178 SECTIONAL ADDRESSES. 



guished, and the same course was adopted by G. S. "West in the ' British 

 Freshwater Algae ' published in 1904 and by most other contemporary 

 authors. Many of these followed Bohlin in regarding the Oedogoniales 

 as a separate class, the Stephanokontae, while the designation 

 Akontae adopted for the Conjugatae by Blackman and Tansley implied, 

 though not quite definitely maintained, a distinct origin also for this 

 group. This practice has been followed by many subsequent writers, 

 although abandoned by Oltmanns in the most recent edition of his great 

 work. He, however, in common with many other authorities, never- 

 theless segregates the Conjugatae from the remainder of the Green Algae. 

 There can be no doubt that any separation of these two groups from the 

 bulk of the Isokontae obscures affinities and cannot reasonably be defended, 

 although it is true that both Oedogoniales and Conjugatae have developed 

 along very specialised lines. 



It is well to realise that the characteristics separating the Green and 

 Yellow-green Algae, like those distinguishing the other great classes of 

 pigmented Protophyta to be mentioned later, are essentially physiological, 

 depending on the colouring matters present in the plastids and the types 

 of metabolism associated with them, as indicated by the nature of the 

 substances stored during photosynthesis. That these diverse classes are 

 also in general characterised by other features, such as the number and 

 arrangement of the cilia in the motile stages, the chemical nature and 

 structure of the cellular envelopes, and sometimes by special peculiarities 

 of the reproductive cells indicates that the physiological distinctions 

 are fundamental and that they go hand in hand with other characters. 

 In separating the Oedogoniales and Conjugatae from the Isokontae we 

 are, however, carrying out a tour de force, since in the pigmentation 

 of their chloroplasts, in the possession of pyrenoids with a ' starch-sheath,' 

 in the storage of starch, and in the chemical nature of their cell-walls 

 these two groups are altogether Isokontan. Nor do they stand more 

 isolated from the bulk of the Isokontae than do many other recognised 

 members of this class, such as the Coleochaetaceae and Vaucheriaceae. 

 As regards the fringe of cilia of the Oedogoniaceous swarmer, which is 

 supposed to have been a feature of the flagellate ancestry of the Stephano- 

 kontae, cilial numbers other than the usual 2 or 4 are not unknown among 

 the motile Volvocales, a matter to which I shall return later. One pecu- 

 liarity of the Conjugatae, viz. the absence of all motile stages, is in no 

 way confined to them, being found for instance in a large number of 

 Chlorococcales, 5 whilst it is not impossible, as Blackman and Tansley 

 first pointed out, 6 to relate the conjugation-process to the sexual fusion 

 found in some species of Chlamydomonas (e.g. C. monadina) in which the 

 gametes are provided with cell-walls. I think there can be no doubt that 

 Isokontae, Stephanokontae, and Akontae all constitute members of the 

 same phylum, however much they may have diverged from one another, 

 and any attempt to separate them must obscure the essence of the present- 

 day concept of the main lines of algal evolution. 



It has been suggested to me by various of my colleagues that, having 

 rejected Stephanokontae and Akontae as separate classes, I should abandon 



5 Protococcalea of other authorities. 6 Loc. cit. p. 168. 



