APPENDIX 



An Illustrated Key to the Common Genera of Desmids and 



Freshwater Diatoms 



Introduction 



The following keys are designed to give the beginning student 

 of the algae an aid in identifying Desmid and Diatom genera. 

 Those which are included here are known to occur in the Great 

 Lakes area, or may be expected. Like most other freshwater 

 algal genera Desmids and Diatoms are widely distributed over 

 the world, although a few genera have some geographical limita- 

 tions. It is well-recognized that an ideal, facile and functional 

 key is not easy to prepare. This is especially true for those genera 

 in which there is a host of species, many of which have inter grad- 

 ing variations. It is hoped that the following keys, together with 

 the illustrations will prove useful. Some of the illustrations are 

 redrawn from previously published works by C. J. Elmore, N. 

 Foged, J. Frenguelli, M. Bourrelly, and G. W. Prescott. 



I. DESMIDS 



Three families of the Order Zygnematales (Phylum Ghloro- 

 phyta) constitute the "Desmids," the Gonatozygonaceae, Meso- 

 taeniaceae and the Desmidiaceae. Characteristic of the Order, 

 these plants have relatively large, ornate chloroplasts, few in num- 

 ber within a cell and bearing large pyrenoids. Reproduction is 

 by cell division, by aplanospores, and by zygotes that are formed 

 by a conjugation type of gametic union. 



The Gonatozygonaceae (sometimes included in the next family) 

 includes Genicularia and Gonatozygon, cylindrical cells commonly 

 occurring in filaments but which may be solitary. They are found 

 in the same habitats as true Desmids. 



In the Mesotaeniaceae the plants are one-celled; have a wall 

 constructed of a single piece, and are not constricted in the mid- 

 region ( as in the Desmidiaceae ) . The contents of the cells usually 

 are divided in two symmetrical portions, with the nucleus median. 

 The wall is smooth and contains no mucilage pores. These are 

 known as the Saccoderm Desmids and include such genera as 

 Mesotaenium, Cylindrocijstis, Netrium, Roya, Spirotaenia and An- 

 cylonema. 



The remainder of the Desmids, 24 or more genera, comprise the 

 true or Placoderm Desmids. The wall is in two sections which 



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