PAPENFUSS: CLASSIFICATION OF THE ALGAE . 117 



For a long time all algae exhibiting movement — diatoms, desmids as well as 

 flagellated green, yellow-green and golden-brown forms — were automatically re- 

 garded as animals. Although the zoologists in the course of time relinquished 

 the diatoms and desmids, the flagellated forms and tlieir amoeboid relatives 

 (but frequently not their nonmotile unicellular and multicellular relatives) are 

 still considered as belonging to the artificial phylum Protozoa. 



Among the early biologists who regarded some of the flagellated organisms 

 as algae rather than protozoa are Siebold (1848, 1849), Braun (1851), and Colin 

 (1852). The bulk of the so-called Flagellata, however, for a long time remained 

 the exclusive domain of zoologists. Klebs (1883) was the first to emphasize the 

 plantlike nature of many of these forms and in the ensuing years the group 

 Flagellata received increasing attention from botanists and was included among 

 the algae in treatises on the plant kingdom (Warming, 1890; Engler, 1898; 

 Senn, 1900; and others). Epoch-making contributions that revealed tlie inti- 

 mate relationship of some of these forms to nonflagellated unicellular and multi- 

 cellular or so-called "algal" types were made by Bohlin (1897a, 1897b), Luther 

 (1899), Klebs (1912), and Pascher (1912b, 1914). The history of this change 

 in the outlook of botanists in regard to the systematic position of the flagel- 

 lates is reviewed in the appropriate sections below, especially in that on the 

 Euglenophycophyta. 



The discoveries and growth in knowledge of sex in plants form an extremely 

 interesting chapter in the history of botany. The algae provide particularly 

 favorable material for the study of sex and have been used to advantage in this 

 connection since the middle of the last century (cf. Kniep, 1928). 



Although numerous observations had been made on the reproduction of algae 

 before 1853 and many botanists had come to believe that some algae exliibited 

 a sexual process, the established facts in support of such a view were extremely 

 meager. Some forms appeared in places and under circumstances that neces- 

 sitated the assumption of spontaneous generation. In this way, Meyen (1827) 

 explained the appearance of small algae, known as "Priestley's matter," in stag- 

 nant water and even in closed vessels. Kiitzing (1833a) and others put forth 

 the view that the simplest algae once produced spontaneously could develop 

 according to circumstances into a variety of algae. When the zoospores of an 

 undisputed alga were seen in the process of liberation from the thallus, the 

 phenomenon was interpreted as a changing of the plant into an animal. The 

 remarkable thing is not so much that such views were entertained but that the 

 majority of biologists of the time combined with them a belief in the immut- 

 ability of species. 



The first alga suspected of showing a sexual process was Spirogyva. Hedwig 

 (1798) was of the opinion that the zygospores, which were discovered by 0. F. 

 Miiller in 1782, were formed as the result of a sexual act. Vaucher (1803) also 

 studied conjugation in Spirogyra and related forms but he, like many later 

 botanists, was not fully convinced of the sexual nature of this process since it 

 was difficult to conceive of a sexual process without the criterion of a morpho- 

 logical difference between the sex organs; and furthermore, it was evident that 

 one and the same filament could both deliver and receive substance from a 

 neighboring filament. During the first half of the nineteenth century and for 

 some time afterward (as recently as 1916 and 1926 by West [1916, p. 135] 



