ISO 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



Certain peculiar types of stem-structure which appear relatively late in the ontogeny of a few 

 isolated genera have a special significance in this relation. 



The sections of Plagiogyria (Fig. 138) have shown that 

 in the younger shoots the leaf-trace separates as a sector 

 of the solenostele, and passes out without complications, 

 except that a centrally-placed strand of sclerenchyma fol- 

 lows its course outwards into the cortex. In older shoots 

 this is much larger, and its tissue is discontinuous at the 

 centre (Fig. 138, D, x). A semilunar space appears which 

 widens out as the trace passes upwards, and opens to the 

 atmosphere in the angle between leaf and axis. Each adult 

 leaf is thus subtended by a deep involution of the outer 

 surface, lined by sclerotic tissue ; but it stops short before 

 the stele itself is reached. Gwynne-Vaughan found a similar 

 structure in Anemia^ Cystopteris^ and Matteuccia Struthio- 

 pteris. But in these Ferns, when adult, the involutions ex- 

 tended into the stele itself, forming deep pockets in the pith, 

 which were lined by epidermis {New Phyt. Vol. iv, p. 211). 



In interpreting these involutions it is important to note 

 how isolated is their occurrence. The genera named have 

 no near affinity to one another. Moreover the involutions are 

 absent from the young plant. As Gwynne-Vaughan points out 

 they appear as "in each particular case the latest expression 

 of a long series of advances from the primitive solid stem 

 with its single solid central protostele." The physiological 

 significance is almost certainly in relation to gaseous inter- 

 change with the atmosphere, which has thus direct access 

 to the inner tissues, together with enlarged surface of ex- 

 posure. It seems possible to interpret them in terms either 

 of transformation or of substitution of the inner tissues. 

 A static change of destination like that seen in forming any 

 involution of endodermiswould affordasgood an explanation 

 of the structure described as some dynamic inflow of tissue 

 from without. The involution may be held to be a last conse- 

 quence of the transformation of theprimitivecylindricalstele 

 into a dictyostele, and not as an indication of the way in which dictyostely arose. 



In some creeping Ferns of large size and relatively primitive character 

 the simple solenostelic structure is maintained, the greatly enlarged pith 

 being surrounded by a distended and very thin solenostele. Examples are 

 seen in Cibotium {Dicksoiiia) Barometz, and in Pteris laciniata, where the 

 soft parenchymatous storage-pith may be over an inch in diameter (Fig. 142). 

 It is likewise seen, though on a smaller scale, in Dipteris conjiigata. Such 

 arrangements when carried to large dimensions are obviously unpractical, 

 and the physiological difficulties raised by the continuous endodermal 

 barrier are met in various ways. One of these is illustrated in Histiopteris 

 incisa (Thunbg.), J. Sm. Here at each leaf-insertion a deep lateral infolding 

 of the cylinder encroaches on either side of the pith. This is merely a 



Fig. 141. StenoihlacnatemiifoUa 

 (Desv.), Moore, after Metten- 

 ius. Stelar system flattened 

 into a single plane, showing 

 perforations. /./. = leaf-trace ; 

 br. = vascular supply to a 

 branch. 



