SOME OF THE PRINCIPLES OP PLANT- BREEDING 



63 



in other characteristics, namely, quality, disease- 

 resistance, drought-resistance and the like, and 

 that we see that all of the good qualities of the 

 variety are retained. To do this properly necessi- 

 tates the use of a score-card, on which each char- 

 acter of the plant which is im- 

 portant is given its relative 

 weight or grade. By the use of 

 such a score-card the breeder can 

 judge each character separately, 

 and by the adding up of the scor- 

 ing get the rank of different 

 plants in a comparative way 

 (Fig. 83). 



Test of transmitting power. 



A factor of primary impor- 

 tance in all breeding work is the 

 testing of what is termed the 

 transmitting or centgener power. 

 It is necessary for us to know 

 that a certain plant, which, for 

 example, gives a heavy yield, 

 has the faculty of transmitting 

 this tendency of producing heavy 

 yield to its progeny (Fig. 84). 

 It is frequently found that two 

 select plants that are equally 

 good so far as their yield is con- 

 cerned will give progeny which, 

 as a whole, differ greatly in this 

 respect. In the progeny of one 

 almost every plant may have in- 

 herited the desired quality, while 

 in the progeny of the other only 

 a few of the plants may show in 

 any noticeable degree the inheri- 

 tance of the quality. To determine the prepotency 

 or transmitting power, it is necessary to grade 

 carefully the progeny of each individual ; and this 

 is the primary reason for planting the progeny of 

 different individuals in separate rows or separate 

 plats, so that they may be examined easily. (Fig. 

 85.) It would seem to be an easy matter, when we 

 plant the progeny of different plants in rows or 

 small plats by themselves, to get the comparative 

 yield, for example, of 100 plants, and from this to 

 figure up the average percentage of the transmitting 

 or centgener power. This matter, however, is very 

 difficult in many cases. In corn, for example, cer- 

 tain individuals may stool and form suckers that 

 have fairly good-sized ears. If the corn is planted 

 thin enough on the ground these suckers will tend 

 to increase the yield, and render the proper judg- 

 ment of the transmitting power very difficult. It 

 would seem at first thought that such suckering, if 

 it increased the yield, would be desirable, and 

 should be considered a favorable character in con- 

 nection with the individual. However, if the soil is 

 heavy enough to have allowed this suckering to 

 give increased yield, it would have been possible 

 on the same soil to have placed the plants closer, 

 and, as seed is of little comparative value, it would 

 be best to have a non-suckering type, and plant 

 the corn as closely as the soil would properly per- 



Fig. 84. 

 A, Result of breed- 

 ingfrom smallest 

 grains ; average 

 head (after 4 

 years). B, result 

 of breeding from 

 the plumpest and 

 heaviest grains ; 

 average head 

 (after 4 years). 



mit. Again, it is almost impossible to get perfect 

 stands, and a change in the stand may affect the 

 yield. Very many difficulties and problems enter 

 into the figuring out of this transmitting power, 

 and it is obviously impossible to give directions 

 for all cases. The breeder must study conditions 

 and determine carefully what policy to pursue in 

 each case. 



The use of hybridization in plant-breeding. 



Ever since the time of Knight, hybridization has 

 been used extensively by plant -breeders, and it 

 seems that this is the only sure means of forcing 

 variations. Whenever it is possible to secure dis- 

 tinct species and races that can be hybridized, it is 

 possible greatly to increase the variation in differ- 

 ent directions, and thereby afford opportunity for 

 greater selection than would otherwise be possible. 

 Plant-breeders have come to understand that when 

 desirable characters are exhibited by different 

 species or races it is possible frequently, if not 

 usually, to unite these characters in a hybrid if 

 the work is done intelligently and on a large scale. 

 (The writer uses the term hybrid here in a general 

 sense, referring to any product of a cross when the 

 parents were noticeably distinct from each other, 

 whether the parents belong to different races, clons, 

 varieties or species. It may be stated that this 

 general or broad use of the term hybrid has become 

 almost universal in recent years.) When plants of 

 different races are crossed, as, for example, different 

 races of wheat, corn or cotton, the hybrid usually 

 comes nearly intermediate between the two parents 

 in the first generation. And this is the case also 

 when different fixed species are crossed. If, how- 

 ever, individuals belonging to unfixed' races are 

 crossed, there is usually a considerable variation 

 in the first generation. This is well illustrated by 

 the crossing of different clons of apples, pears, 

 oranges, and the like, when the different so-called 

 varieties are simply transplanted parts of the same 



jUiii 



Fig. 85. Planting individual grains of flax and other cereals 

 so that the individual growths of the plant may be watched 

 and selection made from the very best. This miicliiiie 

 allows a man to know exactly at what depth each is 

 planted, so that each grain has an equal chance with tlie 

 others. 



individual seedling which have not been bred to a 

 fixity of type. It is well known that if seeds of an 

 apple variety be planted, the resulting plants exhibit 

 many different variations in the first generation. 

 The parents themselves, therefore, not being of fixed 



