THALLOPHYTES 



walls of the two neighboring cells are observed to bulge out. The fila- 

 ment does not grow indefinitely in length, but breaks up now and then 

 by the disorganization of one or more cells, and 

 each fragment begins to construct another colony. 

 This process of fragmentation results in the 

 multiplication of colonies, which may be called 

 colonisation. 



The protoplast of Oscittatoria exhibits the two 

 regions described under Gloeocapsa for Cyano- 

 phyceae in general; in fact, this differentiation is 

 probably more evident in Oscittatoria than in any 

 other common form. The most striking feature 

 of the plant, however, is the characteristic sway- 

 ing and revolving movement of the filaments, a 

 movement which suggested the name. If a mass 

 of filaments be placed on a solid substratum, the 

 filaments begin a creeping movement and become 

 spread out radiately in a film. It is evident that 

 this movement is possible only as the cells of a 

 filament work together, and this introduces into a FIG. (>. Oscillator : 

 colony of cells the idea of an individual composed the ce U s f & e simple 

 of many cells. In fact, the many-celled colony ^"ah" 

 merges so gradually into the many-celled individual p i as t into the peripheral 

 that there is no boundary between the two. pigment region and the 



N ostoc.-In this form the colony is also a ^'g& 

 filament, but when the cells divide, they so nearly appears an irregular ag- 

 separate and round off that they become tangent gregation of dark ma- 

 to one another, resulting in a filament resembling ^^^^T" 

 a string of beads. Each filament has its own 

 mucilaginous sheath, as in Lyngbya, but there is an extraordinary 

 development of mucilage in connection with groups of filaments. As 

 a consequence, Nostoc appears in nature as lumps of jelly, in which 

 numerous filaments are found embedded (figs. 7, 8). 



The most noticeable fact in reference to these filaments is that the 

 cells are not all alike. At intervals cells appear which differ in contents 

 and usually in size from the ordinary working cells. They are derived 

 from ordinary working cells, which usually enlarge, lose their contents, 

 and become thick-walled. The loss of pigments makes these cells 

 very distinctly in the filament. They are called heterocysts, but 



