1914] PETRY—OPHIOGLOSSUM PENDULUM 173 
ditions the protoplasm remains very active. When cells of the 
epidermal layer are killed, the outer walls of the cells immediately 
below begin to thicken in this way. This 
may occur in any of the cells of the outer 
region of the cortex upon destruction of the 
overlying cells. 
Roots originate in the meristematic region 
of the stem by the formation of an apical cell Fina: Vediued 
m the first layer of cells outside the phloem. cell of root; X 236. 
Fig. 3 represents this origin as seen in a 
longitudinal section of the stem. As shown, differentiation of the 
phloem has proceeded to within a few cells of the position of the 
root meristem. The growth of the root proceeds by the segmenta- 
tion of the apical cell, which 
is tetrahedral in form; in 
slowly growing roots the seg- 
mentation is irregular, and a 
definite root cap cannot be 
distinguished. The first 
xylem elements are smaller 
in cross-section than those 
formed later; they are true 
tracheids with walls reticu- 
lately thickened. In a few 
cases of more rapidly grow- 
ing roots, the thickening of 
the walls approaches the 
annular marking. This char- 
acter of the protoxylem indi- 
cates the slow growth of the 
roots. The lignification pro- 
ceeds slowly toward the 
of root, as shown in center of the stele, and xylem 
parenchyma is often found in 
Sitts at a ai the center of the stele of large 
thickenin stance of 3-4 cm. from the tips. There is no secondary 
g of the xylem mass. In a few cases a medullated 
. oa 3.—Origin 
a section of a bud: ph, phloem; 
