64 



SOME OP THE PRINCIPLES OP PLANT- BREEDING 



type, when they are hybridized they produce progeny 

 which in the first generation is variable. An illus- 

 tration is afforded in the crosses made by the writer 

 of the trifoliate orange with the ordinary sweet 

 orange, in which the hybrids of the first generation 

 vary in fruit, foliage and branching qualities, so 

 that almost every individual differs markedly from 

 every other individual of the same combination. In 

 the crossing of races which have been bred true to 

 type, whether of the same or of different species, 

 the first-generation hybrids, however, are nearly 

 uniform in the characters presented, and in such 

 instances it is necessary to secure a second gen- 

 eration of the hybrids in order to accomplish the 

 breaking up of the characters and the production 

 of a large number of variations. Ordinarily, there- 

 fore, desirable variations are looked for in the 

 second generation. This, as has been explained 

 above, is true only in the case of hybrids of species 

 and races that are fixed in type. 



(1) Mendel's law of hybrids. 



The preceding discussion represents fairly well 

 the general understanding of hybrids until about 

 1900, when DeVries and Correns rediscovered 

 what is now termed "Mendel's law of hybrids." 

 While Mendel's laws or principles may not be of 

 great value from an economic standpoint, they 

 have proved of the greatest scientific interest, and 

 the general fundamental principles of the law or 

 laws should be thoroughly understood by every 

 practical breeder of plants. It has been known for 

 many years that a splitting up and redistribution 

 of parental characters occurs in hybrids, and it is 

 on this fact largely that the practical application 

 of hybridization in plant-breeding depended. Ordi- 

 narily, careful plant-breeders would plan to hybri- 

 dize varieties or races having a definite combi- 

 nation of characters in view, as, for example, the 

 combining of the fruit quality of one parent with 

 the hardiness or drought-resistance of the other. 

 Until Mendel's law was discovered, however, we 

 had no understanding of why or how such a com- 

 bination could be made, and it was necessary to 

 experiment extensively in order to determine 

 what could be accomplished. 



Mendel's law includes several important features 

 which must be thoroughly understood before its 

 important bearings can be comprehended. One re- 

 quisite for the application of the law is that the 

 two parents shall possess certain 'characters that 

 are opposed to each other. These two opposing 

 qualities or characters are termed a "character- 

 pair." As illustrations of such character-pairs, 

 may be cited bearded and bald heads in wheat, 

 sweet and starchy kernels in corn, fuzzy and 

 smooth seeds in cotton, and stringy and stringless 

 pods in beans. When parents possessing these 

 opposed or contrasted characters are crossed, the 

 hybrid contains a combination of the potentialities 

 representing both characters, and the first-gene- 

 ration hybrid will thus show an intermediate form 

 of the particular character under consideration in 

 case the two characters are of equal strength or 

 potency. If, however, as sometimes occurs, one of 



the characters is very strong or dominant, only 

 this character will show in the first-generation 

 hybrids, the other character remaining recessive 

 or masked, although present. For example, in 

 crossing a race of wheat having bald heads 

 with a race having bearded heads, all of the 

 first -generation hybrids, or at least the major- 

 ity of them, will have bald heads, this character 

 being strong or dominant over the bearded char- 

 acter. In some instances where the potentialities 

 of these two characters appear to be of nearly 

 equal strength or potency, the beards seem to be 

 produced in the first - generation hybrids but are 

 reduced in length, being intermediate between the 

 bald and the bearded state. A number of inter- 

 mediate cases of this kind were shown to the 

 writer by Dr. C. E. Saunders, of the Canadian 

 Experimental Parms. Frequently, in crossing flow- 

 ers of different colors, the resulting hybrids will 

 show a blend of the two colors, being light pink, 

 for example, when the parents crossed are a white 

 and a red. In other cases, however, one color or 

 the other becomes the dominant character, and the 

 first - generation hybrids show the color of one 

 parent only. 



The second important principle of Mendel's law 

 is what is termed the purity of the germ-cell. It 

 seems certain from the researches that have been 

 conducted that, when the germ-cells of the first- 

 generation hybrids are formed, the potentialities 

 which represent the two different characters under 

 consideration, and which were united by the hybri- 

 dization, ordinarily segregate again in the cell 

 divisions, which lead to the formation of the germ- 

 cells, so that certain germ-cells include the poten- 

 tiality of one only of the two characters. We have 

 thus two kinds of germ-cells formed with respect 

 to this one character-pair. Taking as an illustra- 

 tion a hybrid of wheat having bald heads with one 

 having bearded heads, when the germ-cells were 

 formed a segregation of the two potentialities 

 representing the two opposed characters would 

 take place, and we would have germ-cells of one 

 kind containing the bald-head potentiality and of a 

 second kind containing the bearded-head potential- 

 ity. This segregation, it must be understood, takes 

 place in the formation of both the egg-cells and 

 the sperm-cells or pollen-grains. 



We thus see that the first generation of the 

 hybrid when two such characters are combined 

 contains two kinds of egg-cells and two kinds of 

 sperm-cells, so far as this one character - pair is 

 concerned. 



The third important principle of Mendel's law is 

 what is termed the law of probability, and ex- 

 plains what may be expected in plants of the 

 second generation of such a hybrid. Remembering 

 that we have formed in the first-generation hybrid, 

 as explained above, two kinds of egg-cells and two 

 kinds of sperm-cells with reference to the opposed 

 characters, what would happen if the hybrid were 

 bred with its own pollen; or, in the case of an 

 animal, if it were bred with another hybrid of the 

 same parentage? For the purpose of illustration, 

 suppose that a hybrid of a bald wheat with a 



