EXPEEIMENTAL STUDY OF HEREDITY 551 



1. "The experimental plants must necessarily possess 

 constant diferentiating characters."^ 



2. "The hybrids of such plants must, during the flower- 

 ing period, be protected from the influence of all foreign 

 pollen, or be easily capable of such protection." 



3. "The hybrids and their offspring should suffer no 

 marked disturbance in their fertility in the successive 

 generations." 



Mendel also called attention to the advantage of choos- 

 ing plants which, like the peas, are easy to cultivate in 

 the open ground and in pots, and which have a relatively 

 short period of growth. 



472. Characters Chosen for Observation. — "Each pair 

 of differentiating characters [have been thought to] unite 

 in the hybrid to form a new character, which in the pro- 

 geny of the hybrid is usually variable. The object oj the 

 experiment was to observe these variations in the case of each 

 pair of differentiating characters, and to deduce the law ac- 

 cording to which they appear in successive generations. The 

 experiment resolves itself therefore into just as many 

 separate experiments as there are constantly differentia- 

 ting characters presented in the experimental plants." 

 The following were the characters chosen for observation: 



I. The difference in the shape of the ripe seeds (round 

 and smooth vs. angular and wrinkled). 



' DifEerentiating characters are those in respect to which the two species 

 or varieties to be crossed differ. The possession of chlorophyll by the leaves 

 of peas, for example, is a common character. "Common characters are 

 transmitted unchanged to the hybrids and their progeny." The color of 

 the coroUa (for example, white in one species and purple in the other) is a 

 differentiating character, serving to differentiate or distinguish one species 

 from another. 



