20 Principles of Plant Culture. 



etc., would correspond to families. There would also 

 be copies of the same work in different buildings which 

 would correspond to varieties. 



22. Scientific Names are given to plants and ani- 

 mals because the common names by which they are 

 known are so often local. For example, quack grass, 

 one of our common troublesome weeds, is known by at 

 least seven different common names in this country 

 alone, and yet, in a given locality it is often known by 

 only one name. Its scientific name, however, Agropy- 

 rum repens (a-gro-py'-rum re'-pens), is the same in all 

 languages and countries. Scientific names are usually 

 Latin and consist of two words. The first word is the 

 name of the genus to which the plant or animal be- 

 longs, and is called the generic (ge-ner'-ic) name; the 

 second word designates the species, and is called the 

 specific (spe-cif'-ic) name. For example, Pyrus malus 

 (py'-rus ma'-lus) is the scientific name of common ap- 

 ple, Pyrus being the genus to which the apple belongs, 

 and malus designating which species of the genus is 

 meant. 



23. Crosses and Hybrids (hy'-brids). We have seen 

 that in sexual reproduction, a union of male and female 

 cells is almost always essential (17). When these cells 

 proceed from two individuals of different varieties (21, 

 436), the offspring is called a cross; when they proceed 

 from individuals of different species, it is called a hy- 

 brid. Hybrids are possible only between closely-allied 

 species and are often incapable of reproduction, in 

 which case they are said to be sterile. The mule, which 

 is a hybrid between the horse and the ass, is a familiar 



