PAPENFUSS: CLASSIFICATION OF THE ALGAE 151 



1952) which appear as punctae or areolae. The striae of some Peiinales represent indi- 

 vidual areolae or linear series of closely placed small areolae (punctae). With few excep- 

 tions the markings on the two valves are similar. 



In many representatives of the order Pennales one or both the valves possess a com- 

 plex system of slits and canals, the raphe system. Such forms are capable of independent 

 gliding movement, apparently owing to cytoplasmic streaming. 



In colonial forms the cells are connected to one another in various ways by mucilage 

 that is secreted through pores in the valves. 



Depending upon the genus, the cell contains one, two, or many yellow, olive-green, 

 or brown chromatophores. Naked pyrenoidlike bodies are frequently present. The pig- 

 ment complex consists of chlorophyll a. chlorophyll c, carotenes, and xanthophylls. 

 Reserve food is stored as oil or leucosin. 



The usual method of reproduction is by vegetative cell division. The hypotheca of 

 a dividing cell always becomes the epitheca of one of the two daughter cells. In the course 

 of time there is thus in the vast majority of diatoms a considerable diminution in cell 

 size in a population. Restoration of the maximal size characteristic of a species is brought 

 about sooner or later by the production of rejuvenescent cells, called auxospores. In the 

 majority of species investigated auxospores are formed as the result of a sexual process 

 and two cells are usually involved. Meiosis precedes gametogenesis. One or at most two 

 nonflagellated gametes are produced; or in certain Centrales four flagellated male gametes 

 are formed (Stosch, 1951a). During the process of conjugation the gametes may escape 

 from the parent frustules and fuse with those of the other cell. The zygote (auxospore) 

 enlarges and ultimately produces a new frustule of maximal dimensions. Some species 

 are autogamous and various instances of apogamy are on record. 



Various authors have reported the formation of small anteriorly or laterally biflagel- 

 late cells (microspores) by members of the order Centrales. For a long time the function 

 of these cells was unknown. Stosch (1951a, 1951b) and Geitler (1952) recently produced 

 evidence that at least In some instances they are male gametes. 



Endogenous cysts with a wall consisting of two pieces, comparable to those of the 

 Chrysophyceae and Xanthophyceae, have been observed in several members of the order 

 Centrales. 



The Bacillariophyceae are divided into two orders, Centrales and Pennales, largely 

 on the basis of the shape and ornamentation of the siliceous shell. In most Centrales, 

 the valves are circular, angular, or irregular in outline and are radially or otherwise 

 symmetrical with respect to a central point. In the Pennales the valves are isobilateral, 

 medianly zygomorphic, or dorsiventral with at most only two planes of symmetry — one 

 passing through the longitudinal axis, the other through the transverse axis, of the valve. 

 In some members of this order the valves possess only one of these two planes of 

 symmetry. 



Although a classification into two orders on this basis may seem to be artificial, it 

 apparently is quite natural. In addition to the differences in the shape and ornamentation 

 of the valves, the two orders differ in various other characters. Centric diatoms usually 

 have many plastids, always lack a raphe and hence show no movement, produce cysts 

 and may form motile male gametes. Pennate diatoms, on the other hand, usually have 

 only one or two plastids, many possess a raphe and hence show movement, and do not 

 form cysts or motile male gametes. 



History: The first person who described species of diatoms in a precise enough 

 manner to afford their recognition by later workers was 0. F. Miiller. In 1773 

 he described a species of Gomphonema as Vorticella pyraria. Among the species 

 which he described in later years, the most important is the one which he (0. F. 

 Muller, 1783, p. 81, fn. c; 1786, p. 54) named Vihrio paxillifer. Gmelin (1788, 

 p. 3903) established the genus BaciUaria for Miiller's Vibrio 2:)axillifer, and this 

 name was later proposed by Nitzsch (1817), and following him used for a time 

 especially by zoologists, for the entire group of so-called rod animalcules. 



The resemblance of many colonial diatoms to filamentous algae accounts for 



