K.— BOTANY; 185 



system is, however, characteristic of many Ectocarpales (e.g. Ectocarpus) 

 and Nemalionales (e.g. Chantransia), which include the simplest known 

 members of the Phaeophyceae and Rhodophyceae respectively. In fact 

 it appears that this kind of plant-body represents a definite stage in the 

 evolution of various classes of Protophyta, affording another instance of 

 parallelism. But, whereas in the Isokontae it represents the most 

 advanced type of which we have any knowledge, in the two great marine 

 groups it is seen in the simplest of the present-day forms, since no unicellular 

 or palmelloid members of these classes are certainly known to exist. A 

 consideration of the metabolism and reproductive features of the two 

 groups of Seaweeds, however, clearly supports an origin for each of them 

 distinct from that of any of the classes previously mentioned. 



It will be familiar that, of all the holophytic Protophyta, the two classes 

 Phaeophyceae and Rhodophyceae, which are almost confined to the sea, 

 have alone attained to a high degree of morphological and anatomical 

 specialisation, often affording in one feature or another marked instances 

 of parallel with those groups of the Vegetable Kingdom which are now 

 dominant on the land. We owe to Church' 23 a clear statement of these 

 points of parallel and a suggestion that many, if not all, the fundamental 

 features of higher land-plants were already realised in a marine environ- 

 ment before a terrestrial flora was evolved. The totally differing meta- 

 bolism obviously renders impossible, however, any direct derivation of 

 the land-flora from forms belonging to either class of marine Algae. 



It will be generally agreed that we must seek the origin of terrestrial 

 plants in organisms possessing the same plastid-pigments and the same 

 essential metabolism as they do. The only representatives of such 

 forms among Protophyta at the present day are afforded by the numerous 

 Green Algae, the Isokontae. These, however, as has previously been pointed 

 out, stop short at a level of morphological differentiation of the thallus, 

 at which the two marine groups commence. Roughly speaking, too, 

 the stature of the most highly differentiated Isokontae is approximately 

 equivalent to that of the simpler Brown and Red Algae. Yet, in 

 sexual differentiation and specialisation of the reproductive machinery, 

 there is little to choose between these three classes, although the Red 

 Algae have in part developed post-fertilisation complexities peculiar to 

 themselves. 



We are thus confronted with the situation that in the Isokontae we 

 have a class of great morphological diversity in which almost every con- 

 ceivable type of simple plant-body has been realised and is still existent 

 at the present day, but which stops short at a massive parenchymatous 

 construction and forms of large stature. In the Brown and Red Algae, on 

 the other hand, where no simple forms of plant-body are certainly known, 

 plants of large size and possessed of a highly developed parenchymatous 

 soma are abundantly represented. It appears improbable that a class like 

 the Isokontae, showingsuch extreme capacity for morphological elaboration 

 in every direction and for adaptation to very diverse habitats, should have 

 failed to develop further in the direction generally indicated by Phaeophyceae 

 and Rhodophyceae. Moreover, it must be remembered that they possess 



33 Op. cit. 



