CYANOPHYCE.-E 



431 



times associated in chains, are formed immediately beneath them. The 

 cell-wall of the resting-spores is often coloured yellow or brown, and is 

 sometimes warty. After a period of rest they germinate by the bursting 

 of the cell-wall and protrusion of the protoplasm. The hormogones are 

 strings of from four to eight or twelve ordinary cells, situated between 

 two heterocysts, which detach themselves from the rest of the filament, 

 escape from their mucilaginous envelope, move about with a creeping 

 motion, then come to rest, and develop into new individuals. 



Bornet describes the motion of the hormogones of Xostoc as a 

 creeping movement along a solid substratum at a rate of i /n (•001 mm.) 

 per second. After some hours they come to rest, large refringent 

 globules which had previously been formed in the cells disappear, and 

 they assume the appearance of ordinary filaments. Sometimes they 



B 



Fig. -^60.— An abcpna Jlos-aqiios Fr. ( x 400). 

 (After Cooke.) 



Fig. 359. — N. hyalinum Benn. A, 'frond ' (x 200) ; 

 B, portion of trichome with heterocyst, <: ( x 600). 

 (From nature.) 



invest themselves with a nmcilaginous sheath, and are transformed 

 directly into spores, but usually the filament lengthens, displaying at the 

 same time more or less sinuosity. The heterocysts do not appear at 

 regular intervals. A new filament is thus formed altogether resembling 

 those which spring directly from the germination of the spores. Once 

 formed, it is subject to intermittent periods of growth, a second genera- 

 tion of spores, with thinner cell-walls, being sometimes formed after the 

 first. In other cases the filament springing directly from a hormogone 

 assumes a zigzag form, in consequence of some of the cells dividing in 

 the transverse, others in the longitudinal direction. In X. muscorum 

 (Ag.) all the cells except the heterocysts are sometimes transformed into 

 spores. The spores will occasionally germinate while still within the 

 ' frond.' The filaments of Nostoc are ultimately serpentine, and often 



