NATURAL FAMILIES. GENUS. SPECIES. 51 



All those which seem to be made upon the same plan, 

 with similar stems, leaves, flowers and fruit, are said to 

 belong to the same Natural Family. Thus all the oaks, 

 chestnuts, beeches, and hazel nuts, belong to the Oak 

 Family, because, while they resemble each other in gene 

 ral appearance, in the structure of their flowers and fruit 

 they are still more strikingly alike. 



165. Plants are still farther divided into genera and 

 species. A genus is a subdivision of a family, and a 

 species, a subdivision of a genus. The oak family, for 

 example, is divided into the genera, oak, beech, chestnut, 

 hornbeam, hop-hornbeam and hazel. The genus oak is 

 subdivided into white oak, red, black, post, over-cup, live, 

 willow, and many other species. Speaking of a black oak, 

 we should say ; it belongs to the Class Dicotyledonous 

 Plants, to the Oak Family, to the genus Oak or Quercus, 

 and to the species Black Oak, or Quercus Tinctoria. 



166. An example will show of what practical use these 

 divisions and subdivisions are. I find a grass which I 

 suspect to be Common Hair Grass ; I wish to know cer 

 tainly ; and turn to a volume (Gray s Manual of Botany) 

 which contains a description of every plant in New Eng 

 land. The first part of the volume is occupied with dico 

 tyledonous plants. I find the description of monocotyle- 

 donous plants, to which I know grass belongs, beginning 

 on the 426th page. Not desiring to read the whole of 

 158 pages, I look for the Grass Family, and find it to be 

 the 184th family, and on the 535th page. This family, I 

 find, contains 65 genera. After some examination of a 

 table, I find that the 47th genus of grasses is Hair Grass, 

 (Aira.) Carefully reading the description of the genus, 

 in six lines, and of the first species, (Aira flexuosa,^) in 

 four, I find that the plant I have found belongs to it, and 



