5io Tans ley and Lulham . — A Study of the 
the causes which led to the formation of an internal endodermis and 
internal accessory vascular cylinders, there can be no doubt that they did 
not arise at a correspondingly early stage in phylogeny to that in which 
they are seen in the young Matonia. We must suppose that in accordance 
with Darwin’s well-known principle, this character has been inherited at an 
earlier and earlier stage of development, till it has appeared in the young 
plant at a very early period indeed. 
Judged by the comparative method there is a distinct correlation 
between the type of leaf-trace and the type of stele with which it is 
associated. In general terms we may say that a compact circular, oval, 
or kidney-shaped trace is associated with the protostelic type ( Gleichenia , 
Lygodiutn , Hymenophyllaceae), while the passage from the protostelic to 
the solenostelic type is marked by a broadening and opening out of the 
trace into the arch-shaped form [Lindsay a, Davallia repens , D. pinnata , 
D. aculeata). It is only after the definite establishment of this last type 
that we begin in some cases to get the development of accessory strands 
within the solenostele, and the further evolution of these is associated with 
a further elaboration of the leaf-trace ( Dicksonia ruhiginosa , Pteris etata , 
var. Karsteniana , Pteris aquilina , &c.). It is obvious that in the ontogeny 
of Matonia pectinata the appearance of internal accessory strands is much in 
advance of corresponding elaboration in the leaf-trace. 
Functional Relations of the Vascular System of Matonia 
in connexion with its Evolution. 
In order to attempt to understand the factors which have led to the 
evolution of the Matonia- type of vascular system, we must endeavour to 
realize the conditions existing at each stage of the progressive complication 
described, which, we may now assume, took place along the lines indicated 
by Gwynne-Vaughan’s Dicksonia- series, and confirmed by the history of 
development in Matonia itself. 
Starting with a hypothetical solenostelic ancestor and the arch-shaped 
type of leaf-trace, we have to consider first of all the cause of the origin of 
the local thickening of the xylem of the leaf-gap, such for instance as 
Gwynne-Vaughan has described in Dicksonia apiifolia , &c. This is clearly 
of the nature of a reinforcement of the water-conducting tissue at a point in 
the xylem-cylinder immediately beyond that at which the drain of water 
consequent on the diversion of the transpiration-current up the leaf-trace is 
first felt. It may be supposed that the additional tracheids provided serve 
as an addition to the water-channel supplying the tracheids of the stele 
beyond the node, which have their supply diminished by the relatively 
bulky leaf-trace. This development of an additional xylem in connexion 
with the leaf-gap assumes greater proportions in Dickso7iia adiantoides , 
