K.— BOTANY. 188 



this form has so far been forthcoming. There is no doubt, however, 

 that in the Peridinieae there have again been diverse ' algal develop- 

 ments,' although the main differentiation of the class centres around 

 the motile unicell. 



As an antithesis to classes like the Chrysophyceae and Dinophyceae 

 (as Pascher has styled the whole series of Peridiniean forms) we have 

 the Myxophyceae (Cyanophyceae), where motile types are altogether 

 unknown and all the forms exhibit an algal organisation, progressing 

 from the unicellular through the colonial to filamentous types. Even 

 in this very sharply circumscribed class a considerable degree of parallel 

 with those previously considered can be recognised. Other distinct 

 classes of Protophyta exhibiting holophytic nutrition, but of more re- 

 stricted range and generally showing special development in one direction 

 or another, are the Bacillariales (Diatoms), the Cryptophyceae (including 

 the flagellate Cryptomonadineae and a few little known algal 

 types), the Chloromonadineae, and the Euglenineae. A detailed con- 

 sideration of these is unnecessary, but in all of them one or other organism 

 can be recognised as parallel with types in the classes that have been 

 previously discussed, although none has evolved the branched filamentous 

 habit so far as at present known. As instances of parallelism one may 

 cite the occurrence of the dendroid colony in the Diatom Gomphonema and 

 in Colacium among Euglenineae. In the latter class, too, we have the 

 widespread genus Trachelomonas in which the motile cell is surrounded 

 by a special rigid envelope separated from the cell proper by a space. 

 This encapsuled type is paralleled in the Isokontae t>y Coccomonas and 

 in the Chrysophyceae by Chrysococcus. 



To sum up it seems clear that in all the nine classes mentioned evolution 

 has progressed along similar lines and in many cases has led to the pro- 

 duction of analogous forms of plant-body. Thus, the motile unicellular 

 individual, the motile colony, the palmelloid type, the dendroid colony, 

 the chlorococcoid type, the simple and the branched filament, thesiphoneous 

 type, and others are all to be found in two or more of these classes. In 

 five of them, moreover, the stage of the branched filament has been reached. 



While occasional indications of relationship are to be found (e.g. between 

 Heterokontae, Chrysophyceae, and Bacillariales), 21 they are not very 

 marked, and it is probable that all the nine classes represent as many 

 evolutionary series of uncertain origin. We have in them practically 

 all that is left to us of the early evolution of the holophytic organism. 

 It can scarcely be doubted that there were other phyla which have become 

 extinct, nor is it likely that future research will fail to disclose further series 

 than those at present distinguished. 



The Relation of the Protophyta to the Higher Plants. 



It has been suggested in certain quarters 22 that the simple freshwater 

 Algae are reduced from forms which had a more elaborate parenchymatous 

 soma, being in fact ' starvation-forms ' resulting from a paucity of 

 nutritive salts. It should be stated in the first place that, although 



31 Pascher, Ber. Deuisch. Bot. Qes. xxxix, 1921, p. 236. 

 28 Church, Oxford Bot. Memoirs, No. 3, 1919, pp. 8, 46. 



