HOW TO KNOW THE FRESH-WATER ALGAE 



SPECIES. A particular kind of plant or animal is called a species. 

 For example, a certain rose, or a particular alga such as a "pond silk," 

 or a particular bird is known as a species and is given an identifying 

 or specific name. Because there is so much (although slight) variation 

 between individuals which are in general very much alike, the limita- 

 tions or precise definition of a species of plant or animal is often difficult 

 to draw and is subject to different interpretations of specialists. 



GENUS. All plants which obviously are roses, but not all the same 

 kind, are grouped together and constitute what is known as a genus 

 (plural, genera). Thus all different species of roses are placed in the 

 genus Rosa, the Latin name for the genus. All species of "pond-silk" 

 are placed in the genus Spircgyra. The genus name, Spirogyra, and a 

 species name (a particular kind) together constitute the scientific name. 

 For example, Spirogyra elongaia is the scientific name of a species which 

 has long cells; fiosa cinnamonea is the cinnamon rose. This method of 

 naming each kind of plant and animal with a double name is known as 

 the binomial system of nomenclature. The double name identifies not 

 only a particular kind of individual but also indicates to what group 

 (genus) it belongs. 



FAMILY. The genus flosa has much in common with the strawberry 

 genus (Fragaria), and is much like the prune genus (Prunus). Simi- 

 larly Spirogyra has much in common with another group of species 

 which constitute the genus Mougeofia. Therefore, Rosa, Fragaria, 

 Prunus, and other genera that have characteristics much in common 

 are grouped to form what is called a family. In this case, the Rosaceae 

 or Rose family. Spirogyra, Mougeofia, Zygnema and some other algal 

 genera which have characteristics in common and which seem, there- 

 fore, to be related are grouped to form the family Zygnemataceae. 



ORDER. In turn, families which are distinct from one another but 

 which, nevertheless, have some few characters in common are grouped 

 to form what is known as an order. Thus we have the Rosales, the 

 Zygnematales, etc. 



MOST ALGAE ARE AQUATIC 



