16 
Proceedings of the Royal Society of Edinburgh. [Sess. 
essentially different in their real origin, the physiological effect of them 
is alike. By either means the endodermal surface is increased, and inter- 
change facilitated ; while the complete investment of the vascular tracts 
with endodermis is maintained. The final effect of these several factors, 
separately or in combination, is to break up the vascular tracts as seen 
in transverse section into relatively small circular or oval masses, each 
with a relatively large proportion of surface to bulk. The physiological 
Fig. 13. — Transverse sections of stems, drawn to the same scale, showing that stelar complication 
does not depend directly upon size alone. ( x 2.) 
A = Cibotium Barometz. B = Hemitelia setosa. 
difficulty following on increase of size is thus fully met in Leptosporangiate 
Ferns (fig. 9 : 4, 6, 8). 
If, as the anatomy of the ferns seems to suggest, actual Size is one 
of the factors determining the form which the stelar tissues take, and that 
increase beyond certain dimensions leads to those peculiarities which are 
seen in them, and particularly to the breaking up of the stele into 
meristeles, then tuberous development should lead to such disintegration. 
More especially should the change be apparent where the normal part 
shows a relatively simple stelar structure. A good example of this is 
seen in the tubers borne upon the protostelic stolons of Nephrolepis (fig. 
