Hormogones 



23 



MULTIPLICATION. The general increase of the unicellular and colonial 

 Blue-green Alg?e is by simple cell-division, which may occur in every 

 direction of space or in certain directions only. Indefinite or definite 

 colonies are thus produced, and these are often increased by dissociation 

 into smaller groups. In the filamentous forms, fragmentation of the fila- 

 ments is a frequent method of multiplication in certain genera. The 

 division of the protoplast has already been dealt with, and brief mention 

 has been made of the fact that in many families, such as the Chroococcaceae, 

 Nostocaceas and Stigonemaceae, the cell often divides by a gradually deepen- 

 ing constriction, whereas in others a septum is formed during the division 

 of the cell. The retention of the products of division by persistent envelopes, 

 such as occurs in Gloeocapsa, etc., results in the formation of colonies of con- 

 siderable size, and often of an extended stratum. 



Fig. 15. A and B, hormogones of Stigonema occllatum (Dillw.) Thur. C, hormogones of 

 Phormidium Corium (Ag. ) Gom. and D of Ph. (iiiibii/uuin Gorn. E, portion of extremity of 

 erect tuft of filaments of Symploca muralis Kiitz. showing formation of hormogones. A, B 

 and E, x 520 ; C and D, x 500. 



In the great section of the Hormogoneae, the plants largely multiply 

 by the formation of hormogones. These are short filaments of vegetative 

 cells which are generally set free from the extremities of the main filaments 

 and branches (fig. 15). They are capable of slow spontaneous movements; 

 sometimes in a straight line, as in Lyngbya, Nostoc, Scytonema, Rivularia 

 and Stigonema, or less often in a spiral manner, as in Oscillatoria and 

 Arthrospira. The distance travelled by the hormogones of a species of 

 Phormidium varied, according to Brand, from 2'8 p, to 34*8 /* in ten minutes. 

 Hormogones can be considered as primitive multicellular gemmae, and each 

 one ultimately develops directly into a new plant by vegetative cell-division. 



