MONOCOTYLEDONS AND DICOTYLEDONS 237 



Within a family there are smaller grouj)s, indicating 

 closer relationships, known as genera (singular, genus). 

 For example, in the great family to which the asters belong, 

 the difEerent asters resemble one another more than they do 

 any other members of the family, and hence are grouped 

 together in a genus A sfer. In the same family the golden- 

 rods are grouped together in the genus Solidago. The 

 difEerent kinds of Aster or of Solidago are called species 

 (singular also species). A group of related species, there- 

 fore, forms a genus ; and a group of related genera forms a 

 family. 



The technical name of a plant is the combination of its 

 generic and specific names, the former always being written 

 first. For example, Quercus alba is the name of the com- 

 mon white oak, Quercus being the name of the genus to 

 which all oaks belong, and alia the specific name which 

 distinguishes this oak from other oaks. !N'o other names 

 are necessary, as no two genera of plants can bear the same 

 name. 



In the Monocotyledons about forty families are recog- 

 nized, containing numerous genera, and among these 

 genera the twenty thousand species are distributed. It is 

 OTident that it will be impossible to consider such a vast 

 array of forms, even the families being too numerous to 

 mention. A few important families mil be mentioned, 

 which will serve to illustrate the group. 



131. Pondweeds. — These are submerged aquatics, found 

 in most fresh waters (some are marine), and are regarded 

 as among the simplest Monocotyledons. They are slender, 

 branching herbs, growing under water, but often having 

 fioating leaves, and sending the simple flowers or flower 

 clusters above the surface for pollination and seed-distri- 

 bution. The common pondweed {Potamogeton) contains 

 numerous species (Fig. 220), while Xaia.^ (naiads) and 

 Zannicliellia (horned pondweed) are common genera in 

 ponds and slow waters. 



