506 Introduction to the Study of Science 



Experiments with peas. Mendel selected for his experiments 

 the culinary pea, because it is normally self-pollinating and 

 therefore free from interference by other agencies. He found 

 that the several varieties of the pea have distinctively different 

 characters, and by testing he proved that these characters 

 reproduce true to type. He therefore cross-pollinated different 

 varieties of pea plants, such as tall with short or dwarf plants ; 

 those having pink or red blossoms with those having white; 

 those producing yellow seed with those producing green ; those 

 having hard pods with those having soft; those producing 

 wrinkled seed with those producing smooth, round seed. This 

 he did until he learned exactly how the different varieties, 

 when cross-fertilized, behave in successive generations. He 

 discovered that in the line of descendants of hybridized peas 

 there is never a real blending of opposite characters ; but that 

 a character persists practically unmodified however it may be 

 combined with others. 



Unit characters. Mendel found also that particular char- 

 acters, such as tallness of the plant or color of the blossoms, 

 may be separated from a mass of other characters. Thus a 

 plant grower may single out one character, as the stringless 

 bean pod, from those combined in a bean pod, and by proper 

 fertilization secure its separate continuance in all descendants. 



Such a character, as tallness, color of blossoms, or color or 

 form of the seed, is called a unit character, which means that 

 it persists without blending in cross-fertilized descendants. This 

 may be illustrated roughly by a mosaic made up of many dif- 

 ferent stones, each of which is intact and may be separated from 

 the others and recombined to form other patterns. Or it re- 

 sembles a chemical reaction, as when oxygen and hydrogen 

 (page 130) combine in a definite ratio to form water. When 

 water is decomposed, as in electrolysis, it gives up its con- 

 stituents as they originally were. 



How to produce hybrids. New varieties may be produced 

 by combining desirable characters in cross-fertilization. A 



