348 THALLOPHYTES. 



jelly arranged radially; at the free end the filament runs out into a long hyaline 

 hair, while at the central end is a large heterocyst which gives the whole filament the 

 form of a riding-whip. The filament increases in length by the transverse division of 

 its cells. Reproduction is effected by the cell which lies next to a basal heterocyst 

 becoming thicker, increasing considerably in length and assuming a cylindrical form ; 

 its contents become denser, and invested by a firm membrane. When the whole 

 of the rest of the colony perishes these Resting-spores only remain. They subse- 

 quently germinate, the contents dividing into from 4 to 12 shorter cylindrical pieces, 

 each of which again divides repeatedly, until more than 100 cells are formed which 

 become rounded off and the filament moniliform. During this lengthening the mem- 

 brane of the germinating cell ruptures, the upper end of the filament projects, the 

 lower portion subsequently creeps out of the sheath, and the terminal cells become 

 pointed. The filament, now free, breaks up into several pieces, which become 

 closely packed together into a tuft or ball. Each filament now lengthens at one 

 end into a segmented hair, while the cell at the other end becomes a heterocyst. 

 Such a tuft which springs from a germinating cell forms a young mass of Ri'vularia, 

 the filaments becoming enveloped in jelly. The multiplication of the filaments of 

 a growing mass takes place by a kind of branching ; /. e. one of the lower cells becomes 

 a new heterocyst; the portion of the filament that lies between it and the old 

 heterocyst developes into an independent filament alongside the parent-filament. 



5. The SeytonemesB form branched filaments enclosed in thick gelatinous enve- 

 lopes, which — at all events in their older portions — consist also of several rows of cells. 

 To this family belong Scytonema, Sirosiphon^ &c.^ 



B. The PALMELLACEiE contain pure chlorophyll. The cells live singly or remain 

 in families imbedded in mucilage ; they resemble the Gyanophycese in many ways. Thus 

 Gloeocystis, belonging to this family, has the appearance of a pure-green Glmocapsa; 

 Tetraspora forms Nostoc-VikQ lumps of jelly, but is propagated by zoogonidia; in 

 Occardium the filaments are arranged radially in jelly, as in Ri'vularia. The delicate 

 green growths on damp walls, stems of trees, &c. consist of cells either isolated or 

 grouped into families which are known under the names Protococcus, Palmella, Cysto- 

 coccus, &c. ; Palmella cruenta forming blood-red incrustations. Probably, as has already 

 been said, all these forms are only stages in the development of higher Algae, which 

 attain their further normal development only under favourable conditions of growth. 



FORMS NOT CONTAINING GLOROPHYLL. 



G. The ScHizoMYCETES^ (Fig. 166) live in fluids which contain organic substances 

 (albuminoids) liable to putrefaction, from which they obtain their nutriment, and of the 

 putrefaction of which they are the cause. The greater number consist of extremely 

 small cells without any differentiation into cell-wall and cell-contents, so that in some 

 cases their organic nature can only be determined by indirect methods^. Where 

 they occur, an enormous number of individuals are usually imbedded in a gelatinous 



^ Various forms of the Algse described under A. and B. occur again in the bodies described as 

 gonidia of Lichens. (See Fig. 222.) 



* Cohn, Untersuchungen iiber Eacterien, Beitrage zur Biologic, 1872, Hft, 2. p. 127 ; 1876, Bd. 

 ii. Hft. 2, [Quart. Journ. Micr. Sc. 1873, p. 156; 1877, p. 81 ; 1879, pp. 356-404.] 



^ Since the smaller Schizomycetes, usually called Bacteria, are found also on the slimy surface 

 of living bodies, on wounds, &c., they have recently, from a medical point of view, been regarded as 

 the cause of diseases. A copious literature, generally deficient in even an elementary acquaintance 

 with scientific botany, treats of the Schizomycetes from this point of view ; but it can hardly be 

 doubted that observers have frequently mistaken the mere products of decomposition of organic 

 substances, and every crystalline precipitates of an inorganic character, for Bacteria. 



