The Leaf-Trace. 
3 
vascular tissue in the middle of the adaxial (upper) side. The 
current of water entering an expanded leaf-trace will tend to be 
diverted to the parts continuous with the periphery of the stele below. 
This will lead to a failure to develop tracheids in the acroscopic angle 
between stele and leaf-trace, or, as we may call it, the “axil” of the 
trace. Thus the leaf-trace will acquire a kidney-shaped cross- 
section and the same tendency, if carried further, will lead to the 
formation of the arched type, which sometimes co-exists with a proto- 
stele (Gleichenia flabellata, Fig. 40a). This in turn will necessitate 
a further increase in the diameter of the stele, and will lead to the 
formation of a passive tissue (or pith) in its centre, since if the 
stele remained solid the central mass of xylem would increase in 
amount much faster than would be required. This pith will 
naturally tend to be a basipetal continuation of the passive tissue 
in the concavity of the arched leaf-trace. Thus we arrive at the 
typical solenostele with its typical arched leaf-trace, the departure 
of which causes a gap in the vascular wall of the stele. 1 When 
the leaves are remote and the leaf-trace approximates in size to the 
stele itself, a section taken immediately in front of the point of 
insertion will show two similar arcs of vascular tissue with their 
concavities facing one another, closely resembling the appearance 
of a section taken just in front of the dichotomy of the stele below 
a fork of the stem (e.g., in Gleichenia pectinata). Thus the adaxial 
gap in the leaf-trace is fundamentally a result of the failure to 
develop xylem in the “axil” or region left between the two diverging 
currents of water, accentuated by the broadening of the leaf-trace 
itself and the corresponding broadening of the stele, while the 
gap in the stele and the formation of pith is due to the same cause, 
whose effect is carried down into the internode below as the 
starvation of water is felt by the central portion of the widening 
stele. The stelar gap has to be repaired sooner or later, above the 
departure of the trace, because of the insertion of the leaf-traces 
above. To put the matter more strictly, the failure to develop 
vascular tissue in front of the insertion of the leaf-trace does not 
extend forward along the stele indefinitely. The currents of water 
passing into the next leaf-trace situated in the neighbourhood of the 
same longitudinal line will necessitate the development of vascular 
tissue in front Of the gap. The adaxial gap in the leaf-trace itself 
1 The origin of the Lindsaya- type, which involves the formation 
of internal phloem and typically precedes the formation of the 
solenostele, is here omitted for the sake of simplicity. It has 
been fully dealt with in the fourth and fifth lectures. 
