OF THE DIAT0MEJ2. 31 



characters of a complete cell ; that, iinlike the Desmidieae, they propagate by 

 gonidia; have but one instead of two or more equally-sized starch-grains 

 and a central nucleus ; and that their fronds are not distinguishable into two 

 symmetrical halves. '' The Desmidiese are e\idently multicellular, or pseudo- 

 unicellular, from separation of their cells, whilst (says Braun) Pediastrum 

 is a true unicellular Alga rendered pseudo-multicellular by the cohesion of the 

 cells. The aggregation of cells in Desmidieae is always uniserial, filiform or 

 concatenate ; the fronds of PediastriLm are grouped on a plane of a disc-form 

 or frondose character." 



Braim next traces the affinities of Pediastrum, and remarks that, although 

 it resembles Hydrodictyon in the construction of its fronds (coenobia) by the 

 connexion of motile gonidia, yet since in Hydrodictyon the gonidia are 

 simultaneous, and in Pediastrum successional, it is rather an analogy than 

 an affinity which exists between these two genera. However, he admits the 

 correctness of the association of Pediastrum with the genera Nageh indicated, 

 viz. with Sorastnon, Coelastrum, Scenedesmus, and probably Splicer odesmus, 

 and would add to their number the genus Staurogonia (Kiitz.), the Crucigenia 

 of Morren {Ann. des Sciences Nat. 1830, p. 404). All these genera agree 

 ■with Pediastrum in the successional formation of gonidia, yet differ from it 

 in other particulars except in the construction of the frond from motionless 

 gonidia. Among other genera, Polyedrum may be likened to Pediastrum in 

 the form of its cells, but its propagation is unknown ; lastly, Characium and 

 Cystococcus agree with Pediastrum in the successional genesis and activity of 

 their gonidia. 



To this elucidation of the affinities of Pediastrum we have to add the 

 observation of Cohn (EntwicJc. d. miJcr. Alg.), of the analogy or affinity in 

 general stnictiu-e between this genus and Goyiium. 



The di-^ision of Pediastrum into tribes or subgenera, as proposed by Braun, 

 and the distinction of species of the Pediastreae in general, will receive due 

 consideration in our systematic account of the family. 



II.— OF THE FAMILY DIATOMEJE OR DIATOMACEJE. 

 (Plates ly. to XYII.) 



General and External Charactees of Diatome^. — Testules or Frus^ 

 tides. — Figure : free, concatenated, and fixed Forms. — VaHeties of Filaments 

 and of Pedicels. — Aggregated Frustules. — The Diatomeae, Diatomaeeae, or 

 CymbeUea?, are unicellular organisms composed of two opposite plates or valves, 

 generally convex, and of an interposed connecting thii'd segment, forming 

 together a miniatui^e box of a silicious nature, enclosing a soft organic matter, 

 rarely green, but usually yelloT\ish or orange-brown in colour. They inhabit 

 either fresh, salt, or brackish water. 



They were reckoned by Ehi'enberg among the BaciUaria, and have in con- 

 sequence been sometimes described as silicious Bacillaria. 



Each individual Diatom enclosed in its silicious envelope is spoken of as a 

 fi'ustule, testule, frond, or bacillum, and in general phraseology as a cell. 

 The first term is that now most in use, whilst " testule " and " baciUum " 

 are words rarely employed, except in the works of Ehrenberg and of his im- 

 mediate disciples. 



A rectangular or prismatic figure most widely obtains in this family ; and 

 the angles of jimction of the valves are as a rule acute. Deeply notched 

 fronds, like those in Desmidieae, e. g. Micrasterias and Euastrum, do not occur ; 

 and the production of spines and tubercles on the valves, so common in that 

 family, is rare among the Diatomeae. 



