438 



PROTOPHYTA 



propagation is either by resting-spores or by hormogones. In the former 

 case, at the end of the season of growth, the disc-shaped pseudocysts 

 towards the end of a filament assume a roundish or ovate form, the gela- 

 tinous sheath disappears, and the spores thus formed remain for a time 

 united together in masses. These resting-spores are capable of resisting a 

 high degree of cold and drought ; they germinate after a period of rest, 

 when the membrane bursts transversely. In the formation of hormogones 

 the sheath also becomes absorbed, beginning from the apical cell, and 



the filament breaks up into 

 a number of hormogones, 

 each consisting of many 

 pseudocysts. In the Sti- 

 gonemeae the hormogones 

 are formed only in the 

 lateral branches, which con- 

 tain only a single row of 

 pseudocysts. The hormo- 

 gones move slowly in the 

 water in a straight line ; in 

 some cases an entire fila- 

 ment may be converted 

 into a motile hormogone. 

 During germination the 

 hormogone, enclosed in a 

 delicate transparent mucila- 

 ginous sheath, breaks up 

 into portions of various 

 lengths, or it becomes a new 

 individual without breaking 

 up ; and at this period one 

 of the apical pseudocysts 

 usually becomes a hetero- 

 cyst. 



The branching of the Scytonemaceae takes place in two different 

 ways, characteristic of the two sub-orders into which they are divided 

 the Scytoneme<z and the Sirosiphonece. In the Scytonemeae, which in- 

 clude the genera Scytonema (Ag.), Petalonema (Berk.), Tolypothrix 

 (Ktz.), Plectonema (Thur.), and Drilosiphon (KA.z.\ pseudo-ramuli are 

 formed somewhat in the same way as in Rivularia, but the branches are 

 much stouter, and stand out at right angles to the main axis. In 

 Scytonema Ag. two contiguous pseudocysts separate at indefinite spots 

 on the filament, and each of these then acts as the terminal pseudocyst 



FIG. 366. - Stigonema minutum Hass. A , outline of fila- 

 ment ( x 100) ; B, portion of primary filament ( x 200) ; 

 C, ditto with branch ( x 200). (From nature.) 



