5 1 6 Uphof. — Contributions towards a Knowledge of 
almost entirely thick-walled, with the exception of two or three rows of cells 
towards the endodermis. 
Roots of S. pilifera and 5. lepidophylla are very much the same ; only 
the root-hairs are longer. 
Selaginella rupestris . 
The length of the aerial roots is 4 to 8 millimetres, and the thickness 
J to ■§, sometimes millimetre. The epidermis is thick-walled and 
furnished with a clear cuticle ; the hypodermis also is composed of thick 
walls. The cortex is formed by six to eight rows of thin cells in plants 
which were developed in a moist environment, but are thick-walled in plants 
which were grown on exposed rocks. There is no difference in construction 
of the vascular bundle between that of a root and of a stem, which applies 
to all species of the homophyllous Selaginellas, e. g. S. rupincola , 6 *. densa , 
vS. capensis , and other species, a fact which is also to be noticed in the case 
of the root and stem of Lycopodium . 
The terrestrial root shows a large and thin-celled epidermis, which 
forms short and rather wide root-hairs. These are succeeded by two rows 
of thin-walled cells of the same size; after which a hypodermis is to be 
noticed attaining a thickness of three or four rows of cells. The cortex is. 
usually composed of four rows of cells. The vascular bundle is of the same 
type as that of the aerial root and stem (Fig. XII). 
Summary. 
The species of Selaginella are remnants of older periods in which also 
other related genera reached their highest pitch of development. They 
belong to the primitive vascular plants, and therefore morphologically are of 
much importance as far as the origin of the plant organs is concerned. 
Some of the present living allies of Selaginella are in possession of true 
roots, although anatomically these do not differ very much from that of the 
stem (e. g. Lycopodium) ; some do not have true roots, such as species of the 
Psilotaceae. 
Generally the root is simple in construction and differs from the stem, 
the former lacking the lacunae and trabeculae of the latter. 
There is no important anatomical difference between aerial and 
’terrestrial roots. The difference of the outer tissue is due to environmental 
circumstances. 
Physiologically both kinds of roots have the same characteristics in the 
same degree ; both are negative heliotropic. 
Roots with root-cap always have root-hairs, those without root-cap 
very rarely have root-hairs. 
Of all species, the root originates with regard to the stem exogenously ; 
they branch monopodially. 
