Phylum Phaeophyta [71 



Class Bacillariaceae Engler and Prantl Nat. Pflanzenfam. II Teil; 1 (1889). 



Subdivision and class Bacillariales Engler Syllab. 6 (1892). 



Hauptclasse Diatomeae Haeckel Syst. Phylog. 1: 90 (1894). 



Subclass Bacillariales Engler in Engler and Prantl Nat. Pflanzenfam. Teil I, Abt. 

 la: V (1900). 



Class Bacillarieae Wettstein Handb. syst. Bot. 1: 74 (1901). 



Class Bacillarioideae Bessey in Univ. Nebraska Studies 7: 283 (1907). 



Class Diatomeae Schaffner in Ohio Naturalist 9: 447 (1909). 



Abteilung Bacillariophyta Engler. 



Ahteilung (of Stamm Chrysophyta) Diatomeae Pascher in Beih. bot. Centralbl. 

 48, Abt. 2: 324 (1931). 



Class Bacillariophyceae Auctt. 



Unicellular (occasionally filamentous or colonial) organisms without flagella in 

 the vegetative condition, each cell with one, two, or more plastids, brown, varying to 

 yellow or exceptionally to bluish or colorless, and bearing a siliceous shell of two 

 parts. Globules of oil and granules of something called volutin (the "red granules 

 of Biitschli," apparently protein) are present. Other granules in some examples are 

 said to be of leucosin. 



These organisms, the diatoms, are very common. There are some 5300 species. 

 Microscopic examination of the bottoms of fresh water ponds reveals usually more of 

 diatoms than of any other kind of organisms. Diatoms are frequent prey of many kinds 

 of predators, from amoebas to whales. In using fish-liver oils as a source of vitamin 

 D, man adds himself to a long chain of predators of which it is believed that diatoms 

 are the usual ultimate prey. 



The shells of diatoms are not subject to decay. In certain places which were in 

 the geologic past arms of the sea, there are enormous deposits of diatom shells in 

 the form of a white earth. The oldest deposits are of the Cretaceous age. Thus it ap- 

 pears that diatoms are a modern offshoot, no more ancient than the flowering plants. 

 Diatomaceous earth is mined for various uses. It is an effective insulating material, 

 and was the inert material first used in connection with nitroglycerine in the manu- 

 facture of dynamite. 



The two parts of the shell of a diatom are called valves. They fit one over the 

 other "like the parts of a pill box" (ZoBell, 1941, objects to this traditional simile, 

 on the ground that in current language a pillbox is a concrete structure with loop- 

 holes). The shells consist basically of something of the nature of pectin heavily im- 

 pregnated with silica and characteristically sculptured. The cells appear markedly 

 different in different aspects: the aspect which is in effect top or bottom view is 

 called valve view, and that which is in effect side view is called girdle view. When a 

 cell divides, each of the daughter cells receives one of the valves and generates an 

 additional valve fitting within it. Diatoms in culture undergo a gradual diminution 

 in size; there is an old hypothesis that this is caused by the fact that one of each pair 

 of sister cells receives a slighly smaller valve than the other. 



Lauterborn (1896) described mitosis in Surirella and other diatoms. He found a 

 centrosome, with radiating strands, near the nucleus. At the beginning of mitosis, 

 the centrosome generates a disk-shaped structure which enters the nucleus and grows 

 in such fashion as to become a cylinder extending through it. The cylinder is recog- 

 nizably a spindle, but the chromosomes, instead of appearing within it, form a ring- 

 shaped mass about its middle and divide into two ring-shaped masses which move 

 along it to its extremities. The nuclear membrane ceases to be recognizable early in 



