152 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



the attention that these organisms received from various early botanists, who 

 often described species under the generic name Conferva. Although as far as 

 known, De Candolle did not especially investigate members of this group, he {in 

 Lamarck and De Candolle, 1805, p. 48) was the first to regard the species previ- 

 ously known by the name Conferva floccuJosa as representative of a distinct genus 

 which he named Diatoma, and thus furnished the name that C. Agardh (1824) 

 adopted for the group (as Diatomeae) and by which it is now commonly known. 

 The most significant contributor to knowledge of this group during this early 

 period was Nitzsch (1817). He gave the first useful illustrations of members of 

 the class and also recognized their prismatic quality, which he considered a ma- 

 jor character of the group. He carefully studied the multiplication of the rod- 

 like forms by longitudinal division and pointed out, among other things, that 

 the individuals did not lose their form after death. Nitzsch divided diatoms into 

 two groups, animal and plant, according as they exhibited movement or not. 

 Until 1832 members of this class were regarded partly as animals (the motile 

 forms) and partly as algae (the nonmotile forms), although several botanists 

 (C. Agardh, 1817, 1824, 1830-1832; Lyngbye, 1819; and others) had no hesi- 

 tation in referring the entire group to the algae. In fact, C. Agardh in 1824 

 established for them the order Diatomeae, one of six which he recognized in the 

 algae. Ehrenberg (1832, and many later publications), to the contrary, re- 

 garded all diatoms as animals, without reservations, placing them in the family 

 of rod animalcules (Bacillaria). 



C. Agardh, Ehrenberg, and others grouped the desmids with the diatoms. 

 Kiitzing (1833b) was the first to recognize clearly the differences between these 

 two groups of organisms, especially as regards the nature of the cell wall. Later, 

 in a comprehensive monograph on the diatoms, Kiitzing (1844) elaborated on 

 his earlier observations on the composition of the shell and pointed out that it is 

 composed of silica. In this monograph Kiitzing also concerned himself with the 

 classification of these organisms, and on the basis of the structure of the frus- 

 tule, recognized a total of nineteen families (including one comprised largely 

 of silicoflagellates). 



Thwaites (1847, 1848) was the first to observe the process of conjugation 

 in diatoms. At first (1847) he did not comprehend the significance of his ob- 

 servations but in 1848 he fully suspected that these phenomena were instances 

 of a sexual process. 



In a monograph on diatoms published in 1853, Eabenhorst corrected cer- 

 tain of Ehrenberg's and Klitzing's errors with respect to the structure of the 

 frustule. In this publication, Eabenhorst considered the diatoms as constitut- 

 ing an autonomous class of algae, which had no equal among living things as 

 regards the sharpness of characters as shown by their peculiar type of shell. 

 Previously, however, Harvey (1836) had regarded the diatoms (including the 

 desmids) as forming one of the four divisions into which he divided the algae. 

 Turpin in 1828 (b) expressed the view that the diatom shell consisted of 

 three pieces, instead of two as had previously been believed, two valves, and 

 a girdle. This view was adhered to until Wallich (1858, 1860b) pointed out 

 that the girdle actually consisted of two connecting bands, one fitting over the 

 other. 



From the point of view of the distribution of these pieces at cell division, 



