Ulotrichacese 287 



The genus Uronema differs little from Ulothrix, but the filaments are 

 mostly rather short and fixed at the base ; moreover, the apical cell is always 

 conical. In Geminella the cells develop convex, and often hemispherical, 

 poles, and are scarcely adherent, being held in position very largely, and in 

 some cases entirely, by the copious mucous investment (fig. 180). Binuclearia 

 is a montane and subalpine genus characterized by the great thickness of 

 some of the transverse walls of the filaments, the cells having the general 

 appearance of being arranged in pairs. In Rhaphidonema the filaments, 

 which are attenuated at both ends, are the shortest of any genus of the 

 family, consisting of only a few (4 to 7) cells. This genus was first described 

 by Lagerheim ('92 c) from the snow-flora of the Andes, but both the original 

 species (Rh. nivale} and another one (Rh. brevirostre Scherffel, '10) are known 

 to occur in Europe. 



The genera are: Geminella Turpin, 1828; em. Lagerheim, 1883 [inclus. Hormospora 

 Brebisson, 1840 ; Glceotila Ku'tzing, 1843 ; Planctonema Schmidle, 1903] ; Ulothrix 

 Kutzing, 1833 ; Stichococcus Nageli, 1849 [inclus. Hormidium Klebs, 1896 (non Kiitzing, 

 1843) and Hormococcus Ohodat, 1902] ; Binuclearia Wittrock, 1886 ; Uronema Lagerheim, 

 1887 ; Rhaphidonema Lagerheim, 1892 ; Radiofilum Schmidle, 1894 ; Psephotaxus W. & 

 G. S. West, 1897 ; Catena Chodat, 1900. 



With the exception of a few subaerial species of Ulothrix and Stichococcus all the 

 members of the Ulotrichacese are aquatic. 



Most of the species of Ulothrix pass the winter in the vegetative condition in the form 

 of short filaments in which both the transverse and lateral walls have much increased 

 in thickness. In the British Islands active cell-division generally begins by the middle 

 of January (in the absence of prolonged frosts), the old thick walls being burst apart 

 as the cells divide. In the winter state of U. asqualis and U. Subtilis oil-drops are of 

 frequent occurrence. 



Quite recently Brunnthaler ('13) has attempted to show that Radiofilum should be 

 transferred to the Desmidiacese, but there is little doubt that such a position would not be 

 in accordance with the affinities of the genus. In describing some Australian forms of 

 R. conjunctivum the present author (G. S. W., '09) instituted a comparison between the 

 structure of the cell-wall in this Alga and that which occurs in the Placoderm Desmids. 

 ' The wall of each individual cell is composed of two halves, and cell-division appears 

 to take place much as it does in some of the simpler types of Desmids, such as Penium, 

 by the interpolation of two new half-cells between the old ones. The line of junction 

 of the old and new halves of the wall is distinctly visible in most specimens, and is 

 particularly obvious at the marginal a'piculations, the latter owing their prominence to 

 the projecting suture at this region. Each fully-grown half is helmet-shaped, but in its 

 earliest stages the young half is much flattened. The chloroplasts are parietal and 

 cup-shaped, occupying about two-thirds of the interior of the cell-wall. They are 

 disposed very largely back to back in pairs of adjacent cells.' This statement apparently 

 induced Brunnthaler to transfer the entire genus (with its 3 species) to the Desmidiaceee, 

 but such a view suggests lack of direct knowledge of the Algse in question. Neither 

 R. Jlavescens G. S. West ( ; 99) nor R. irregular e has any near affinity with the Conjugate, 

 and the similarity in structure of the wall between R. conjunctivum and many Desmids 

 can only be regarded as a parallelism of modification. 



