Vascular System of Matonia pectinata. 513 
demand on the second cylinder, thus leading to the appearance and increase 
in complexity of the third. 
It is legitimate to inquire why precisely should the complicated vascular 
system whose development we have described be developed in this particular 
species. Such a question is never easy to answer satisfactorily, but the 
following considerations may be put forward. It is necessary, in the first 
place, to assume a progressively increasing demand by the leaf on the con- 
ducting and storage capacity of the rhizome. If the frond of Matonia 
pectinata has originated from the compact Gleichenia - type existing in 
G. flabcllata , G. qnadripartita , &c., as we have suggested (pp. 479-82), the 
greatly increased extent of lamina produced by the repeated addition of 
peripheral pinnae would certainly involve such an increased demand on the 
conducting- system, as compared with that of the hypothetical protostelic 
ancestor. Such an increased demand might be met by a corresponding 
increase in the breadth of the arch of the leaf-bundle and a correlated 
increase in the diameter of the stem-cylinder after it had become solenostelic. 
Up to a certain point no doubt it has been so met ; but it is easy to see 
that an indefinite expansion of the leaf-bundle and rhizome-stele would 
involve a correspondingly indefinite increase in thickness of petiole and 
rhizome, and thus a constantly increasing thickness of relatively useless 
pith and a wasteful accumulation of material. An increase of the leaf- 
bundle by the development of its incurved free edges into the lateral loops, 
and a correlated development of accessory internal cylinders (by the 
elaboration of an original ‘ compensation thickening ’ on the edge of the leaf- 
gap) connected with the lateral loops would certainly economize space and 
material. The initiation of such a line of evolution may be attributed to 
the ease with which the original compensation-strand (second cylinder) can 
be drawn into supplying the free edges of the trace. 
Roots. Nothing has hitherto been said about the connexions of the 
root-steles, which are presumably the sole means of the original supply of 
water, with the vascular system of the rhizome. The root-steles are attached 
to the outer cylinder exclusively, opposite the protoxylems. The roots are 
scattered along the whole length of the rhizome, without any particular 
relation to the nodes, except in the young plants, where there are nearly 
always two or three roots at the node (two often dorso-lateral with their 
steles attached close to the actual base of the leaf-trace), and often not 
more than two along the considerably greater length of internode. In the 
adult rhizomes the roots are in some cases, though by no means always, 
mainly attached to the ventral and ventro-lateral portions of the surface of 
the rhizome. The water-supply thus apparently enters the outer cylinder 
all along its course, though sometimes mainly from the ventral side. As 
we have pointed out above, it is clear that water can only enter the second 
cylinder through its nodal connexion with the outer one, and the third 
N n 2 
