XLV] 



PLATYCERIUM 



209 



Figs. 16, 17). But this orderly sequence is soon departed from, and the adult 

 stems show a high degree of vascular disintegration with numerous per- 

 forations. The complexity of the result is related to size. In P. aldcorne 

 (-^ p bifurcatum (Cav.) C. Chr.), one of the less robust species, the axis is 

 traversed by a simple dictyostele, but highly perforated. The leaf-trace 

 arises from this as a group of small strands rather irregularly disposed 

 (Fig. 718, yJ). But in P. aethiopicum {=P. stcmaria (Beauv.) Desv.), one of 

 the most robust species, there is a complex medullary system, in addition 

 to the outer circle of strands which represents the primary solenostele: 

 this medullary system contributes its quotum to the outgoing leaf-trace 

 (Fig. 718, E). As Miss Allison remarks, "it would be quite consistent with 



are 



wou 



Fig. 718. .-/= sections of the rhizome oi Platyceriiim 

 aldcorne showing the relation of two leaf-traces to the 

 ring of meristeles of the axis. j9 = similar sections from 

 Platyceriiim aethiopicum showing the greater structural 

 complexity, with numerous medullary strands enlarged. 

 (After Miss Allison, New Phytologist, 1913, Vol. Xli, 

 P- 31'-) 

 the structural facts if we were to consider Phxtycerimn with its complicated 

 dictyostele as the dictyostelic type of a series of which Dipteris and Matonia 

 the solenostelic types." The inner cycles of the polycyclic axis oi Matonia 

 Id then be regarded as providing the prototype of the medullary system 

 seen in the larger P. aethiopicum. The leaf-trace of Platyceriiim consists of 

 many strands, some ten in P. Hillii: but already in Cheiropleuria there are 

 two, and Dipteris Lobbiana has four (see Vol. II, Fig. 574, p. 316). It thus 

 appears that the vascular structure of Platyceriiim may be held as cognate 

 with that of the Dipteroids, allowance being made for the larger size and 

 higher specialisation. 



As the numerous strands of the leaf-trace pass up the broadening leaf- 

 stalk of Platyceriiim they spread out like a fan, with many anastomoses, 



