the Anatomy of the Genus Selaginella. 513 
The terrestrial root has a large-celled epidermis, which forms relatively 
long root-hairs ; it is followed by two other thin-walled layers, whose cells 
are of the same size ; this tissue is succeeded by three rows of cells belong- 
ing to the hypodermis, which as usual are thick-walled. There is no 
difference in the vascular bundle in comparison with that of the aerial root, 
only that its elements are relatively fewer the thinnef the root in the soil 
becomes. 
Selaginella atrovirens. 
The rhizophore has a length of 35 to 65 millimetres and a diameter of 
i to 1 millimetre. All the cells of the epidermis, hypodermis, and some- 
times of the outer cortex are furnished with secondary walls, although these 
are not very thick. The cortex is composed of eight to twelve rows of cells, 
which are larger than the hypodermis cells. The vascular bundle is as 
a rule surrounded by a distinct endodermis and pericycle. The xylem 
takes considerable room in the vascular bundle, and is more developed than 
the phloem. The metaxylem is composed of twenty to twenty-eight 
tracheides; the protoxylem sometimes contains eight to fifteen of such 
vessels, and is strongly developed in the primary rhizophore. 
The terrestrial roots are thicker than those of most other species, 
and attain a thickness of \ to | millimetre. The root-hairs are not very 
abundant. The epidermis, as a rule, is succeeded by two other thin-walled 
layers ; the hypodermis is composed of cells which are of rather consider- 
able thickness, especially in those which grow close to the surface ; those 
which have developed considerably deeper in the ground are in possession of 
a hypodermis which has not as many secondary layers in its walls. 
Selaginella grandis. 
The aerial root reaches a length of 8 to 12 millimetres ; in one instance 
I found one of 25 millimetres ; its diameter varies from \ to 1 millimetre. 
There are two different types of rhizophores, one of which has developed 
a root-cap and root-hairs ; the other has no root-hairs and no root-cap. The 
primary rhizophore, which is in direct connexion with the stem, very rarely 
develops root-hairs, and never a root-cap, as far as I was able to observe. 
While the rhizophore with root-cap develops all its epidermis cells into 
root-hairs, the other one without root-cap occasionally forms a root-hair 
from an epidermis cell, leaving the other cells of epidermis to their normal 
development. 
The aerial root without root-cap is composed of small cells ; those of 
the epidermis, hypodermis, and cortex are of the same size. The two 
former tissues are thick-walled ; the cortex is composed of fen to fifteen 
rows of cells. The vascular bundle is very small, although it is very 
strongly developed in the stem. The endodermis cannot be clearly distin- 
M m 
