494 Uphof.- — Contributions towards a Knowledge of 
the family of the Isoetaceae, also shows in some of its characteristics slight 
resemblance to Selaginella, and possesses true roots. Under the fossil 
forms, Stigmaria has been extensively studied ; especially Scott (11) has 
greatly distinguished himself ; he considers the subterranean organs of this 
genus as a .primitive stage in the differentiation of root and shoot. . 
The aerial roots of Selaginellas have been considered by various in- 
vestigators as leafless shoots, as roots, and as organs standing between root 
and shoot ; others consider them as special organs (1). 
The first investigators who have drawn considerable attention to the 
aerial roots are Leitgeb (8) and Nageli, who call them Wurzeltrdger 
(= rhizophores) and consider them morphologically as stem-like, leafless x 
organs, which only perform the function of bearing true roots ; both 
investigators consider it as of much importance that the rhizophores have 
no root- cap and therefore could not be roots ; on the other hand, there are 
nevertheless roots, whose morphological significance is beyond doubt, 
which do not develop any such root-caps, as has been studied under the 
H ippocastanaceae and Sapindaceae by Waage (15). Also Pfeffer (9), 
Treub (14), Bruchmann (2), Fries (5), Campbell (3), and Worsdell (17) 
consider them as leafless stems. On the other hand, van Tieghem (12), 
Douliot (13), and Sarauw (10) suggest they are roots ; whereas Goebel 
(6 and 7) supposes they have the characteristics of both ; also Bower (1). 
states that the structure is like that of a root, but that in some cases the 
rhizophore has a structure resembling that of an axis. Those who are, 
in favour of a stem-like nature, such as Pfeffer (9), Bruchmann (2), and 
Worsdell (17), claim that sometimes leafy shoots are seen instead of leafless 
rhizophores, although on the other hand it has been observed that true roots 
may give rise to branches, as in A nthurium longifolium, Asplenium 
escu ten turn , and Neottia Nidus-avis (16). 
Generally the root of the genus Selaginella is simple in construction 
and differs from the stem. Although there is much discussion as to the 
morphological value of the rhizophore, as has been shortly outlined above, 
the writer does not hesitate to consider them as roots which have been 
developed in the air instead of in the soil, and have therefore had to adapt 
themselves to their particular environment, which necessitated a change in 
some of the tissues. Instead of being called rhizophores, the name 1 air- 
roots ’ would therefore be more appropriate ; this name will be frequently 
used in this sense in the present article. 
Both kinds of roots have been examined by the author in 262 different 
species, of which about forty have been studied anatomically. 
There is very little prospect of arranging this genus phylogenetically in 
accordance with the appearance of the roots. The aerial roots are always 
exogenous in origin, whereas the branching of the roots in the soil is mono- 
podial, although apparently dichotomous. There is apparently more diver- 
