Ontogeny. 7 
petiolar vascular system have clearly arisen, as Gwynne-Vaughan 
has shown, in a different way from the internal strands of the 
petiole in the Leptosporangiate Ferns. They do not form 
elaborations of the main petiolar curve, but arise individually from 
the dorsal (lower) strands of the curve. They are, however, con¬ 
nected (see Vol. VI., p. 230 and Fig. 72E), through the ventral 
strands of the curve, with the pinna-traces, and no doubt fulfil the 
same function, in part at any rate, as the lateral folds of the 
Cyatheaceae. 
The breaking of the leaf-trace into separate strands, which is 
found in all the higher dictyostelic ferns and in the Marattiaceae, is 
probably somewhat the same phenomenon as the “perforation” of 
the cauline stele (p. 192) with which it often co-exists. In a 
certain sense it is to be regarded as a reduction-effect, since it is 
clear that less conducting tissue is produced than would be the case 
if the petiolar curve or stelar cylinder formed a continuous plate of 
vascular tissue. The tendency is to the formation of a separate 
vascular strand (a divergent) in connexion with each protoxylem,—or 
sometimes a bi-polar bundle between two protoxylems—leaving 
spaces of ground-tissue between. This would seem to be the result 
of some factor, such as an increase in the circumference of the 
entire system, leading to the disappearance of such parts of it as 
are no longer required for conduction. Here again the petiolar 
system is often in advance of the cauline stele, for we often have a 
“ broken ” leaf-trace associated with a non-perforate stele, but not 
the reverse case. Of the factors which actually determine the lines 
of flow of the water current and hence the formation of the isolated 
strands, we are, however, ignorant. An investigation of a few 
simple cases from this point of view would be a useful piece of work. 
The Ontogeny of the Filicinean Vascular System. 
The characteristic difference between the construction of the 
typical animal body and that of the typical plant is the compactness of 
the one and the elongated branched form of the other. This depends 
directly upon the distributed growth, limited in time, of the animal 
as contrasted with the localised but continuous growth at the 
apices of the axes in the plant. The fertilised egg gives rise to a 
miniature plant which in the higher forms at least grows almost 
entirely by means of localised meristems as they are called, primary 
meristems situated at the ends of the axes, and in some cases 
secondary meristems arising within the axes. Confining our attention 
