1915] PETRY—OPHIOGLOSSACEAE 355 
Periderm formation.—The formation of cork at the point of 
injury has commonly been accepted as a direct response to condi- 
tions. In several of the rhizomes of B. obliquum, injuries affected 
all the tissues of the stem. In these cases, periderm is formed by 
every tissue that is still capable of growth; that is, by cortex, 
endodermis, pericycle, cambium, and pith. Periderm formation by 
the pith is shown in fig. ro. 
The formation of vascular tissue by the pith—In three specimens 
of B. obliquum, tracheids scattered through the pith were found in 
considerable numbers; fig. 9 shows a section through such a stem. 
In these cases the injury is at some distance above the point of 
appearance of the pith tracheids. 
In both branching specimens of B. virginianum and in three of 
the four of B. obliquum, a considerable development of secondary 
xylem occurs within the pith; figs. 10 and 11 show examples of this. 
In all cases this development of centripetal secondary xylem occurs 
below and opposite the point of departure of a leaf trace. The 
cambium is always directed toward the center of the stele; well 
developed sieve tubes are present in most cases. 
The formation of vascular tissues by the cambium.—In every 
injured rhizome of B. virginianum and B. obliquum a renewed 
activity of the cambium has occurred. Lignification does not 
always take place in the first elements produced by this renewed 
activity; this produces a narrow strip of cells of rectangular cross- 
Section just outside the original xylem of the stem. Usually, how- 
ever, tracheids resembling those of the original growth are produced 
by this renewed cambial activity (fig. 10); these make up the 
accessory secondary xylem mentioned above. It is to be noted 
that no such accessory xylem was found in either of the two species 
of the section EvsotrycutuM that were examined, but that such 
formation occurs in B. Lunaria as reported by Lane. 
The formation of vascular tissues by the pericycle-—In every 
injured rhizome of B. virginianum and B. obliquum the pericycle has 
produced xylem in greater or less amount. As already stated, these 
masses of pericyclic xylem sometimes contribute to the vascular 
Supply of the branches. When the amount of xylem that occurs 
is relatively small, only a tangled mass of tracheids is formed; but 
