82 



BOTANY 



In the Scytonemataceae (Fig. 56, E) the filaments branch, and in 

 the Rivulariaceae (Fig. 58), the filaments are much attenuated and 

 have a single basal heterocyst. 



Cell-division. The cells multiply by simple cell-division, which 

 is accomplished by the formation of a ring-shaped wall at the 

 equator of the cell, which grows inward and gradually cuts the 

 protoplast in two. The central-body, apparently, is quite passive, 

 and takes no active part in the cell-division. In one family, Chamae- 

 siphonaceae, internal cell-division has been observed, resulting in 

 the formation of numerous " conidia " (Fig. 55, C). 



E 

 C 



B 



FIG. 56. A, mass of Oscillatoria sp., showing the radiating arrangement of the 

 filaments (X'2). B, & single filament, showing the central-body in each cell 

 (X500). C, three cells included in a fragment of the sheath. D, Nodularia 

 major ; h, heterocyst. E, Tolypothrix sp. ; h, heterocysts. 



Reproduction 



In the Chroococcacese (Fig. 55, A) there is no distinction between vegetative 

 and reproductive cells, each cell-division resulting in the formation of two 

 individuals. In the filamentous forms there is usually a breaking up of the 

 filament into lengths Hormogonia which are usually motile, and creep out 

 of the sheath or gelatinous envelope in which they are enclosed. In case 

 heterocysts are present, they become detached, leaving the portions of the fila- 

 ment lying between them, as free hormogonia, which then separate to form the 

 beginning of a new plant or colony. It is in this condition that they usually 

 infect the plants with which they may be associated. 



Resting-spores, or arthrospores, occur in many species. These are formed 

 by a simple enlargement of a vegetative cell, or occasionally, e.g. Rivularia, 

 apparently by a fusion of two or more into one large cell. The contents of the 

 arthrospore become very dense by the accumulation of reserve-food, and a thick 

 wall is developed about it. The spores may be formed from almost any cell in 

 Nostoc and Anabsena, but in certain genera like Cylindrospermum and Rivularia 

 they occupy a definite position in the filament. 



