562 FILICALES 



species the xylem-core is fluted, the protoxylems being mesarch in its 

 slightly projecting ridges. The only other widely different type of structure 

 found in the genus is the solenostelic, which has been observed in G. 

 pectinata alone : here the stele is larger than in any other species which 

 have been examined : in addition to the structure as described the centre 

 of the fluted xylem is replaced by a mass of sclerenchyma, surrounded by 

 a ring of endodermis, pericycle, phloem, and conjunctive parenchyma 

 (Fig. 3136). Another type which takes an intermediate position as 

 compared with those already mentioned is seen in G. (Platyzoma} micro- 

 phyllutn, in which the leaves are densely crowded and polystichous on the 

 rhizome. Here there is also an inner endodermis surrounding a central 

 sclerenchyma, but there is no internal phloem between the xylem and 

 endodermis. 



In this last-named species the leaves are small, and the leaf-trace 

 separates as a small collateral strand from the periphery of the stele 

 without disturbance of the underlying tissues. This appears also to be 

 the mode of origin in the seedling of the more complex G. circinata ; 

 but in these larger-leaved species the leaf-trace of the mature leaf takes 

 in the petiole an almost cylindrical form bounded by an endodermis, 

 with ( Mtrtensia) or without ( Eugleichenia) an involution on the adaxial 

 side (Fig. 314). G. dichotoma is exceptional in Mertensia in having 

 no involution. The whole petiolar bundle may be regarded as a single 

 flat ribbon widened laterally, but closely compressed and crumpled so as 

 to take a cylindrical form : in that case the condition of Mertensia with 

 the endodermal involution would be more primitive than Eugleichcnin 

 where there is none. But G. dichotoma is an exception in the latter 

 section, showing the more advanced state. 



The node of insertion of these larger leaf-traces may be marked by 

 complications, islands of tissue (composed of phloem, endodermis, and 

 sclerenchyma) appearing in the xylem of the stele as cut transversely : 

 these correspond actually to pocket-like encroachments of those tissues, 

 extending down from the centre of the petiolar trace into the stele of the 

 axis. Such pockets are only slightly developed in Eugleichenia, but more 

 so in Mertensia, and especially so in G. dichotoma, which leads suggestively 

 on towards the continuous solenostely seen in G. pectinata. They have 

 their relation to the theory of stelar structure, and on the facts two views 

 are possible : either that the protostelic condition of most Gleichenias is 

 primitive, and that the solenostelic type has been derived from it, or that 

 the protostelic Gleichenias might be regarded as showing the reduced 

 remnants of a previous solenostelic structure. The former view appears 

 the more probable : in the first place the seedling is protostelic, and offers 

 no suggestion of reduction to. produce that primitive state: analogy with 

 Lygodiitm corroborates this. Further, the nodal pockets may naturally 

 be held to be local complications of the stele, directly connected with 

 the insertion of the peculiarly complicated leaf-trace of an unusually 



