8 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Hymenophyllacese, it is *5 mm. in diameter. In all of them its form is 
conical at first, but after reaching a certain size it retains that size through 
life, as a cylinder traversing the cylindrical rhizome. The limiting factors 
have come into play, one of which is the proportion of surface of the stele 
to its bulk. When the stele attains larger dimensions, as it did in certain 
fossils while still retaining its protostelic state, is is seen to have undergone 
a modification of form. For instance, in Ankyropteris Grayi (fig. 6, ii), 
which is 2-8 mm. in diameter, it is corrugated, the insertions of the leaf- 
Fig. 5. — Transverse section of a stem of Botryopteris cylindrica, showing 
a protostele with a solid central cure of xylem, and peripheral 
phloem. The endodermis is not clearly shown in this fossil Fern. 
traces projecting, and the surfaces between being hollowed. Moreover, the 
curvatures of the hollows are deeper in the larger than in the smaller 
specimens (fig. 6, iii). A still more extreme case of this is seen in the stele 
of Asterochloena laxa, which may be as much as 15’5 mm. in diameter 
(fig. 6, iv). Here the stele is thrown into deep involutions of the surface. 
It is obvious that this will give a very greatly increased proportion of 
surface to bulk. It seems natural to conclude in such cases that the more 
elaborate form of the stele has made the larger size possible, by overcoming 
the limiting factor. But notwithstanding the complicated outline, and the 
well-known differentiation of the xylem of these fossils, their steles are 
still of the nature of protosteles: their non-medullated structure is 
maintained. 
