18 Proceedings of the Royal Society of Edinburgh. [Sess. 
A somewhat similar result is found in the tubers formed on the 
rhizomes of certain species of Equisetum. Each tuber results from the 
dilatation of an internode of the rhizome to a size beyond the normal, 
and this is found to be accompanied by a disintegration of the stele. E. 
arvense is one of those species in which the stele is strictly circumscribed 
by an unbroken external endodermis. In the internode of the rhizome 
the stele is relatively compact, and in an average case it measures about 
1 mm. in diameter. The individual vascular strands may vary in number. 
In fig. 15, A, six are seen. In the 
distended tuber the strands appear 
widely apart, each surrounded by its 
own endodermal sheath, which closes 
round it as the strands separate at 
the base of the tuber. The ring 
which they form may be 3 mm. or 
more in diameter, fully three times 
that of the original stele (fig. 15, B). 
As in Leptosporangiate Ferns, the 
stele has divided into meristeles, and 
as in them the disintegration accom- 
panies an increase in bulk, which 
may be held to be one of the factors, 
or perhaps the chief factor in deter- 
mining it. As the next node is 
B 
Fig. 15. — Equisetum arvense. 
Transverse section of internode of rhizome : stele approached the tuber Contracts, and 
with encircling endodermis, 1 mm. in diameter. 
Transverse section of tuber, to the same scale, 
showing ring of meristeles, each with its own 
endodermal sheath, 3 mm. in diameter. (xlO.) m 0i»isteleS 
the stele is reconstituted from its 
The significance of these 
two examples in supporting the theory cannot be mistaken. 
In the petiole of the fern-leaf the leaf -trace has as a rule the form of 
an arc concave on the adaxial side. In many relatively primitive ferns 
the leaf-trace is undivided. Being thus a continuous curved tract limited 
by endodermis, it may form in large leaves a formidable obstacle to com- 
munication from the outer surface inwards, and especially at the leaf -base, 
where naturally interchange would be active and the petiole is at its 
largest. In many ferns the leaf-trace is broken up at, or near to, the 
base into separate strands. There is often a median slit dividing the 
trace into equal halves, as in the type of Asplenium, Athyrium, or 
Gymnogramme (fig. 16). But often the subdivision may be carried further, 
as in the case of many Cyatheoid Ferns (fig. 17 : 6). Sometimes the slits 
close again upwards and downwards, as is very well seen in Plagiogyria f 
