24 Myxophycese 



Sometimes the hormogone is terminated at each end by a heterocyst. 

 Frequently entire filaments will break up into hormogones owing to the 

 formation at intervals of biconcave discs of intercellular substance, each 

 of which marks the limitations of the extremities of two consecutive 

 hormogones. These separation-discs are much more frequent in some forms 

 than in others. Each disc consists of an intercellular substance secreted 

 by the two adjacent cells, and is often so thin in the middle as to appear 

 almost like a ring. At first the discs are dark green, but they may become 

 colourless. They are coloured yellow by chlor-zinc-iodine ; they are not 

 contracted by glycerin, nor do they stain with congo-red. (Consult Brand, 

 '03; '05.) 



Filaments are frequently broken, and thus multiplied, by the death of vegetative cells, 

 which lose their turgor and are compressed by the neighbouring turgid cells. The dead 

 cell finally decays, causing the dislocation of the filament. Such cells (called by Brand 

 * necridia ') are strongly contracted by glycerin and stain with congo-red. 



Quick growth of the filaments, accompanied by repeated hormogone - 

 formation, enables many of the Hormogonese to form a considerable stratum 

 in a relatively short time. The hormogones of Nostoc, Phormidium and 

 Lyngbya frequently contain pseudovacuoles. In the developing hormogones 

 of Nostoc the division of the cells produces a more or less spirally twisted 

 filament within a firm and relatively wide integument, at each pole of which 

 is a prominent heterocyst. 



In two species of the Rivulariacese (Galothrix adscendens and Gloeotrichia 

 natans), Teodoresco ('07) has described the formation of hormogones of 

 a peculiar kind. The upper piliferous part of the filament grows greatly 

 in length and becomes differentiated into wider portions with thick, short 

 cells, and narrower portions with much thinner, longer cells. A dislocation 

 then occurs in the middle of each thin portion, and the two narrow dislocated 

 ends grow past each other. In this way there may be formed several 

 filaments, each gradually attenuated from the middle to the hair-like 

 extremities, by which they are adherent. Soon the filaments completely 

 separate and two adjacent vegetative cells in the thick middle part of each 

 filament become transformed into heterocysts. The final separation occurs 

 between these heterocysts, two complete Calothrix-filaments thus arising 

 from each hormogone. 



ASEXUAL REPRODUCTION. Reproduction takes places asexually in many 

 genera by the formation of gonidia or by resting-spores. 



GONIDIA occur in many genera of the Myxophycese, and are usually 

 formed within gonidangia which have arisen from vegetative cells. They 

 have long been known as the only method of increase in the various genera 



