1920] Setchell-Gardner: Chlorophyceae 163 



FAMILY 5. DERBESIACEAE (thuret) kjellm. 



Thallus of erect, simple or sparingly' branched coeiiocytic filaments, 

 arising from more slender creeping filaments which are attached to the 

 substratiini by sliort, branched, rhizoid-like holdfasts; chromatophores 

 small, discoid, oval or elliptical in sliape without or with one or two 

 pyrenoids; non-sexual reproduction by zoospores provided with a 

 crown, or circlet, of cilia, produced in special, lateral, globose to pyri- 

 form zoosporangia, sexual reproduction unknown. 



Derhesiaccae Kjellman, Algae Arctic Sea, 1883, p. 316. Derbesiees 

 Thuret, Rech. sur les zoospores des algues, etc., 1850, p. 231 (p. 22 

 Repr.). Dcrhesieae Thuret, in Le Jolis, Liste des alg. mar. de Cher- 

 bourg, 1863, p. 14. 



The family of the Derbesiaceae differs from all others of the 

 Isokontae in the possession of zoospores with a circlet of cilia similar 

 to those of the Stephanokontae. In spite of this seemingl}^ funda- 

 mental difference, all writers have placed it among the Isokontae 

 rather than among the Stephanokontae. Davis, in his paper on "Spore 

 formation in Derhesia" (1908), has followed out the nuclear behavior 

 during zoospore formation and its relation to the development of a 

 blepharopla-st as well as the resulting circle of cilia. Unfortunately 

 the development of the zoospore and of the circlet of cilia is not as 

 yet known for Oedogonium or any other of the Stephanokontae. 

 Davis {loc. cit., p. 16) states that the zoovspores of Dcrbesiu and of 

 Oedogonium- are of similar structure and ventures to predict that 

 those of Oedogonium will be found to develop a blepharoplast closely 

 similar to that of Derbesia. Nevertheless, he warns against the danger 

 of classifying the algae on the basis of the structure of zoospores and 

 gametes and expresses as his idea that Derbesia should not be removed 

 from the Siphonales. Davis also expresses the opinion that no one 

 will be bold enough to suggest a relationship between Derbesia and 

 Oedogonium on account of the resemblances of the zoospores. 



It seems to us, however, that in the Stephanokontae, there exists 

 a peculiar tyjie which may be as early, or as primitive, as any of 

 those under the Isokontae. Possibly there may have existed many 

 forms of Stephanokontae, now lost, or possibly not yet discovered. We 

 may assume then that as the Isokontae have advanced along several 

 lines from multicellular to septate and then to unseptate coenoeytic 

 condition, the Stephanokontae may have done the same. It seems 

 to us neither impossible, nor wholly* inconsistent with what we find 



