456 Harvey-Gibson. — Contributions to Knowledge of 
a root on its own account. In 5. Lobbii the dorsal papilla 
may, apparently, either develop into a third proliferous shoot 
giving rise to a root, or the root may arise from the papilla 
directly, the intermediate shoot being suppressed. 
Another argument advanced by those who uphold the stem- 
nature of the aerial root is that it is exogenous in origin and 
bears no root-cap. I have not as yet proceeded far enough 
with my own investigations into the origin of members in the 
genus to offer an independent opinion, but it has been shown 
by Van Tieghem (12) that no great weight can be laid on this 
argument, since neither in regard to exogenous origin nor 
absence of root-cap is Selaginella exceptional. Anatomically, 
it will be seen from the present paper that the aerial and 
subterranean parts of the root exhibit practically identical 
features. On all these grounds, therefore, I follow Van 
Tieghem in considering the root of Selaginella as in some 
cases subterranean only, in other cases partly subterranean 
and partly aerial, developed at the base of the stem only, and 
without relation to the branching of the stem, or at the 
bifurcations of the stem, sometimes singly and either from 
the dorsal or the ventral side, or both from the dorsal and 
ventral sides. 
Anatomically the root in Selaginella shows comparatively 
little variation, being, in the great majority of cases, composed 
of a more or less sclerotic cortex covered externally by two or 
more layers of thin-walled cells, the most external of which 
is piliferous. These superficial layers are absent from the 
aerial parts of the root. The vascular cylinder is enclosed by 
a usually well-marked endodermis and a two- or three-layered 
pericycle, and consists of one xylem and one phloem bundle. 
It will be most convenient to study the structure of the root 
in one species somewhat in detail, and thereafter add such 
further notes as may be necessary in explanation of the 
variations in anatomical structure seen in other species. 
6'. atroviridis , Spr., was found a convenient type for 
examination, as being fairly robust and possessing the morpho- 
logical characters most commonly met with in the genus. 
