152 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



ridge along the internal face of the vascular cylinder from one node to the 

 next : while in the internode the ridge contains a distinct strand of xylem 

 surrounded by a phloem-sheath of its own (Fig. 144, /}). In Dennstaedtia 

 riibiginosa there are one to three separate vascular strands traversing the 

 internodal pith, but connected with the main vascular cylinder at the nodes 

 (Fig. 144, B). In the very large rhizome of Dennstaedtia dissecta (Sw.), Moore, 



Fig. 144. A, B. Diagrams of the vascular system of rhizomes ai Dennstaedtia, including a node in 

 each. A = D. {Duksonia) adiantoides. B = D. [Dicksonia) rnbiginosa. L. 7"= leaf-trace ; /.j'// = lateral 

 shoot arising from the basiscopic margin of the leaf-trace; /= lacunae or perforations of the soleno- 

 stele not related to the departure of a leaf-trace; /.j = internal meristele, initiated from a ridge 

 upon the internal surface of the solenostele. The upper surface of the rhizome would face the 

 observer. (After Gwynne-V^aughan.) 



there is an inner solenostele, which shows occasional openings. In Pteris 

 elata, var. Karsteniana, the internal system is still more elaborate. Within 

 the typical outer solenostele the accessory system may consist of a second 

 cylinder which opens by occasional perforations. This regularly gives off a 

 large flat strand which, passing forward at the node, inserts itself on the 

 anterior margin of the leaf-gap of the outer 

 cylinder. It is sometimes called the "com- 

 pensation strand." Within the inner cylinder 

 a rod-like strand, or even in large plants a 

 third cylinder, may be found (Fig. 145). 

 This structure is essentially the same as that 

 of the large upright Fern, Saccoloma elegans, 

 Klf, long ago described by Mettenius. A 

 series of transverse sections from a large 

 plant shows the detail of separation of the 

 leaf-trace from the outermost cylinder (Fig. 

 146). The compensation strand is seen to 

 pass off from, the inner cylinder (sees. <f or^): 

 it moves outwards {d, e or h — k), and finally joins with the outer cylinder 

 so as to close the leaf-gap (/or /). The acme of these complications hitherto 

 observed is seen in Pteris {Litobfochia) podophylla, in which within three 



^j'^A'^Atrf.*' 



Fig- 145- Pteris elata, var. Karsteniana. 

 Diagram showing the vascular tissue 

 at the insertion of a leaf A piece is 

 supposed to be cut out of the side of 

 the solenostele, so as to show the in- 

 ternal vascular system. (After Gwynne- 

 Vaughan.) 



