adjoin at the midregion where there is a constriction or sinus (in 

 most forms). There is an isthmus, therefore, between the two 

 cell halves (semicells). The walls are often ornamented with 

 spines, teeth, granules, verrucae, scrobiculations, etc. and in many 

 forms there are mucilage pores. 



In studying and in differentiating Desmid genera ( and especially 

 species), it is often necessary to view the cell from the top or from 

 the side as well as from the 'front', the position in which the cell 

 most commonly lies. It is found to be desirable to use a weak 

 glycerine solution (3/f), dilute agar, or some other such medium 

 which will permit the cell to remain in a given position to which it 

 may be manipulated by the observer. 



Desmids occur principally in soft water or acid habitats. Spha- 

 gnum bogs are especially suitable, but standing water which is 

 rich in organic acids and low in calcium may contain an abundance 

 of these plants. Some forms are subaerial, growing on soil and 

 among mosses, especially in high altitudes. One genus (Oocar- 

 dium) is found only in limey concretions. It is noteworthy that 

 the greatest number of genera, the greatest number of individuals, 

 and the largest cells occur in highly acid habitats. See: G. M. 

 Smith, Freshwater Algae of the United States, McGraw-Hill Co. 

 and G. W. Prescott, Desmids in Botanical Review, 14(10): 644- 

 676, 1948 for information on this group of the algae. 



A Key to the Common Genera of Desmids 



1. Cells joined side by side or end to end to form filaments 2 



1. Cells solitary, or inclosed in a common mucilaginous 



-sheath to form a colony, but not occurring as filaments 11 



2. Cells adjoined only by the interlocking of short, straight or hooked 

 spines or horn-like processes (prong-like extension) at the polar walls . 3 



2. Cells adjoined by their end walls, either along the entire apical mar- 

 gin, or by the adjoining of blunt arms projecting from the ends of 

 the cells (the arms being extensions of the cell, not merely out- 

 growths from the wall mentiond in 2 above) 5 



3. Interlocking processes simple, slender, hornlike and straight Onychonema 



PI. 1, Fig. 5, 6 



3. Interlocking processes curved hooks, or stubby, wart-like or tuber- 

 culate processes 4 



4. Interlocking processes curved hooks which are sometimes forked (fila- 

 ments incidentally formed and not true filaments ... Micrasterias foliacea 



PI. 4, Fig. 5 

 4. Interlocking processes short and stubby, tuberculate outgrowths 



on the wall Sphaerozosma 



PI. 1, Fig. 14 



[935] 



