178 THE PHENOMENON OF 



persistent, double or triple, firmly connected sheaths 

 are not so clearly distinguishable as in Ulothriss Braunii. 

 The escape from the mother envelope is exhibited in a 

 different way in the Scytonemeae (especially finely in 

 Arthrosipkori) and Rivularieae (especially in Schizosiphon 

 and Euactis). Here it is not the individual cell, but a 

 whole series of cells, which breaks through the enveloping 

 membrane. In these genera a common coat of either 

 leathery or gelatinous consistence is formed around the 

 whole row of cells, through the cells separating only 

 by their side-walls (not at the joints of the link-cells.) 

 With the progress of longitudinal growth, this produc- 

 tion is repeated, so that numerous layers are formed, the 

 outer of which, however, are successively broken through 

 at the upper end, remaining as open funnel-shaped 

 sheaths, one fitted inside another, no longer shutting up 

 the upper end of the filament. In Arthrosiphon these 

 funnels are short, pale-coloured, and transparent, whence 

 this elegant genus is especially fitted to exhibit the true 

 structure of the compound sheaths in the said group of 

 Algae. In Scytoncma the sheathing funnels are longer, 

 thinner, and very firmly connected, so that the entire 

 sheath assumes merely the appearance of one many- 

 layered sheath, clearly streaked with oblique lines, 

 however, in the longitudinal section. In Euactis, lastly, 

 the delicate and soft funnels separate into loose lamellae, 

 whence the lateral view which is obtained by the 

 microscope exhibits the appearance of a fibrous breaking- 

 up of the outer surface of the sheath. The phenomenon 

 of unilateral rupture of the coats is exhibited again in 

 another way by the remarkable genus Urococcus,* of the 

 family of the Palmellaceae. 



The large, globular, brownish-red or blood-red cells of 

 this genus, throw off colourless layers of cell-membrane, 



* Vide Hassall, 'British Fresh-water Algae,' (1845,) t. Ixxx, especially 

 figs. ^ and 6. A form belonging to this genus, apparently standing between 

 U. Hookerianus and U. insignis, occurs on the elevated moors of the Black 

 Forest. 



