512 Tans ley arid Lulham. — A Study of the 
of the incurved edges of the trace l . In Matonia the case seems to be that 
as the leaf-trace increases in size, a number of tracheids from the internal 
strand run up into the wings of the leaf-trace, as these are departing from 
the stele. This diversion does not, however, interfere with what appears to 
be the original function of the internal strand, namely the compensation of 
the external xylem-cylinder, a compensation which from this time forward 
takes the form of an actual filling of the leaf-gap by the dorsal xylem of 
the internal strand (pp. 488-9). The free edges of the leaf-trace at this 
stage are distinctly thickened, and their further progressive elaboration into 
the complicated lateral loops characteristic of the adult type goes hand in 
hand with the progressive elaboration of the internal cylinder, the whole of 
both limbs of the lateral loop on each side being supplied from this cylinder, 
which itself becomes solenostelic and increases greatly in size, while its roof 
regularly closes the leaf-gap of the outer cylinder. 
The second cylinder now finds itself in the position of the original stele 
at an earlier stage of development. Not only has the whole of its roof gone 
to supply the lateral loops of the trace, and to fill the gap in the outer 
cylinder, but the tracheids occupying the edges of the gap made by this 
departure are in actual continuity with those of the free edges of the trace, 
the water-supply here running backwards from the second cylinder to the 
trace. This channel is established at an early period of development, as is 
shown by the continuity of protoxylems between the flanges of the cylinder 
and the gutters of the trace (p. 508), and must aid considerably in the depletion 
of the second cylinder just in front of the node. It is this state of things, 
no doubt, which gives the condition for the development of the third cylinder 
as an appendage of the edge of the gap of the second, bearing exactly the 
same relation to it as the second originally bore to the first, and serving in 
the same way as a supplementary reservoir of water, drawn upon when 
transpiration is active. If this view be accepted, we need not suppose that 
those cases in which the third cylinder ends blindly in the pith represent 
cases of reduction and loss of function ; they can be interpreted as earlier 
stages in the evolution of the third cylinder, which starts as an elaboration 
of the edge of the gap in the second and extends in both directions through 
the internodes till it becomes continuous throughout the rhizome. It has 
been shown (p. 495) that the complication of the third cylinder is nearly always 
greatest in the region of its junction with the second, where its evolution 
may be supposed to be furthest advanced, and is correlated with the 
diameter of the outer cylinder and consequently with that of the leaf-trace. 
The increase in the diameter of the leaf-trace gives space for the increase in 
size and importance of the lateral loops, and these in their turn increase the 
1 In specimens of this plant obtained by Mr. Boodle since the publication of his paper, 
the thickening of the edges of the leaf-gap is entirely a downward extension of the leaf-trace 
edges. 
