Thalassiophyta and Algal Ancestry of Higher Plants 169 
Protococcales : numerous forms (species of Chlorococcum , Chlorella , 
Trochiscia , etc.). 
Ulotrichales: species of Hormidium, Prasiola, Stichococcus, 
Microspora, Rhizoclonium. 
Chaetophorales : Trentepohlia and other members of Trentepoh- 
liaceae; Pleurococcus (?). 
Oedogoniales: Oedocladium. 
Siphonales : F aucheria . 
Conjugatae : species of Mesotaenium and Cylindrocystis; Zygogo- 
nium ericetomm. (See also Bristol, of Bot. 34 , 1920, p. 35.) 
The extent to which such terrestrial Algae are developed is 
probably not generally familiar. Everyone is of course acquainted 
with the ubiquity and abundance of Pleurococcus, but Zygogonium 
ericetomm, Hormidium flaccidum, species of Prasiola, and species of 
Trentepohlia are scarcely less common and the first-named at least 
often covers whole acres of ground 1 . On the other hand I know of 
no records of indubitable Brown or Red Algae as inhabiting equi¬ 
valent terrestrial situations. Where conditions are less favourable to 
the growth of Green Algae it is Cyanophyceae that dominate as 
terrestrial algal forms 2 . 
A large number of these terrestrial Chlorophyceae and Cyano¬ 
phyceae possess a faculty for resisting drought, without any assump¬ 
tion of special resting-stages, that is only paralleled elsewhere among 
Lichens and Mosses 3 . It is certainly not the bulky parenchymatous 
forms at the present day that are best equipped to resist desiccation. 
If Church cites Pelvetia and the Fuci of salt-marshes as instances 
of bulky forms able to maintain themselves in semi-aquatic environ¬ 
ment (p. 21), the numerous terrestrial Trentepohlias and other 
terrestrial forms, such as Prasiola, Hormidium, and Zygogonium, 
show the great capacity of filamentous Green Algae to maintain 
themselves, even in a dry atmosphere. The terrestrial Green Alga is 
indeed highly adapted to its conditions of life and it may have been 
this faculty, apparently possessed otherwise only by the Blue-green 
Algae and some Diatoms, that primarily resulted in the success of 
Green forms in establishing themselves on dry land. 
A leading hypothesis in Church’s theory may be summarised in 
his own words (p. 33): “The whole of the fundamental framework 
1 Recent Antarctic exploration has shown that Prasiola is probably the 
most important green plant in the land-vegetation of these latitudes. 
2 Fritsch, loc. cit. p. 203 et seq. 
3 Cf. West and Starkey, in New Phytol. 14 , 1915, p. 201; Piercy, in Annals 
of Botany, 31 , 1917, p. 527 et seq.; Fritsch, in Annals of Botany (in the press). 
