result of branching 

 from it or of disintegration of any former polycyclic state. That such new 

 formations may occur is shown by the occasional appearance of a short and 

 isolated vascular strand, surrounded by endodermis, which ends blindly in both 

 directions, in the centre of the large leaf-trace of PteiHs podopJiylla (Fig. 147). 



ig- 



Cyathea Lnrayana, Hook. Transverse 

 section of stem. Natural size. At b, c, d, foliar 

 gaps ; all the black bands and parts are stereom, 

 all the paler bands are vascular strands in section. 

 a = vase, strands of the main cylinder; s, s' = 

 outer and inner plates of the sclerotic sheath. 

 (After De Bary.) 



