the Anatomy of the Genus Selaginella . 451 
directed faces. Divisions of the apical cell cease when the 
rhizophore is about 1 mm. long, and by rapid divisions of 
the segments the apex swells and differentiation takes place 
within, so as to give rise to the true roots which at this stage 
do not exhibit further development. The cells of the swollen 
apex have their walls thickened, and they are filled with 
watery contents. Intercalary growth now becomes energetic, 
so as to bring the apex of the rhizophore into contact with 
the soil. The cells of the apex become disorganized, and the 
apices of the true roots enter the soil covered at first by 
a homogeneous slime derived from the disorganized apical 
cells. 
In section the rhizophore exhibits a clearly marked cortex, 
whose cells are sclerotic save in the innermost layer, and 
a central vascular system. The first vascular elements arise 
centrally in 5. Kraussiana ; scalariform elements develop 
round these, and are surrounded in turn by three to five layers 
of small cells. The vascular cylinder of the rhizophore arises 
from the vascular strand of the stem of the same side at 
or near the conjunctive strand. The markedly centrifugal 
development of the vascular system in the rhizophore of 
Kraussiana is, according to Nageli and Leitgeb, a unique 
feature. 
In S. Martensii one rhizophore (the lower one) is alone 
developed, although occasionally the upper one is also. The 
latter is always represented by a swelling. The vascular 
bundles of the rhizophores unite in the stem and run parallel 
with its long axis to the point of bifurcation of the stem, and 
there unite with the cylinders of the branches. The air-space 
of the stem-cylinder is continued up through the rhizophore 
for a short distance. Nageli and Leitgeb believe that the 
rhizophores are independent in origin. In the case of the 
true root the development of the protoxylem is lateral. 
Where the root branches the protoxylems face each other, 
hence the bundles undergo torsion to the extent of 90 degrees. 
Branching of the vascular cylinder takes place before the 
forking of the root itself, hence the end of the unbranched 
1 i 
