359 
appear, thick-walled cells, alternating with the xylems. Düistinct 
sieve tubes, even in the older stages, could not be traced. At 
this stage the endodermis begins to develop. The pericycle 1s 
absent between proto-phloem and endodermis. Between xylem 
and endodermis there is a pericycle consisting of one layer, 
the cells of which are filled with dense protoplasm. Still later 
the two proto phloems join (Fig. 30) to form a linear strand of 
xylem between the two phloems. The xylem increases in bulk 
before the leaf trace joins it. The point of their junction can 
be seen in Fig. 31. The endodermis, previously broken by 
the entering leaf-gap, surrounds the stele again entirely. The 
phloem follows the inside of the endodermis, leaving only 
an occasional pericycle cell between. 
Bower (4) in his work on the medullation of the Ophioglossa- 
ceae remarks that the pith is initiated below the departure of 
the first leaf trace; but that the medullation is intra-stelar. 
There is no evidence that the medullation in B. 
simplex is anything else than the enclosure of a cer- 
tain number of parenchyma cells between two 
xylem strands. 
