495 
the Anatomy of the Genus Selaginella . 
sity in the location of the roots of the air than those developed in the soil. 
The former have their origin always at the forking off of an ordinary branch, 
either at the upper or lower side of the stem ; they may be found over the 
entire length of the stem, e.g. 5. Kraussiana , S. Baker iana , and X. serpens ; 
in other species they are only seen below the middle of the stem, as in 
S. atrovirens and S. viticulosa . Xerophytic species such as S. cttspidata , 
Fig. I. Different types of rhizophores or aerial roots, i, S. Wildenowi ; 2, S. pilif era \ 
3, S. rupestris ; 4, S. rubella ; 5, S. serpens. 
S . pilifera , and S. lepidophylla , only form aerial roots towards the base of 
the stem when they are grown in a very moist environment, which also 
may be said of the hygrophytes, 5. Vogelii, S. caulescens , and 5. viridangula. 
On the other hand, the xerophytic species, 5. sanguinolenta , forms under 
natural conditions aerial roots at several places of the entire stem. 
The thickness of the aerial roots, which differs but slightly in the same 
species, is fibre-like (about \ mm. in thickness) in X. molliceps , 5. Bakeriana , 
X. apus , and X. haematodes ; they are string-like (about i mm. and more in 
