of Vascular Strands in Angiopteris evecta . 385 
Outside the phloem is a well-marked tissue bounded 
externally by a characteristic endodermis. The extra-phloic 
(pericyclic) parenchyma is cut off from a primary layer which 
also gives rise to the endodermis, as is so common in ferns. 
The parenchymatous layers internal to the actual endodermis 
become more numerous and easier to trace in older plants, 
and the common origin of the respective tissues is exceedingly 
clear. 
The leaf-traces which pass out from this region of the 
stem are very simple, forming single strands, and their exit 
from the axile stele does not occasion the formation of foliar 
gaps in the ordinary sense of the expression. What happens 
is simply that a sector of the stele bends outwards towards 
the leaf, and then the loss of the tissues thus detached from 
the stele is at once made good above the exit of the trace. 
But sooner or later the xylem ceases to form a solid axile rod. 
One or two parenchymatous cells appear in the centre, and form 
the commencement of a pith (Figs, it, 12). The time at which 
this pith first appears varies greatly. It may be present in 
a rudimentary form when the first leaf-traces are formed, 
but more commonly it does not occur until after several of 
these have been given off ; when it has once been differentiated 
it increases rapidly in importance, and then the subsequently 
produced leaf-traces begin to become associated with foliar 
gaps, inasmuch as the entire thickness of the now cylindrical 
vascular strand becomes involved in the formation of the 
leaf-trace. This occurrence of foliar gaps commonly begins 
in connexion with the sixth or seventh leaf-trace. But there 
is as yet no permanent disturbance of the ‘ monostelic * 
arrangement, for the gaps continue to be repaired as fast 
as they occur, and no phloem or endodermis is to be dis- 
covered within the cylinder of xylem. As the leaf-traces, 
however, become more numerous and the foliar gaps larger, 
the cylindrical vascular strand loses its first character. Traces 
succeed each other rapidly, and before the gaps left by older 
ones have quite closed. In this way the annular appearance, 
as seen in transverse section, disappears, and several isolated 
