126 THE VASCULAR SYSTEM OF THE AXIS [CH. 



\\^ell as in the sporelings of Botrychium, Helminthostachys, Osinunda, and 

 GleicJie7tia pectinata.'va all of which the ontogeny has been followed in detail, 

 the whole vascular system is completely surrounded by endodermis during 

 the establishment of the pith. The process is wholly intra-stelar. There is 

 no indication of any readjustment of tissues in the nature of a flow or 

 intrusion of parenchyma from outside the stele. The pith appears to owe its 

 origin in all of these relatively primitive Ferns to an early change of destina- 

 tion of certain of the procambial cells from the tracheide-nature as seen in 

 the solid protostele to that of parenchyma as seen in the medullated stele. 



This conclusion, which follows from the ontogenetic facts observed in 

 numerous examples, is in accord with what is seen in more mature steles of 

 related Ferns. Isolated tracheides are frequently to be found in the paren- 

 chymatous pith near the inner boundary of the xylem in normal stems of 

 Botrychium virgiiiianum, ternatum, and Lunaria (Fig. 119, A). Sometimes 

 they may be scattered throughout the pith, in stems where it consists nor- 

 mally of parenchyma only, as in Osmiinda (Fig. 120). This state is described 

 by some writers as a "mixed pith." It has been found to follow occasionally 

 on traumatic injury, as \x\ Botrychium ternatum. Such facts derived from adult 

 stems point clearly to the conclusion that in them the pith is of intra-stelar 

 origin, and that it is a consequence of a change of procambial destination of 

 the central tract of the xylem from development as tracheides to development 

 as parenchyma. The isolated tracheides are then held to be residual cells in 

 which the change has not been perfectly carried out, or has been for some 

 reason reversed. The physiological probability of this will appear from the 

 following considerations. In the larger types of medullated protostele, such 

 as the Ophioglossaceae and Osmundaceae, the protoxylem when recognisable 

 as such lies within the xylem, but near to its periphery. A distinction thus 

 arises between the outer or centrifugal primary wood external to the proto- 

 xylem, and the inner or centripetal primary wood lying within it. The former 

 being more directly continuous with the leaf-traces will act functionally as 

 conducting wood: this is the region of the wood which is retained when medul- 

 lation first takes place, and its retention is probably related to its functional 

 importance. The centripetal wood being less closely associated with the leaf- 

 traces will tend to become a place of water-storage. The centre of the en- 

 larging stele, being thus a storage-place for more or less stagnant water, it 

 is immaterial for the carrying out of this function whether the tissue-elements 

 are thick-walled or thin-walled, provided the mechanical strength be suffi- 

 ciently maintained. On the principle of economy therefore the place of the 

 thick-walled tracheides would be equally well filled by thin-walled paren- 

 chyma. Accordingly with increasing size of the stele the central region of 

 the wood is liable to be converted into parenchymatous pith. Whether or 

 not this is the true physiological reason, it accords with the structural facts 



