688 CONCLUSION 



the modern Lycopods, but it is quite different in origin from medullation. 

 Intrusive bands of phloem invade somewhat irregularly the central xylem, 

 giving it sometimes the form of a fluted column, or of a series of 

 plates connected at intervals, or of a continuous xylem-sponge (Fig. 171, 

 p. 329). Such conditions, which are characteristic of modern Lycopods, 

 are probably secondary derivatives of the simple protostele, since they are 

 absent in the early fossils, as well as in the early condition of the plants 

 that show them when adult. 



A somewhat similar intrusion of tissues from without leads, in many 

 Ferns, to the condition which is described as the solenostelic. But here 

 it is regularly at the point just above the exit of the foliar strands from the 

 stele that the intrusive tissues enter; it thus comes about that phloem 

 and endodermis and ground parenchyma come to occupy continuously 

 the centre of the stele, which accordingly takes the form of a hollow 

 tube, with openings opposite each leaf-base (Figs. 95, 97, 100). This 

 formation of a solenostele has probably occurred along more than one 

 phyletic line, and it lies at the base of those complex types of dictyostelic 

 structure of the stem seen in Leptosporangiate Ferns. These follow upon 

 the overlapping of the foliar gaps, which results in dictyostely formerly 

 described as a polystelic state (p. 190). A similar condition in some 

 species of Selagindla, though phyletically quite distinct, shows interesting 

 analogies ; but its origin appears to be in relation to the departure of 

 supplies to axes, not to leaves; these are, however, referable also by origin 

 to a primitive monostelic structure. 



Still further complications occur in certain Ferns which are associated 

 with the formation of accessory vascular tracts ; these arise in relation to the 

 foliar gaps as described on pp. 568, 600, and lead to a doubling or even 

 trebling of the solenostele (Figs. 319, 342), or accessory strands may arise 

 in pith or cortex (Fig. 339). The condition of the modern Marattiaceae 

 and of the fossil Psaronius may also be mentioned as extreme cases of 

 complexity of vascular structure based probably on a scheme allied to 

 those above noted (p. 525). Into these details it is not necessary to enter 

 further here, they concern us chiefly as illustrating some of the extreme 

 methods of amplification of the vascular system seen in the axes of 

 Pteridophytes. 



In some degree parallel with this progressive dilatation and disintegration 

 of the stele goes also the disintegration of the foliar trace. In all the 

 smaller-leaved, and in many of the larger-leaved forms, the leaf-trace consists 

 of a single strand ; in the Lycopodiales this is uniformly so, with exception 

 of certain Sigillarias described by Kidston. 1 It is a single strand also in 

 Isoetes? and in the Equisetales. In the Sphenophyllales and Ophio- 

 glossales (except Ophioderma, and perhaps Cheiroglossa), the leaf-trace 

 comes off always as a single strand, but branches frequently while still 



1 Proc. Roy. Soc., Edin., vol. xxvii., part iii., p. 203. 

 z Studies, ii., Fig. 105. 



