ALGJE. 103 



and then the fragments of the trichomcs escape, 

 leaving the heterocysts behind. The escaped frag- 

 ments, or hormogones, become endowed with motion, 

 similar to that observed in the threads of Oscillaria. 

 The cells of the hormogones increase by division at 

 right angles to the filament, ultimately separating 

 longitudinally, and becoming the centre of new plants. 

 Thus it will be seen that the Nostoc has in itself the 

 power of reproducing the species by means of the 

 hormogones, as well as by another process, which is 

 the conversion of certain privileged cells into spores, 

 these latter germinating, and giving origin to new 

 plants. 



Brief allusion may be made to the theories linked 

 with the Nostocs by different authors. Professor 

 Sachs has recorded that he watched a quantity of 

 Nostoc commune^ and saw it develop into the lichen 

 called Colleina pidposuni. He states that the peculiar 

 collema threads first appeared as little lateral off- 

 shoots from the cells of the nostoc filament, and 

 rapidly developed into well-formed collemoid fila- 

 ments. He says that every possible stage from the 

 typical Nostoc to the typical Collema was seen re- 

 peatedly. These observations have, however, never 

 to our knowledge been confirmed. 



Another author, M. Baranetsky, professes to have 

 seen the reverse process of Nostoc developed out of 

 a Collema. He placed sections of actively growing 

 fronds of Collema pnlpjsum on smooth damp earth, 

 using proper precautions to prevent external influence. 

 After some days the sections became less transparent 

 and intensely green from the crowding of the gonidia. 



