, 9I5 ] 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. 10. 



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 1 1 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 Eubotrychium that were examined, but that such 

 formation occurs in B. Luuaria as reported by Lang. 



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 relativelv small, only a tangled mass of tracheids is formed; but 



