CYANOPHYCE^ 429 



several species of Oscillaria (Bosc.) and Tolypothrix (Ktz.). The 

 hormogones consist of a number of cells, often considerable, but usually 

 variable in the same species. Only in a few species is the number 

 constant, and then usually two, three, or four. Their contents are 

 generally of a yellowish -green colour. According to Borzi, the hormo- 

 gones of the Nostochineae are of two kinds, straight and spiral. The 

 latter kind occur only in the Oscillariacese, and are confined to the 

 terrestrial species of Oscillaria, Microcoleus (Desm.), and Spirulina 

 (Lk.). They are invested in a thin gelatinous sheath, and their movement 

 is of a spiral nature, lasting much longer than that of the straight 

 hormogones. These latter occur in all the families ; they are not in- 

 vested in a gelatinous sheath ; their motion is rectilinear and of short 

 duration. The cells have excessively thin cell-walls, and contain 

 abundance of cyanophycin, slightly coloured by phycocyanin. The 

 cells of the hormogones are in communication with one another in the 

 same way as the ordinary vegetative cells. 



The spores of Nostoc and other Nostochineae are, according to 

 Borzi, formed as follows. The cells which become transformed into 

 spores cease dividing transversely and increase somewhat in size. The 

 gelification of the outer layer of the cell-walls ceases at the same time 

 and a new cell is formed inside the old one by a process of rejuvenes- 

 cence, the wall of the mother-cell ultimately disappearing altogether. 

 The membrane of the spores thus constituted is formed out of their 

 protoplasmic contents, and is homogeneous, without differentiation into 

 exospore and endospore. The spore is, therefore, not homological with 

 those of the higher Cryptogams, but is regarded by Borzi as partaking 

 more of the nature of a cyst. Gomont, on the other hand, finds that 

 the spores of the Nostochineae, which are always the result of the 

 encysting of ordinary vegetative cells, possess a distinct exospore and 

 endospore, the former being again composed of two distinct layers, the 

 outer of which is frequently warty or otherwise marked. 



LITERATURE. 



Thuret Mem. Soc. Sc. Nat. Cherbourg, 1857, p. 29 ; and Classification des Nosto- 



chinees, 1875. 



De Bary Beitr. zur Kenntniss d. Nostocaceen, Flora, 1863, pp. 553 and 577. 

 Bornet and Thuret Notes Algologiques, 1876-1880 (especially Fasc. ii.). 

 Borzi Nuov. Giorn. Bot. Ital., 1882, pp. 272 and 384 ; and Malpighia, 1886. 

 Bornet and Flahault Ann. Sc. Nat., iii.-vii., 1886-1888; and Mem. Soc. Sc. Nat. 



Cherbourg, 1887, p.,195- 

 Gomont Morot's Journ. de Bot., 1888, p. 43; and Bull. Soc. Bot. France, 1888, 



p. 204. 



