Vascular' System of M atom a pectinata. 5 1 1 
where the ridge is continued throughout the internodes, and the xylem is 
separated from the xylem of the stele except at the nodes, and greater still 
in D. rubiginosa , where, according to Gwynne-Vaughan, it takes the form 
of from one to three separate strands, which only come into connexion with 
the stele at the leaf-gap. Here we may suppose that three separate internal 
xylem-strands serve as a reservoir of water to supply the tracheids of the 
stele, when they are depleted, during active transpiration, by the diversion 
of the current passing along the stele. This view is supported by the fact 
that the xylem of the accessory internal strands is in connexion with the 
xylem of the stele only at the point where the supply of water, to com- 
pensate for that diverted to the leaf-trace, is required. This is simply the 
physiological expression of the view that these internal accessory strands 
correspond to some extent with compensation-strands (Ersatzstrange, 
faisceaux reparateurs) serving to £ compensate ’ the vascular system of the 
stem for the loss caused by the departure of the leaf-trace. This idea also 
finds support in the phenomena seen in the young plants of Matonia , as 
detailed on pp. 486-8, and summarized on p. 497, in which the internal 
accessory strand either supplies tracheids to the xylem of the stele at the 
point from which the forward wing of the leaf-trace has departed, or sends up 
its whole dorsal half to fill the gap in the xylem made by the departure of 
the trace. It may be pointed out that the internal tracheal reservoirs have 
here no independent connexion with the roots, and therefore presumably 
no independent water-supply. We must suppose that they are filled from 
the xylem of the stele, when the water-supply is in excess of the needs of 
transpiration, and the pressure in the tracheal system consequently in- 
creased; depleted again when transpiration is active, and the pressure 
in the tracheids in the neighbourhood of the base of the leaf-trace con- 
sequently lessened. There seems no objection on physical grounds to such 
an assumption of reversible action, and no other way of giving a functional 
meaning to the internal accessory strands. 
The tracheal system thus originating cannot, however, to speak meta- 
phorically, long resist being drawn into supplying the leaf-trace itself. At 
least such a diversion obtains in the young Matonia , in Dipteris conjiigata , 
where a small part of the conspicuous thickening of the leaf-gap edge forms 
the incurved hook of the leaf-trace 1 , and it has probably occurred exten- 
sively in many of the advanced polycyclic Ferns 2 , though it is not mentioned 
by Gwynne-Vaughan as existing in his Dicksonia- series. In Gleichenia 
pectinata , on the other hand, the thickening of the leaf-gap edge appears 
from Boodle’s figures and descriptions 3 to be mainly a decurrent extension 
1 Seward and Dale (’01), PL XLVII, Fig. 4 . 
a e. g. Pteris aquilina ; see Tansley and Lulham (’04). It is possible that in some cases the 
development of the internal accessory system may be exclusively related to elaboration of the 
leaf-trace. 
3 Boodle (’01), PI. XXXIX, Figs. 26 , 27 . 
N n 
