Vascular System of Matonia pectinata. 515 
a presumption that pith first arose in connexion with the cortex at the leaf- 
gaps. The only evidence which would establish the origin of pith by 
intrusion of the cortex would be proof that the cells of the cortex actually 
pushed into the stele during development from the growing point, or 
had done so at some time during the course of descent. The possibility of 
this has been admitted by Gwynne-Vaughan 1 > but all available evidence 
points to a formation of pith by meristematic cells which would otherwise 
have given rise to stelar elements. In default of evidence of actual intrusion, 
the fact that pith occupies a region of the axis previously occupied by 
a totally distinct kind of tissue leads, in our view, necessarily to the 
conclusion that it is a new tissue. The statement about the intrusion 
of cortex becomes not only metaphorical, but misleading ; a true and 
useful statement, on the other hand, would be that the ground-tissue 
of the plant has now extended so as to occupy the axis of the stem as well 
as the periphery, by the development of a new tissue, the pith, whose 
histological characters and opposition to vascular tissue bring it within the 
wider concept of ground-tissue. 
This view necessarily carries with it the consequence that pith is morpho- 
logically part of the stele, since it is the phylogenetic successor of vascular 
tissue. We find it impossible to understand how this conclusion can be 
escaped if morphology is to have an evolutionary meaning. The case of 
the young Matonia appears to show, further, that pith does not in all cases 
arise in connexion with cortex. In nodes 2, 3, 4, 6, 7, 8, 9 of plant E there 
is no connexion between the pith of the stele and that of the cortex, though 
there is a connexion between the pith of the stele and that of the leaf-trace 
(when the latter is present), the trace being a closed cylinder with its pith 
shut off from the cortex, and sometimes consisting of a few cells only 
confined to the base of the trace. In nodes 6, 7, and 8 the leaf-trace 
possesses at its base an internal endodermis only which is connected with 
the internal endodermis of the stem cylinder. In the open type of trace, 
the pith of the stele is at first in connexion with the cortex only through 
that of the trace (e. g. E, nodes 10, 11, 13, 14 ; F, nodes 1 to 7) ; while later 
on (F, node 8, &c.) the pith of the stele opens directly to the cortex of the 
stem at the base of the trace, as is the case in typical solenostelic ferns. 
I11 Schizaea malaccana 2 and in 5 . dichotoma 3 , where the leaf-trace has 
no pith, the non-continuous ground-tissue pith of the stem is normally in 
continuity with the cortex in the form of the endodermal or ground-tissue 
‘ pockets.’ Isolated strands of internal endodermis enclosing pith are 
sometimes met with, and these may, with Boodle 4 , be interpreted as the 
result of reduction from a type with continuous pith ; though, for reasons 
given 5 , such an interpretation does not appear to be absolutely necessary. 
1 Gwynne-Vaughan (’ 03 ), p. 737, 8 cc/ 2 Tansley and Chick (’ 03 ). 3 Boodle (’ 03 ). 
4 Boodle (’ 03 ), pp. 519, 521. 5 Tansley and Chick (’ 03 ), pp. 500-2. 
