INTKODUCTION TO CKYPTOCJAMIC BOTANY. 14:9 



and constitutes a branch, a character by which the species are 

 at once known from Scytonema. 



121. We come now to the compound gelatinous forms. 

 And here we have jelly of very different tenacity in different 

 species. In Dasyglwa the masses are so slippery, that they 

 can scarcely be handled ; the common tube of each chain 

 of endochromes being exceedingly thick and very tender. In 

 some of the marine species, hoAvever, the jelly is so firm that 

 thin slices can readily be obtained for microscopical examina- 

 tion, while Rivularia nitida is often so tender as to make a 

 clean section almost impossible. The great peculiarity of Rivu- 

 laria and its allies consists in the mode of branching, which 

 arises from the alternate turning aside at tolerably regular inter- 

 vals of a connecting cell. In consequence of this singular mode 

 of origin, the branching is oblique, the main thread being 

 always diverted from a straight course at the point at which 

 the new branch is set on. Many of the species of RivuloAHa, 

 in consequence of the peculiar branching, are very interesting 

 objects under the microscope. They occur on the stumps of 

 aquatic plants, on rocks in rapid streams, sometimes where 

 they are exposed constantly to the force of cataracts, and in 

 calcareous water, in consequence of which crystals of carbonate 

 of lime are deposited in their substance ; other species again 

 grow on exposed rocks, subject to the constant wash of the 

 waves, while one of the most beautiful {Rivularia nitida) 

 occurs amidst other Algae on the flat rocks which are exposed 

 at low tides, though not in every state of the moon. A very 

 pretty species floats on the surface of fresh water lakes, 

 like minute green stars. 



122. OscillatoricG occur in every part of the world, and 

 are especially common in temperate regions, though by no 

 means confined to them, and often in waters of considerable 

 temperature. The curious Rhizonema interruptum has 

 been found in Ceylon, by Mr. Thwaites, exactly resembling 

 the specimens which he has figured so beautifully in the 

 Supplement to English Botany, t. 2954-. The Rividarice 

 appear to be confined to the northern regions. 1 do not find 

 any trace of them amongst the Indian Algie, nor does any 



