Factors in the Evolution of Solenostely. 151 
not find adequate attachment in the external phloem of the rhizome 
stele, and an internal strand of phloem in the rhizome might arise 
hy the decurrency of the internal phloem of the trace into the 
xylem of the stele. But here a difficulty arises, for while it is, as 
we have seen, quite easy to understand the rationale of origin of 
blind ground-tissue pockets, the function of a blind phloem pocket, 
such as we find in Gleichenia flabellata and other species, is not so 
intelligible. We know far too little about the actual function of the 
phloem of ferns to dogmatise on this subject. We certainly cannot 
be sure that a discontinuous series of phloem pockets has no function. 
They may act as storage reservoirs, they may feed the developing 
xylem, but till we know more it is useless to discuss the matter 
further. Nevertheless, on purely morphological grounds there 
certainly seems every likelihood that the establishment of phloem 
pockets in a protostelic type formed the point of departure for the 
evolution of the Lindsaya-type, just as the formation of ground-tissue 
pockets within the internal phloem formed the point of departure 
for the evolution of the solenostele. 
The formation of a “ leaf-gap ” in the stele is a necessary con¬ 
sequence of the great dilation of the stele, and the concomitant 
thinning of the vascular ring, involved in the formation of a soleno¬ 
stele. The vascular ring is “ broken ” by the departure of the 
trace, or in other words the vascular elements in the “ axil ” of the 
trace, are not developed, owing to the deflection of the water- 
current on that side of the stele into the tracheae of the trace 
itself. The gap so formed in the wall of the stele is “ repaired ” or 
“closed,” in the typical case, by the development of vascular 
elements, completing the solenostelic ring, at a higher level, above 
the area affected by the departure of the trace; in many cases, 
however, particularly where the vascular ring has a large diameter, 
the closing of the gap is effected in another way, as we shall see in 
a future lecture. The gap may be confined to the base of the leaf- 
trace or it may extend for some little distance forward along the 
rhizome-stele. A few solenostelic ferns have no leaf gaps, but this 
is pretty certainly always due to reduction of the trace in relation 
to the stele so that a smaller relative demand is made upon the 
latter for the supply of any given leaf. In the Lindsaya -type a gap 
is usually made in the dorsal arch of xylem covering the internal 
phloem. It is, in most cases, of small size and quickly repaired, 
but this depends upon the size of the leaf-trace in relation to the 
stele, 
