Algal Ancestry of the Higher Plants. 24Q 
(1) In all the more advanced groups of the Algae, the thallus 
exhibits frequent differentiation into a creeping base and an upright 
system. 
(2) Among the Chaetophorales, which plainly show such a 
differentiation of the thallus, there are evidences , of unlimited 
potentialities; we also find in this group (a) a whole series of 
terrestrial forms, (b) the only member of the Isokontae with a distinct 
main axis bearing laterals which carry on assimilation and 
reproduction and (c) in the species of Trentepohlia, forms showing 
relegation of the sexual organs to the base and of asexual organs to 
the upright system. 
(3) The available evidence is regarded as pointing to the 
Isokontae for the ancestry of the higher plants, and for the reasons 
mentioned under (2) this ancestry is thought to lie among forms 
resembling the Chaetophorales. 
(4) The Pteridophyta are supposed to have arisen from such 
forms by the gradual divergence of two generations, the sexual 
derived from the creeping base, the asexual from the upright 
system. The Bryophyta are supposed to have arisen from forms 
resembling Coleochcete by gradual elaboration of the zygote. 
(5) Cutleria, in a side-line of evolution, fully illustrates the 
way in which two generations can be derived from the type of 
thallus common in all the main groups of the Algae, after the manner 
postulated for the case of the Pteridophyta. 
(6) The cases of alternation among the Alga may be 
distinguished as follows :— 
(a) The two generations arise from different parts of the 
ancestral sporo- and gametogenetic thallus ( pseudo-homologous 
alternation ):— 
(1) The gametophyte is prostrate, the sporophyte erect 
(Pteridophyta, possible cases to be found among the 
Chaetophorales). 
(2) The gametophyte is erect, the sporophyte prostrate 
[Cutleria). 
(b) The two generations arise from the same part of the 
ancestral thallus, the other portion aborting [strictly homologous 
alternation ):— 
(1) The two generations arise from the upright system 
[Dictyota). 
(2) The two generations arise from the prostrate system 
[Zanardinia ?). 
