26 Myxophyceae 



In Phormidium, Brand has described the formation, by further divisions, 

 of microgonidia (fig. 16 E), and Lemmermann ('02) has observed similar 

 microgonidia in Plectonema capitatum. Solitary gonidia have been de- 

 scribed by Brand ('03) as occurring in Phormidium autumnale (= Pit. 

 imcinatum). 



The gonidia, such as those described above, usually germinate at once 



without any resting period. 



Spratt ('11) found that the heterocysts of Anab&na cycadearum formed a variable 

 number of gonidia by the rejuvenescence and subsequent division of their contents. The 

 gonidia were spherical and each one was capable of forming a new vegetative plant. This 

 observation is interesting from the support it lends to the suggestion that heterocysts are 

 merely relics of reproductive organs of the nature of gonidangia. It must not be forgotten, 

 however, that the production of gonidia is not a normal function of heterocysts, and in 

 this particular case it may simply be reversion due to degeneration. 



Fritsch has observed the production of ' gonidia ' in old culture material of Anabsena 

 azolliK. One large gonidium was described as being formed within each mother-cell by 

 rejuvenescence, and before liberation it was surrounded by a well-marked membrane. It 

 is evident that these bodies differ much from the gonidia described above, and Fritsch 

 himself states that they appeared to him to be the result of arrested spore-formation, 

 which is certainly not the case with ordinary gonidia. Moreover, the so-called ' gonidia ' 

 of A nabsena uzollse apparently passed through a resting period. 



Reproduction by zoogonidia does not take place in the Myxophycese 1 , 

 and the few motile blue-green unicells which are known to exist appear 

 to be Flagellate forms with no place in the Myxophycese. 



RESTING-SPORES occur in the Chroococcacese (in Gloeocapsa), in the 

 Oscillatoriacese (in Oscillatoria, Phormidium, Lyngbya and Microcoleus), 

 in the Nostocaceae (in Anab&na, Cylindrospermum, Nostoc, etc.), in the 

 Scytonemacere (in Scytonema, Tolypothrix and Microchtete), in the Stigo- 

 nemaceoe (in Hapalosiphon and Stigonema) and in the Rivulariacese (in 

 Calothrix and Glceotrichia). In certain genera of the Nostocaceoe and 

 Rivulariaceas resting-spores are formed with sufficient regularity to become 

 of specific importance. 



The spores arise in all cases from vegetative cells, which generally 

 increase in size, become largely filled with reserve materials, and develop 

 two distinct membranes, the endospore and the exospore. The latter is 

 the stronger membrane. It is often coloured yellow or brown, and some- 

 times, as in Cylindrospermum majus and G. tropicum (fig. 11 F), it has a 



1 Zukal ('94) has described the formation of small motile bodies of two sizes in Tolypothrix 

 Idiiata, which he termed 'gametes,' since they associated themselves in pairs, a large one with a 

 small one, although he was not certain that they fused. Goebel, also, has described zoogonidia 

 in Merismopedia. Neither Zukal's nor Goebel's observations are in any way substantiated by 

 the investigations of other authors, and for the present they must be regarded as exceedingly 

 doubtful. It seems highly improbable that 'gametes' could occur in any group of organisms 

 in which the nuclear differentiation is imperfect. 



