174 
F. E. Fritsch 
and dwarfing of parts whereby the exposure of any large surface or 
the elevation of any part of the thallus into the drier air some little 
way above the surface was obviated. Such dwarfing may be seen 
in the spray zone of larger pieces of water at the present day, in the 
case of Cladophora, Oedogonium, Stigeoclonium, etc. The gradual 
origin of an erect growth from such short tufted filaments, as they 
became accustomed to terrestrial conditions, is not as difficult to 
picture as the persistence of a large erect form. 
It is, however, possible that an erect growth was not the most 
important feature of the transmigrant Alga. In each of the three 
great series of Algae we encounter a similar central type of organisa¬ 
tion, in which the thallus is differentiated into two distinct portions, 
a prostrate attached dorsiventral base, which not uncommonly 
possesses a parenchymatous construction, and an “erect” radial 
portion. This is well exemplified in the species of Stigeoclonium and 
Trentepohlia among the Chaetophorales (Chlorophyceae), in Ecto- 
carpus , Cutleria , etc. among the Phaeophyceae, and in several of 
the Nemalionales among Florideae. In the Seaweeds such an 
organisation appears on the whole to be most marked among the 
simplest members of their respective series, whilst the Chaetophorales, 
the bulk of which are freshwater or terrestrial, are among the most 
advanced members of the Green Algae 1 . It seems probable that this 
type of construction marks a stage in the evolution of the Algae, 
the highest attained by the Green forms of the present day. 
In each of the series just mentioned we find, side by side with 
forms with a practically equal development of the prostrate and 
“erect” portions, others in which the one or the other is more or 
less completely reduced (among Chaetophorales compare Drapar- 
naldia and Protoderma) . In the aquatic Green forms the reproductive 
organs are usually borne on the upright system, but in some of the 
terrestrial Trentepohlias the spherical sporangia, which have been 
shown in several cases to be gametangia, are confined to the creeping 
base 2 , whilst the asexual stalked sporangia, so highly evolved for 
aerial dispersal, are found on the “erect” system. Among Phaeo¬ 
phyceae, in Myrionema vulgare and Cutleria (in the latter on two 
distinct generations), the converse is the case. I have previously 
suggested 3 that the Algae from which land-plants arose possessed 
1 Fritsch, loc. cit. p. 235. 
2 Cf. Heering, in Pascher, Suesswasserfl. 6, 1914, Fig. 173, a; De Wildeman, 
Algues Bnitenzorg, 1900, p. 72; Schmidle, in Engler’s Bot. Jahrb. 30, 1902, 
p. 63, Tab. II, Figs. 8, 10. 
3 Loc. cit. p. 240. 
