506 Uphof*— Contributions towards a Knowledge of 
is followed by two layers of thin-walled cells which are of the same 
appearance. The next tissue met with is the hypodermis, which is composed 
of about three to five layers of cells ; the walls are thick on account of the 
presence of secondary walls. The cortex is thin-walled, though sometimes 
its outer cell-layer may be slightly thick-walled. The endodermis is not as 
Fig. VII. Section of the top part of a young rhizophore of S. Wildenowi. 
easily visible as that of the primary air-root or rhizophore. The phloem 
contains but few sieve-tubes in comparison with the aerial root. The xylem 
is the same as has been described above. 
Selaginella rubella. 
The aerial root reaches a length of 30 to 65 millimetres and a diameter 
of | to 1 millimetre. The epidermis has rather a thick-walled cuticle, and 
the other walls of the epidermis and hypodermis are thick, those of the 
former differing but slightly in size from those of the latter, except that 
