8 INTRODUCTION 



and further observes that in Hantzschia amphioxys, Scoliotropis lalestriata and Acknanthes 

 brevipes, three widely separated forms, the chromatophores are essentially the same. 



In one of the earliest classifications of diatoms, the individual cell received less con- 

 sideration than the nature of the filament or thallus in which many species occur in the 

 first stages of their growth. Those, however, which exist in colonies at first are, sooner or 

 later, broken up into separate frustules, either before or at the time of their maturity or 

 previous to conjugation, while very many species are never seen except in a free state. The 

 union of frustules, therefore, is of secondary importance and the group must be considered 

 as filamentous or unicellular algse. Their relation to other algse is not well determined. 

 Among the Desmidiacece, a family of the order Conjugates, of the class Chlorophycece, the 

 cells are in many forms divided by a constriction into symmetrical halves. The Conjugales 

 are starch forming, with walls of cellulose. In the Diatomacese the starch is replaced by oil 

 globules, while the walls of cellulose are more or less filled with a deposit of silica. The 

 Conjugales, however, reproduce by zygospores and usually contain pyrenoids, as may be 

 seen in the parietal chromatophores of Spirogyra. In the class Heterokontce we have the 

 reserve material in the form of oil, instead of starch, but there are no pyrenoids. To this 

 class belongs the order Confervacece, in which the cells are unicellular or filamentous, and to 

 which all of the Diatomaceae were referred. While, therefore, Diatomaceae have a close 

 affinity to the Desmidiaceae and to the Confervacese, the determination of their origin, one 

 from another, or from a common ancestral type, appears to be a matter of conjecture. 



