192 ISIMTISH DKSMIIH AI'K/E. 



//. Point of division always fixed (at tlic isthmus). 

 Tribe 5. ( -OSMAKIK.K. 



The Desmids of tins tribe exhibit great diversitv of 

 si/e and form, and tliey possess at least three planes 

 of symmetry. Most of them are solitary, but. others 

 arc colonial, forming filamentous or more or less 

 spherical a.o-oreffates. The cell-wall consists of two 



I -~ it o 



firm layers furnished with minute pores. The outer 

 layer is frequently ornamented with spines, warts, 

 granules, or other excrescences, and the cell itself 

 is often lobcd. 



No periodical growth takes place, the cell becoming 

 adult very soon after division by the mature growth 

 of the younger semicell. 



The wall of the older semicell overlaps the wall of the 

 younger semicell, and the edges of both new and old 

 walls are bevelled so as to fit firmly together and at 

 tlic same time present a plane surface. Thus, the two 

 scmicells, which are of different ages, are firmly joined 

 along an oblique plane which runs round the isthmus. 



It is much the most important tribe of the Desmid- 

 iacere, and it includes some of the largest and best- 

 known genera. 



The three genera Streptonema, Desmidium, and 

 Gymnozi/ga are at once separated from the rest of the 

 genera in this tribe by a peculiarity in the division of 

 their cells, a girdle-like thickening being developed 

 from the point of division at the isthmus, which pro- 

 jects backwards into each of the older semicells during 



division. 



* 



Sec f ion a. The cell-wall at the point of division 

 (which is always at the isthmus) remains perfectly 

 plane during division. 



