138 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



tain species of a genus are colorless. These forms, and also some of the pigmented 

 species, ingest solid food or are saprophytic. 



The majority of the species are uninucleate; a few are multinucleate. "Where present, 

 the cell wall is composed of pectic compounds, and it frequently consists of two equal or 

 unequal overlapping pieces. 



Multiplication is by cell division, akinetes, aplanospores, or zoospores. Cysts with 

 a silicified wall composed of two pieces have been observed in a number of species (cf. 

 Pascher, 1937, pp. 71-78). Sexual reproduction has been observed with certainty (cf. 

 Pascher, 1937, pp. 150-154) only in Trilionema (Scherffel, 1901, p. 149), Botrydium (Rosen- 

 berg, 1930; Moewus, 1940) and YaucUeria. Vaucheria is oogamous, Trihonema is isogamous, 

 and Botrydium is isogamous or anisogamous (Moewus, 1940). 



History: Despite the fact that at least one member of this class has been known 

 since the time of Linnaeus (1753), who described Ulva granulata {^^^ Botry- 

 dium granul-atum) , the bulk of our knowledge of the group has been acquired 

 in the course of the present century, mainly through the efforts of Pascher. The 

 distinguishing characteristics of the class remained unrecognized until the lat- 

 ter part of the past century. Owing to their greenish color, the few species 

 which had become known previous to 1899 were regarded as Chlorophyceae. 



Alexander Braun in 1855 (p. 49) recognized certain features of correspond- 

 ence between Opliiocytiiim, Sciadium, and Tribonema; but it was Borzi who in 

 1889 (see also 1895, p. 199) first convincingly pointed to the alliance of sev- 

 eral genera which had been placed in widely separated families of the Chloro- 

 phyceae. He erected an order Confervales for these algae and credited it with 

 the three families Sciadiaceae, Confervaceae, and Botrydiaceae. In his work 

 of 1895 Borzi considered the Confervales as comprising nine genera, all of which 

 are still regarded as belonging to the Xanthophyceae. These forms were brought 

 together by Borzi mainly on the basis of three characters: (1) they possessed 

 discoid, yellowish-green plastids; (2) they did not store starch; and (3) their 

 zoospores had only one flagellum ( as he believed ) . 



Some j^ears later, Bohlin (1897a) published a significant study of certain 

 cytological characters of members of the Confervales, pointing out that the two 

 overlapping pieces that form the lateral wall of the cells have a layered structure, 

 that the wall is not composed of cellulose but of a pectic acid derivative, that 

 the plastids contain a preponderance of yellow pigments and that the storage 

 product is not starch (as Borzi had already established) but a fatty substance. 



In this paper Bohlin also described a remarkable amoeboid flagellate {ChJor- 

 amoeha) which recalled the zoospores of Conferva sensu Lagerheim (^ Tribo- 

 nema). He regarded it as the progenitor of the genera comprising the Confer- 

 vales. In a footnote (p. 48) Bohlin remarked that Chloramoeha at times possessed 

 two flagella — one much shorter than the other. In a later paper Bohlin (1897b) 

 described Chloramoeha in some detail. It was found that if the cell lay in a cer- 

 tain position, two flagella — one much shorter than the other — could be observed 

 in the majority of instances, that the cells possess two to six discoid plastids of 

 a yellow-green color, and that the assimilatory product is stored as oil. 



Two years later, Luther (1899) described another remarkable genus {Chloro- 

 saccus) belonging to this complex. This genus was palmelloid in habit, its zoo- 

 spores were provided with two flagella of unequal length, and the cells contained 

 several yellow-green, discoid plastids. In connection with his work on Chloro- 

 saccus Luther also investigated the zoospores of Trihonema and Botrydiopsis 



