90 COMPARATIVE MORPHOLOGY OF FUNGI 



germ tubes, whereby their contents pass out of the papilla as a ball. 

 Bremia Lactucce causes a rot of various composites, especially lettuce and 

 artichokes. It is divided into several biological forms which may be 

 partially separated biometrically (J. Schweizer. 1919). Peronospora is 

 divided into a large number of smaller morphological and biological 

 species, which are usually specialized on single host genera and often 

 on single host species (Gaumann, 1918a, 1923). P. cannabina causes a 

 disease of hemp, P. Brassicae of cabbage, P. Schleideni of onions, P. 

 Schachtii of sugar beets, P. Spinaciae of spinach. 



Having reached the end of the Oomycetes we will now return to the 

 question of their source. We face the same dilemma as in the Archi- 

 mycetes and the Chytridiales. According to one concept, the Oomycetes 

 are derived from the Algae; thus Bary (1884) considers the Chloro- 

 phyceae, especially Vaucheria; also Tavel (1892) names Vaucheria and 

 related genera as ancestors but connects the Monoblepharidaceae with 

 Oedogonium; Davis (1903) considers them derived from isogamous or 

 slightly heterogamous Chlorophyceae at the stage of development which 

 approximately corresponds with Cladophora and the isogamous Sipho- 

 nales. According to the other concept, held by A. Fischer (1892), 

 Dangeard (1906), Atkinson (1909a) and Scherffel (1925), the roots must 

 be sought in the Chytridiales. 



The fundamental question is whether the structure of the zoospores 

 is of phylogenetic significance or not. Most authors have answered this 

 question negatively; and yet one must admit that in the fungi where the 

 number and insertion of the fiagella have been accurately observed, they 

 afford a systematic character with which other morphological facts 

 entirely agree. Numerous ideas of this type have been recently upset by 

 the fact that they have been based upon incorrect observations; thus 

 Vuillemin (1907, contradicted in 1912) and Lotsy (1907) consider incorrect 

 the observations of Hesse that Pyihium is uniflagellate. Since then, it 

 has been substantiated that he made his observations on abnormal 

 material. Similarly Thaxter has described biflagellate Monoblephari- 

 daceous zoospores which, according to Woronin (1904), belong to a 

 parasite. Even the justification for Myrioblepharis (Thaxter, 1895) 

 has been questioned by Minden (1915). Further the observation of 

 Atkinson that Pythium intermedium may form two uniflagellate zoospores 

 is inconclusive, because the fate of the halved zoospores is unknown and 

 because Butler (1907) could not find this in the same (?) species; further- 

 more Dastur (1913) is inclined to consider abnormal similar phenomena 

 which he observed in Phytophthora parasitica. It may easily be (and fungi 

 offer several examples) that because of mode of life, parasitism etc., there 

 appears a hereditary loss of fiagella, but there is no suggestion that only 

 one of two fiagella is lost, and it is equally improbable that from two lateral 

 fiagella a terminal flagellum could arise, or vice versa. Hence it appears 



