180 SECTIONAL ADDRESSES. 



to be represented by Botrydium. The less vigorous development of tbe 

 Heterokontae, which is thus manifest, accords with the fact that only 

 a few of the more specialised members of the class exhibit sexual repro- 

 duction and that this has not passed beyond the phase of isogamy. The 

 oogamous Vaucheriaceae, at one time referred by some to the Hetero- 

 kontae, are now by practically common consent regarded as outlying 

 members of the Siphonales among the Green Algae. 



The few ciliated members of Heterokontae, that are at present known, 

 without exception show ' flagellate ' characteristics ; that is to say, they 

 are devoid of a cell-wall, their plasma-membrane (periplast) is more or 

 less rigid but usually admits of some change of shape, multiplication 

 is effected by longitudinal division, the protoplast readily encysts, and 

 sexual reproduction is not known to occur. Some of the palmelloid 

 members (e.g. Chlorosaccus), possibly all, also show these features. The 

 many motile and palmelloid types among the Isokontae are, on the other 

 hand, for the most part on a higher plane of organisation and reproduction, 

 being true Algae provided with a firm cell-wall and usually exhibiting 

 sexuality. When, however, the parallelism between the two classes is 

 recognised, the distinction between flagellate and algal organisation loses 

 force, and it is realised that the assumption of ' algal ' characteristics 

 has taken place at an earlier stage in the evolution of the one and at a 

 later stage in that of the other. 



These conclusions, however, do not apply only to Isokontae and Hetero- 

 kontae. It is now clear that, in all the classes of pigmented Protophyta, 

 an analogous evolutionary sequence has been followed, but that the 

 features associated with what may be called ' algal organisation ' have 

 appeared, if at all, at different points in the sequence in the diverse classes. 

 It is no longer feasible to separate the Algae from the holophytic 

 Flagellata as distinct groups of Protophyta. There is reason to believe 

 that every series of holophytic Flagellates could potentially have 

 acquired algal characteristics, although on the present evidence some 

 have failed to do so. 



These points are well illustrated by a consideration of Pascher's Chry- 

 sophyceae which, until relatively recent times, were only known to include 

 a wealth of flagellate types, the Chrysomonadales, whose members on 

 the whole favour pure waters and seem to attain a maximum development 

 in the cold streams and pools of mountainous tracts. They are not, however, 

 without their marine representatives (Coccolithophoridae) and also appear 

 to play a conspicuous part in certain kinds of salt-marsh. 11 



The Chrysomonadales are distinguished by a golden-yellow pigmen- 

 tation of their plastids due to the presence of various accessory pigments, 

 and by storage of the photosynthetic products in the form of oil and of 

 usually rounded lumps of a highly refractive substance known as leucosin, 

 the chemical composition of which is unknown ; endogenously formed 

 silicified cysts with a very distinctive structure 12 are also peculiar to this 

 class. The chromatophores are parietal, relatively large, and generally 

 only one or two in number. In the numerous motile individuals the 

 cilia are always borne at the front end, but three distinct series can be 



11 Conrad, Archiv f. Protistenk. lvi, 1926, p. 167. 



12 Scherffel, Archiv f. Protistenk. xxii, 1911, p. 334. 



