344 THE POPULAR SCIENCE MONTHLY 



nished the pollen and half from the plant to which it has been applied. 

 If one studies the characters of several such hybrids, he finds many sur- 

 prising facts. It usually makes no difference which plant is the mother 

 plant, the result is the same. Certain characters are found in the hybrid 

 that are identical with those possessed by the male parent and other 

 characters the same as those possessed by the female parent. Other 

 characters appear to have resulted from the blending of those of the 

 two parents, while still others appear to be entirely new. The plant 

 may be sterile if the cross is between widely differing species, but if it 

 is fertile and the flower of the hybrid is self-fertilized, the plants re- 

 sulting from this seed present still more surprises. For example, if 

 one has crossed a pear-shaped yellow tomato with a round red tomato, in 

 the second hybrid generation he will find individual plants bearing fruit 

 of four kinds, pear-shaped yellow and round red, as were the two parents, 

 and in addition pear-shaped red and round yellow. In other words 

 all possible combinations occur and in definite proportions. Stated as 

 a principle it may be said that where either of the parent plants pos- 

 sesses characters absent from the other, the potential characters remain 

 pure in the germ cells of the hybrid and recombine as if by chance. 

 This is the most important feature of the only law of heredity of which 

 there is any exact knowledge — the law of Mendel. Let us illustrate the 

 action of the law. Such a character as starchiness, as shown in " flint " 

 maize, is either present or it is not present. The flinty appearance of 

 the seed is due to the possession of some character that causes the 

 maturation of plump starch grains. When this character is absent, the 

 seeds dry up without maturing their starch grains, and present 

 the wrinkled appearance common to sweet maize. Pairs of char- 

 acters such as these, affecting a certain plant structure, are called con- 

 trasted or allelomorphic pairs. When a sweet maize is crossed with a 

 flint maize, the resulting seeds are all flint like. That is, the dominant 

 character or the character that calls for the presence of the structure 

 or compound in question, manifests itself in the first hybrid generation. 

 Complete dominance, however, is not a general phenomenon in crosses 

 and as its importance is slight as compared with the second law, that 

 of segregation of the pure characters (potentially) in the germ cells 

 of the hybrid, we will discuss it no further. The second law predicts 

 that in the generations succeeding a cross, plants grown from the self- 

 fertilized seeds of the hybrid reproduce both contrasted characters in 

 the proportion of three of the dominant or " presence " characters to 

 one of the recessive or absent character. Furthermore, inbred or self- 

 fertilized plants bearing the recessive character continue ever after to 

 breed true, while of those plants bearing the dominant character one 

 third are pure and breed true while two thirds are hybrids and again 

 throw the recessive character in one fourth of their offspring. 



