OF THE DESillDIE^. 23 



tion, and on the other to the Palmelleae, by the usually complete transverse 

 division, and by the presence of a gelatinous investment. Indeed the relation 

 to the latter is so intimate, that it is difficult to say to which family some 

 genera belong. . . . Some species of Scenedesmus may be allowed to have an 

 almost eqnal claim to rank with either." Again, they are related to the 

 Diatomese by similarity in the reproductive process. 



In Ehrenberg's system of Polygastria, the Closteria were placed together 

 as a distinct family, imder the name of Closterina, whilst aU the other genera 

 of Desmidieae were ranged as a section of the Bacillaria. This separation, 

 based as it was upon presumed sti-uctural peculiarities, is no longer accepted 

 by microscopists, who cojoin Closterium with the several genera included in 

 Ehrenberg's section Desmidiaeea in one group — the Desmidieae. 



The division of this family proposed by Mr. Ralfs is made according as — 

 1. The plant forms an elongated jointed filament (by incomplete division of 

 its cells) ; or — 2. The frond is simjile, from complete transverse division, and 

 distinctly constricted at the junction of the segments, which are seldom longer 

 than broad ; sporangia spinous or tuberculated — rarely, if ever, smooth ; or — 

 3. The fi^ond is simple, as above, generally much elongated, never spinous, 

 frequently not constricted at the centre; sporangia smooth; or — 4. Cells 

 elongated, entire, fasciculated ; or — 5. The fi'ond composed of few cells, de- 

 finite in number, and not forming a filament. 



This last section is so exceptional in general characters, and especially in 

 the mode of reproduction, that Braun detaches it from the Desmidieae and 

 associates it with the Palmelleae. In this plan we coincide, and have there- 

 fore treated separately this last section of Mr. Kalfe, comprehending Pedias- 

 trmn and Scenedesmus. 



Kiitzing {Species Algarum) includes the Desmidieae in his subclass Mala- 

 coPHYCE^, suborder Chamaephyceae. 



Mr. Ealfs enumerated 20 genera ; viz. — In Sect. 1, Hyalotheca, Didymo- 

 jirium, Desmidium, Aptogonium, Splicerozosma. Sect. 2, Micrasterias, Eiuxs- 

 trum, CosmaHum, Xanthidium, Arthrodesmus, Stcmrastrum, Didymocladon. 

 Sect. 3, Tetmemorus, Penium, Docidium, Clostermm, Spirotcenia. Sect. 4, 

 Aiikistrodesmus {Kliaphidiimi). Sect. 5, Pediastrum, Scenedesmus. 



When compiling his systematic work, Kiitzing appears not to have seen 

 Mr. Ralfs's monograph, but only his detached papers in the Magazines, and 

 consequently was imable to compare the genera established by the English 

 author ^ith those described by himself. The consequence is that Kiitzing 

 describes several genera not admitted by Mr. Ealfs, who has otherwise dis- 

 posed their representative species, disallowing the supposed distinctive generic 

 character. Nevertheless it seems desirable to enumerate the additional 

 genera of Kiitzing, since several are new (unnoticed by our English authority), 

 and derived from the papers of Ehrenberg or of other obsen^ers, or from his 

 own researches. Those instituted by Ehrenberg were introduced in our last 

 edition. 



The additional genera are: — Trochiscia (K.), Tetraedron (K.), Pifhiscus 

 (K.), Stauroceros (K.), Polysolenia (E.), Microtheca (E.), Polyedrum (jS'ageh), 

 Zygoxantliium (E.), Phycastrum (K.), Asteroxanthium (K.), StepTianoxcni- 

 thium (K.), Grammatonema (Agardh), Bamhushm (K.), Isthmosira (K.), 

 Spondyhsum (Brebisson), Eucampia (E.), Geminella (Turpin), Mo-nactinus 

 (Corda), Staurogonia (K.), Sphoirastrum (Meyen), Sorastrum (K.), Coelas- 

 trmn (Nageli), Bliaplddium (K.), Oocardium (NiigeH). 



The value of the several genera instituted and their characteristics form 

 the subject of the systematic historj^ of the Desmidieae by Mr. Ralfs in the 

 subsequent portion of this treatise. 



