PROTOCOCCOIDEJE 419 



Reinsch unites Polyedrium with three other genera to make up a sepa- 

 rate family, Polyedriace/e, belonging to Palmellacese. 



Richter connects Gloeocystis with the Chroococcacese, and hence 

 genetically with higher forms of algae. Cienkawski regards Pleuro- 

 coccus, Glceocystis, and probably other genera of Protococcaceae, as 

 resting conditions of Chlamydomonas, or of similar organisrns classed 

 among the Coenobie» which multiply by conjugation. Under suitable 

 conditions he states that they can all be made to produce biciliated 

 zoospores with two contracting vacuoles and a nucleus. The part taken 

 by some Protococcaceae in the development of lichens has already been 

 discussed on p. 318. 



LiTEKATURE. 



Cohn — (Protococcus) Nov. Act. Akad. Cses. Leop. -Carol., xxii., 1850, p. 605 (see 



Ray Soc, Bot. and Phys. Mem., 1853, p. 515). 

 Cienkowski — Bot. Zeit., 1865, p. 21. 



Kostafinski — (Haematococcus) Mem. Soc. Sc. Nat. Cherbourg, 1875, P- '42- 

 Lagerheim — Oefv. Svensk. Vetensk. Akad. Forh., Stockholm, 1882, p. 47 ; and 



1883, p. 37 (Bot. Centralbl., xii., 1882, p. 33). 

 ■Richter— Hedwigia, 1880, pp. 154, 169, 191 ; 1884, p. 65 ; and 1886, p. 249. 

 Dangeard — (Chlamydococcus) Ann. Sc. Nat., vii., 1888, p. 105. 

 Reinsch —{Polyedrium) Notarisia, 1888, p. 493. 



Class XXVI.— Diatomaceae. 



The family of Diatoms — called by the older writers Bacillariaceae — 

 includes a very large number of genera and species, all microscopic, 

 some of them extremely abundant in running, stagnant (but not putrid), 

 and salt water. The individuals are strictly unicellular, and are either 

 free-swimming and isolated, or attached to one another in a linear series 

 or in zigzag chains, adhering to one another by means of small annular 

 cushions, or fixed to some solid object by a simple or compound 

 gelatinous stalk. They are, with very few exceptions, characterised 

 by the presence in the cell-wall of a deposit of silica, by which it becomes 

 converted into a hard but thin and perfectly transparent shell ; and this 

 is always invested in a thin gelatinous envelope. Some species are 

 closely adherent to submerged plants by the whole of one side ; in 

 other cases whole colonies are enclosed in a common gelatinous en- 

 velope, which assumes the form of a simple or compound tube, flattened 

 plate, or globular mass. This is especially the case with the marine 

 species. 



Each individual or frustule consists of two more or less symmetrical 



