380 Conjugate 



borne in mind that the direct transference by natural means from one country 

 to another distant country of any living Desmid is in most cases an utter 

 impossibility, since desiccation, or in many cases even partial drying is 

 quickly followed by death, and submergence in sea-water is equally fatal. 

 Moreover, zygospores, which might possibly withstand the entailed vicissitudes 

 if circumstances arose by which they could be transferred from one country to 

 the other (such as by the long flight of a wading bird), are so rarely produced 

 that distribution by their means across an expanse of ocean is almost 

 impossible. 



Borge ('92) has recorded a number of sub-fossil Desmids from the glacial 

 clays of the Isle of Gotland, and a few subfossil forms of existing species 

 have also been noted from an ancient peat deposit near Filey in E. Yorkshire. 



Desmids only thrive in soft water, and they are very numerous in peaty 

 water which has a trace of acidity and in the almost pure water of rocky lakes 

 on the old geological formations. There are a few terrestrial or partially 

 terrestrial species, such as Mesotsmium caldariorum and sometimes Cylindro- 

 cystis Brebissonii. There are also others which live in well aerated positions 

 among Bryophytes on wet and dripping rocks. 



The classification of Desmids is on a very sound basis, largely owing to 

 the researches of Liitkemtiller ('02 ; '05), and the work of the present author 

 (G. S. W., '99; etc.) mostly verifies Liitkemiiller's conclusions. There are two 

 well-defined sub-families in which the genera are arranged as follows : 



Sub-family I. SACCODERM^:. 



Cell-wall unsegmented and without pores, soluble in an ammoniacal solution of cupric 

 oxide. Point of division of cells somewhat indefinite and unknown previous to the actual 

 division. The young half of the cell is sometimes developed obliquely and its walls are 

 absolutely continuous with the walls of the older half. 



Tribe 1. Gonatozygae. Cells elongate, cylindrical and unconstricted, solitary or 

 forming loose filaments. Cell-wall with a differentiated outer layer of which the small 

 roughnesses or spines form a part. Oonatozygon De Bary, 1856 ; Genicularia De Bary, 

 1858. 



Tribe 2. Spirotsenieae. Cells solitary, relatively short and unconstricted 1 . 

 Cell- wall a simple sac, destitute of a differentiated outer layer. The cell becomes adult 

 by periodical growth. Cylindrocystis Menegh., 1838; Spirotsenia Breb., 1848; Netrium 

 Nag., 1849; em. Liitkem., 1902; Mesotsenium Nag., 1849 [inclus. Ancylonema Berggren, 

 1870] ; Roy a W. & G. S. West, 1896. 



Sub-family II. PLACODERM^E. 



Cell- wall segmented, with a differentiated outer layer, only slightly soluble in an 

 ammoniacal solution of cupric oxide. Cell-division follows a fixed type, with the interpo- 

 lation of younger sernicells between the old ones. The younger portions of the cell-wall 

 are joined to the older portions by an oblique surface. 



1 All the supposed constricted species of Cylindrocystis have now been shown to belong to 

 the genus Cosmarium. 



