PAPENFUSS: CLASSIFICATION OF THE ALGAE 121 



jugatae, Siphoneae, and Confcrvoideae, respectively) remained in force until 

 the beginning of the present century. 



The classification of the unicellular and colonial green algae (other than the 

 desmids, whose relationshij) with the Zygnemataceae is clear) and of the siphon- 

 ous forms has always been fraught with many difficulties. It is the sorting 

 out of these algae that will be considered especially in the following pages. 



In agreement with Kabenhorst, Kirchner (1878) grouped the families Volvo- 

 caceae, Protococcaceae, and Palmellaceae in a common order which he named 

 Protococcoideae. Ilansgirg's (1888a) and Wille's (1890-1891) circumscriptions 

 of the order Protococcoideae were much the same as that of Kirchner, except 

 that Hansgirg merged the Protococcaceae in the Palmellaceae whereas AVille 

 recognized a larger number of families. 



The far-reaching contributions by Klebs (1883, 1892) and L^^ther (1899), 

 which resulted in the removal of the euglenids from the Protococcoideae and the 

 establishment of a separate class Heterokontae for those "green" algae with two 

 unequal flagella, are considered under the Euglenophycophyta and Chrysophy- 

 cophj^ta, respectively. 



Blackman (1900) and Blackman and Tansley (1902) arrived at the signifi- 

 cant conclusion that the order Protococcoideae comprised families representa- 

 tive of three divergent vegetative tendencies which furnished the phylogenetic 

 lines on which the different types could be arranged. (1) Those forms in which 

 the plant body is motile by means of flagella during the vegetative phase. (2) 

 Those in which the plant body is not motile and in which the cells are uninu- 

 cleate and divide during the vegetative phase. (3) Those in which the plant 

 body is nonmotile and in which the cells do not undergo vegetative division but 

 the nucleus divides and the cells consequently become multinucleate. 



As delimited in the arrangement at the end of this section, these three lines 

 are represented by various orders and families as follows. The first group cor- 

 responds to the Volvocales. The second group is represented by the suborder 

 Tetrasporineae of the order Volvocales and the family Pleurococcaceae of the 

 Ulotrichales. The third group is represented by the order Chlorococcales. (In 

 the breaking up of the heterogeneous order Protococcoideae [= Protococcales 

 Kirchner orth. mut. Engler, 1892], the ill-fated designation Protococcales has 

 fortunately been relegated to synonymy. Silva and Starr, 1953, have produced 

 convincing evidence that the type species of Protococcocus C. Agardh is actu- 

 ally a species of Haematococcus rather than of the plant that is commonly known 

 as Protococcus but which probably should be known as Pleurococcus. Further- 

 more, it is now known that the unicellular habit of this genus is a derived rather 

 than a primary condition and that it belongs in the Ulotrichales.) 



The classification of the multinucleate segmented (exclusive of the Ilydro- 

 dictyaceae) and the siphonous Chlorophycophj-ta has also been a matter of much 

 confusion and disagreement. Egerod (1952) has recently given an excellent 

 treatment of the taxonomic history of these algae; a brief review will conse- 

 quently suffice here. 



Greville (1830) established the "order" (family in the modern sense) Siphon- 

 eae to accommodate certain green algae {C odium, Bryopsis, Vaucheria, Botry- 

 dium) that possess a tubular, nonseptate thallus. (He placed Caiderpa in its 

 own "order.") In the course of time a number of additional families were erected, 



