K.— BOTANY. 179 



the name Isokontae and revert to the old designation Chlorophyceae 

 for the Green Algae. Under this name, however, there were also originally 

 included the Heterokontae and its use might thus prove misleading. In 

 the following I shall therefore continue to use the designation Isokontae 

 for all true Green Algae. 



Already at the end of the last century practically every conceivable 

 type of simple plant-body was known in the Green Algae, ranging 

 from the motile or motionless unicell, through manifold colonial 

 forms, to a more or less highly elaborated filament. This extremely 

 varied somatic development corresponds to a remarkable range 

 of habitat and goes hand in hand with a great diversity in reproductive 

 processes. There is in fact no other group of simple organisms showing 

 such a wide scope in all these respects. By contrast the Heterokontae, 

 when first distinguished, included only relatively few forms. By 

 degrees, however, many additional members have been discovered, and 

 in the course of this century it has become increasingly apparent that 

 there exists a far-going parallelism between these two classes, Isokontae 

 and Heterokontae, which are so sharply segregated by their metabolism 

 and other features that the vast majority of algal workers have regarded 

 them as quite separate evolutionary lines, in no way related to one another. 



Thus, in each class we have a series of motile unicells, Chlamydomonas 

 and its many allies in the Isokontae, Chloramoeba and Heterochloris 7 in 

 the Heterokontae, while the palmelloid type, with large numbers of cells 

 embedded in a mass of mucilage, is represented respectively by the Tetra- 

 sporales and Chlorosaccus, &c. (Heterocapsales). The motionless, often 

 spherical, unicell — we may speak of it as the chlorococcoid (or protococcoid) 

 type of plant-body — is well represented in both classes, and in part the 

 relevant forms are so similar that, prior to the clear recognition of the 

 Heterokontae, they were classed in the same genus. Thus, many species 

 of Characiopsis were first included in the Isokontan genus Characium, 

 while the type-species of Chlorobotrys was first described as a Chlorococcum* 

 The unbranched and the branched filamentous habits are met with in 

 both classes, while the coenocytic Botrydium is now clearly established 

 as a siphoneous variant of the Heterokontan type analogous to Protosiphon 

 among the Isokontae. 9 On detailed scrutiny it is not difficult to find 

 other points of parallel ; thus, in both classes there are colonial forms 

 with an analogous dendroid construction (Chlorodendron, Mischococcus), 

 while in each the chlorococcoid type is represented by two series of forms, 

 the one reproducing by zoospores and the other (azoosporic) in which 

 motility is completely suppressed. We are indebted to Pascher 10 for first 

 drawing attention to this striking degree of parallel. 



The Heterokontae do not, however, exhibit anything approaching 

 the multiplicity of forms that are seen among the Isokontae, in particular 

 they do not appear to have evolved in the direction of the motile colony 

 which is so well developed in some of the other classes. Also the fila- 

 mentous members are few, and the siphoneous type is as yet only known 



7 Susswasserflora, xi, 1925, p. 23. 



8 Bih. K. Sv. Vet.-Akad. Handl. xxvii, 1901, Afd. 3, No. 4, p. 34. 



Kolkwitz, Ber. Deutsch. Bot. Gas. xliv, 1926, p. 539. 

 10 Hedwigia, liii, 1913, p. 6. 



N 2 



