Myxophycece 313 



Hegler, and later, Fritsch, regard the heterocysts as storehouses 

 for reserve substances, the latter passing into the heterocyst along 

 the protoplasmic threads which communicate with the adjoining 

 cells. It must be remarked, however, that only under very un- 

 favourable conditions would this function be taken up by the 

 heterocysts, as under normal circumstances the cell-contents of a 

 heterocyst are absolutely homogeneous. 



Brandt 1 has observed (in Nostoc commune and N.microscopicum) 

 the contents of heterocysts set free as gonidia, which have sub- 

 sequently developed into new filaments. This germination of the 

 contents of a heterocyst must have taken place under very excep- 

 tional and abnormal conditions, and it has not been confirmed. It 

 suggests the possibility that heterocysts are the lingering and 

 abortive relics of a type of spore once possessed by certain of the 

 Myxophyceae, but which long ago ceased to be functional. 



The multiplication of the unicellular and colonial blue-green 

 Algae is brought about principally by simple cell-fission, division 

 occurring in every direction of space or in certain directions only. 

 Definite or indefinite colonies may thus be produced, which dis- 

 sociate into smaller groups on attaining their maximum size. 



Asexual reproduction takes place in a variety of ways. In some 

 families, such as the Nostocaceae, Stigonemaceae and Rivulariaceae, 

 spherical or cylindrical spores are formed by the rejuvenescence and 

 growth in size of certain of the vegetative cells. In the Chamaesi- 

 phoniaceae the elongated unicell divides into a number of small 

 spores which are then liberated from the free apex of the cell. 

 Among the plants of the entire order Hormogonieae reproduction 

 occurs by the formation of hormogones. These are short filaments 

 of cells which are set free from the extremities of the vegetative 

 filaments, and they ultimately develop into new plants. They can 

 be considered as a primitive type of multicellular gemmae. Repro- 

 duction may occur in certain species of Nostoc by means of cocci, 

 which are small cells about the same size as the vegetative cells 2 . 

 They form a scum on the surface of the water, and each one is 

 capable of producing a colony by simple cell-division. Reproduction 

 by zoogonidia does not take place in the Myxophyceae, although a 

 few motile, blue-green unicells are known to exist. 



Sexual reproduction is unknown amongst the blue-green Algae, 



1 Brandt in Berichte Deutsch. Bot. Gesellsch. 1902, xix. 



2 Sauvageau in Ann. Sci. Nat. Bot. iii, 1897, p. 367. 



