436 



PROTOPHYTA 





.n §" '.? .'^y S -.£j' &fi::'.'« ft ^ri3')SaK. ■',g'i8Bj| 



(Thur.) while still within the sheath, the cell-contents passing from a 

 heterocyst into the basal cell of a hormogone. 



Multiplication by quiescent resting-spores has been observed in some 

 species of Rivulariacece. The lower portion of the green part of a fila- 

 ment immediately above the basilar cell is transformed into an elliptical 

 thick-walled spore, which escapes from its investing membrane, and, 

 after a period of rest, either develops directly into a new filament, or 



breaks up into a number of hormo- 

 gones. The spores of Gloeotrichia 



ir.) are rough. 



' conids ' Bornet 



describe special 



propagative ceus which become 



detached from the lower part of the 



filament. 



By far the larger number of 

 species of Rivulariacese grow in 

 fresh water, especially stagnant, also 

 on damp soil and on wet rocks. 

 The species of the typical genus 

 Riviilaria Roth are especially 

 abundant in both running and 

 standing w^ater ; R. fluitans (Cohn, 

 Hedwigia, 1878, pp. i and 2iZ) floats 

 free on the water, forming a blue- 

 green scum which enters largely 

 into the composition of what is 

 known as ' flos aquae.' Some species 

 have a red tinge. Calothrix Ag., 

 though placed by Rabenhorst among 

 the Scytonemacese, has all the cha- 

 racteristics of the Rivulariacese. 

 The filaments of some species are 

 comparatively thick, as much as 

 o'oi mm. in diam.eter, and are invested in a copious, often coloured, 

 mucilaginous sheath. Alicrochate Thur. is nearly allied to Calothrix, 

 as is Glaotrichia Ag. to Rivularia. Several species of Rivularia 

 and Calothrix grow^ in salt or brackish water, and Isadis Thur. is ex- 

 clusively marine. In Hor7nactis Thur., which is also marine, the fila- 

 ments are curved in a serpentine manner, and this character, together 

 with the interstitial heterocysts, appears to indicate an approach to the 

 Nostocaceae. Other genera included in the Rivulariacese are I.eptoch^te 



Fig. 365. — Isactis plana Thur. (x i6o). 

 (After Bornet.) 



