the Anatomy of the Genus Selaginella . 501 
the outer cortex are thick- walled, which is caused by secondary layers in the 
cell-walls. Treatment with potassium hydroxide gives a yellow colour and 
suggests the presence of suberin ; its amount depends upon the exposure ; 
organs developed in a moist and shady climate contain considerably less 
suberin than those grown in a dry exposed place. 
Rhizophores or aerial roots which approach the soil become even more 
thin-walled, and having touched the soil the cell-walls are only composed of 
their primary walls. Such roots, soon after entering the soil, form root-hairs. 
Such behaviour was studied by the writer on Lobbi, S. rubella, S. Wilde- 
nowi, S. Kraussiana , S'. Douglasi , and S. Martensi. Of the latter species, 
the following observations may be made: The aerial root has a small- 
Fig. IV. S. rubella, i, junction of vascular bundles of rhizophore and stem ; 
2, young rhizophore. 
celled and rather thick-walled epidermis ; the cells of the hypodermis are 
also thick-walled ; the outer rows of the cortex show some secondary 
thickening of their walls, and in very exposed places these cell-walls of the 
cortex may be considerably thicker (Fig. V, 3). When the aerial root 
approaches the moist soil, at a distance of about 2 millimetres, its structure 
soon shows some differences, e. g. the epidermis is not as thick-walled, and 
the space in the cells is larger ; further, there is no great difference in size 
between the cells of the epidermis and of the hypodermis (Fig: V, 4), such 
difference as exists having no doubt been caused by the moisture of the soil ; 
there is, however, no difference in the cells of the cortex, whether developed 
in the air or near the surface of the soil. When the aerial root actually 
touches the soil the epidermis does not show any secondary layers in its 
walls, although they are still present in those of the hypodermis ; the 
epidermis, however, is often followed by another, sometimes two layers of 
