510 Uphof . — Contributions towards a Knowledge of 
thin-walled for a long time, only being thick in the older rhizophores and in 
two or three layers of the outer cortex (Fig. V, i and 2). The secondary 
layers in the walls of younger rhizophores are not formed as soon as those in 
several other species. The vascular bundle is like that of S. rubella\ there 
are, however, not as many tracheides in the metaxylem. 
The terrestrial root has, with the exception of the epidermis and the 
two following rows of cells, which are large, the same construction as the 
stem. The cells of the hypodermis are slightly thick-walled. 
Selaginella viticulosa. 
This species develops aerial roots very close to the base of the stem ; 
they attain a length of 3 to 8 millimetres and a diameter of J to § milli- 
metre ; consequently very little is exposed to the air. 
The cells of the epidermis, hypodermis, and cortex are almost of the 
same size, and those of the first two tissues are thick-walled when they become 
older. The epidermis has no developed cuticle. All the walls of the 
epidermis and hypodermis are encrusted with a red pigment ; the walls of 
the cortex cells are colourless. The cortex is composed of eight to ten rows 
of cells. The vascular bundle is comparatively little developed ; its xylem 
is composed of three to four tracheides belonging to the protoxylem and four 
to five wide ones belonging to the metaxylem. 
Of the terrestrial root, which shows very much the same characteristics, 
it may be mentioned that there also the hypodermis is red-coloured for 
some length, but this colour disappears when the roots penetrate deeper into 
the soil. 
Selaginella serpens . - * 
The aerial roots attain a length of 6 to x 2 millimetres and a diameter 
of J to •§ millimetre. The epidermis and hypodermis are composed of 
cells whose walls are slightly thickened ; the cortex is composed of three to 
five layers of thin-walled cells. The endodermis and pericycle are composed 
of small cells which are frequently difficult to distinguish from those of the 
outer cortex. The xylem is composed of only seven to eight tracheides, of 
which two to three belong to the protoxylem. 
The terrestrial roots (Fig. X) are very thin ; the epidermis and the two 
following rows of cells are rather large ; root-hairs are formed abundantly 
and are relatively long ; the hypodermis is composed of about three rows of 
cells, which contain some secondary walls ; the cortex is formed by about three 
to four cell- layers, of which all the cells are much smaller than those of the 
hypodermis, and especially those of the epidermis. The vascular bundle is 
is formed by, as a rule, only three tracheides, which seem to belong to the 
protoxylem. 
