186 SECTIONAL ADDRESSES. 



the photosynthetic equipment which has evidently proved to be the 

 only successful one on the land, and that practically every group and 

 family of Isokontae has its terrestrial representatives. 



What then, it may be asked, has become of the more highly elaborated 

 members of this class ? It seems to me that there is every reason to 

 suppose that, approximately at the level of morphological differentiation 

 and stature reached by the Isokontae of the present day, the terrestrial 

 habit was adopted in the remote past, that the more highly elaborated 

 Green Alga became a land-plant, the early forms of which are perhaps 

 yet to be disclosed by palaeontological research. The facts of relative 

 development of the three large algal classes just considered appear to 

 indicate that the first land-plants were probably forms of small stature, 

 although not necessarily quite as simple as the most advanced Isokontae 

 known to us at the present day. In this connection it is not without 

 significance that the oogamous members of this class for the most part 

 occupy a peculiarly isolated position, appearing as outliers well in advance 

 of the rest, although for none of them is there to my thinking any possible 

 connection with the higher land-plants. On the little available evidence 

 it seems possible that oogamy may have been undeveloped or in an incipient 

 stage in the first land-plants. 



If one recognises among Phaeophyceae and Rhodophyceae many features 

 of anatomy, life-history, &c, that recall the characteristics of land-plants, 

 I can see in that only a confirmation of the belief that environment has 

 little to do with the broad evolution of the plant-organism and that 

 these features are a natural outcome of the evolutionary trend in the 

 Vegetable Kingdom and not any positive evidence for the view that they 

 must necessarily have originated in a marine environment. The com- 

 parative study of the simpler forms of plant-body in the different classes 

 of Protophyta lends great support to such a concept of a general evolu- 

 tionary trend. Before we adopt the idea of a mythical group of Thalas- 

 siophyta as ancestral to the land-flora, the blind termination of the Iso- 

 kontae must be accounted for. A more intensive investigation of the 

 Chaetophorales, and in particular of the terrestrial Trentepohliacae, than 

 has hitherto been undertaken may well afford data bearing upon the 

 relation of the Isokontae to higher plants. 



It is scarcely possible to touch on the problem of the origin of the 

 latter without some reference to that striking universal phenomenon in 

 the life-history of the land-plant, the two alternating generations. It 

 is just in this respect that the Isokontae are too incompletely known to 

 afford any good points of contact. We know now, thanks to Dr. Knight's 

 researches on Pylaiella, that an homologous alternation exists in this 

 simple filamentous Brown Alga, and it is possible that analogous cases 

 are yet to be discovered in the Chaetophorales among Isokontae. In this 

 connection attention may be drawn to certain observations made by 

 Meyer' 24 on Trentepohlia umbrina which appear to indicate a segregation 

 of sporangia and gametangia on distinct individuals, and other like cases 

 are suspected. These merit a fuller investigation. As a matter of fact, 

 except in a few special instances, the cytological features of the life-cycles 



M Bot. Zeit. lxvii, 1909, Abt. 1, p. 26. 



