514 Uphof. — Contributions towards a Knowledge of 
guished from the small cells of the inner cortex. The xylem is composed 
of six to nine tracheides ; those of the protoxylem are very narrow. Aerial 
roots which are developed in a very moist environment always develop root- 
hairs and root-cap. As has been stated, every epidermis cell changes into 
a root-hair ; these hairs are narrow and long. The epidermis is followed by 
only one thin-celled layer, which is succeeded by a thick-walled hypodermis. 
There are less rows of cells forming the cortex than in a root without root- 
hairs (Fig. Ill, 3 to 9). 
Terrestrial roots are the same as the latter kind of roots. 
Selaginella cuspidata. 
Rhizophores are only developed towards the base of the stem, and only 
when growing in a very damp environment ; plants which grow in their 
Fig. XI. Section of root of S. Bakeriana . Fig, XII. Cross-section of root of rupestris. 
native environment do not develop such roots, which is also the case with 
other xerophytic species, e. g. X. pilifera and .S'. lepidophylla % They attain 
a length of 5 to 8 millimetres and a diameter of 1 to ij millimetres. These 
are very tough and velvety ; the former quality is caused by its well- 
developed sclerenchyma, the latter by the presence of innumerable root- 
hairs. The writer was not able to observe aerial roots without root- 
hairs. 
A section of the root (Fig. XIII, 1 and 2) shows the following: the 
epidermic cells are all transformed into root-hairs, after which follows 
one layer of thin-walled cells ; this tissue is followed by layers of hypo- 
dermic and cortex cells, which are composed of eight to nine rows of cells. 
These are succeeded by six to eight rows of thin-walled 4 cortex cells ; 
