142 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



Transverse sections of the rhizome of Gleichenia 

 flabellata showing the detachment of the leaf-trace from the 

 protostele, with its included protoxylems. (After Tansley.) 



through a node the leaf-trace is seen to separate on the upper side as a 

 C-shaped tract of tissue, at first surrounding an involution which is continued 

 downwards as a shallow pocket. This is delimited by internal phloem, and 

 again lined by internal endo- 

 dermis, while centrally there 

 lies a mass of sclerenchyma 

 (Fig. 132). Passing upwards 

 from the base of this pocket 

 the leaf-trace separates with- 

 out break of endodermal con- 

 tinuity, as a single C-shaped 

 tract, carrying out with it 

 three protoxylem groups 

 {A). The pocket does not 

 here extend into the inter- 

 node, and consequently the 

 stelar xylem is not indented 

 by the departure of the trace. 

 The condition is typically protostelic. 



In Gleichenia pectinata a section through an adult internode shows typical 

 solenostely. The outer tissues down to the xylem are as in G. flabellata, but 

 towards the centre, in place of the continuous xylem, there appear succes- 

 sively a band of internal phloem, a ring of internal endodermis, and a central 

 mass of medulla or pith, which is indurated. This core of sclerotic pith is 

 continuous through the adult stem, and the hollow cylinder of the stele 

 surrounds it throughout the internodes (Fig. 133). But transverse sections 

 cut at intervals in the neighbourhood of the node show that the leaf-trace 

 passes off in essentially the same way as in G. flabellata. Here, however, 

 in place of the shallow pocket there is seen directly above its exit a deep 

 channel, filled by sclerotic tissue, and limited by the still continuous endo- 

 dermis, around which is a sheet of phloem connected with that outside. 

 A direct continuity is thus established at the departure of each leaf-trace 

 between the outer cortex and the pith, which happens in this case to be 

 strongly sclerosed. The vascular structure of the adult is accordingly that 

 described as the amphiphloic solenostele, since the stele is tubular, and is 

 lined within and without by phloem, and is delimited completely by unin- 

 terrupted endodermis. The openings of the ring at the departure of the 

 several leaf-traces are cdiWed foliar gaps (Fig. 133). 



It would then appear probable that an examination of the young spore- 

 ling of G. pectinata should reveal facts of the ontogeny which will shed 

 light upon the origin of solenostely from that protostelic state so prevalent in 

 other Gleicheniaceae. This is found to be the case (Fig. 134). Starting from 



