INTRODUCTION 7 



plant, the impossibility of culture, the fallacies of microscopical vision and the chaotic 

 condition of Algology itself to-day." 



The words of Agardh, written in 1824, are almost as true to-day. The lack of 

 authentic specimens, which we hope will be remedied in time by the collections of the 

 Smithsonian Institute, numerous incorrectly labelled slides in amateur collections/ the 

 imperfections of figures copied and recopied, without regard to relative size or correct 

 references, and the confusion in the attempts to harmonize different descriptions, deter 

 the student at the outset. The remaining difficulties mentioned by Agardh add, how- 

 ever, to the remarkable interest these forms have always had, since no increase in 

 optical perfection of the microscope serves to lessen the mystery of their structure 

 and mode of growth. 



CLASSIFICATION 



The few species of diatoms first discovered were included by Lyngbye, Dillwyn, and 

 others in the genus Conferva. In 1824, the species, increased to forty-eight, were separated 

 by Agardh into eight genera distinguished partly by their mode of growth. But little change 

 was made until Heiberg, in 1863, advocated the division into symmetrical and asymmetri- 

 cal forms. Without entering upon a general review of the later classifications, including 

 Pfitzer's and Petit's divisions according to the number and location of the chromatophores, 

 or the arrangement of Prof. H. L. Smith, because of the presence or absence of a raphe, or 

 that of Mereschkowsky into motile and immotile forms, the modification of all of these 

 methods by Schuett is here adopted, varied in accordance with certain monographs which 

 appear to offer advantage. 



It is customary, especially among writers who are familiar with other classes of plants, 

 to decry any classification of diatoms according to the markings of their siliceous envelopes. 

 As, however, one of the chief distinctions of the class is the possession of a more or less 

 siliceous and indestructible frustule, and as the cell and its contents are never seen except 

 within the valves, their variety forms the only available method of identification. The cell 

 contents, owing to the difficulty of observing their living condition, their continued change, 

 their lack of distinct variation and their entire absence in fossil forms, render their con- 

 sideration as a complete method of classification an impossibility. If, however, the cell 

 contents can be brought into relation with the markings of their siliceous envelope, it will 

 be a consummation for which the future student of these complicated forms ought to be 

 grateful. That this result is one to be expected may be inferred from the fact that the 

 arrangement of protoplasmic masses in the interior of the cell is coincident in some cases 

 with markings on the valve, and the character of the endochrome is assuming a certain 

 value in accentuating the difference between such forms as Pkurosigma and Gyrosigma, or 

 in the resemblance between Hantzschia and Nitzschia, or between Surirella and Campy- 

 lodiscus. Mereschkowsky, however, states that it is necessary to be careful in "establish- 

 ing the relationship between diatoms based on the resemblance of their chromatophores," 



