156 



HORTICULTURE 



covering an interior tissue of tliorniness. 

 When a "thonilcss" blackberry' is prop- 

 agated from stem cuttings, the resulting 

 plants are thomlcss because the thorn- 

 less tissue still continues as the other 

 layer. But when propagation is bv root 

 cuttings the resulting plants are fre- 

 quently thorny, for the reason that 

 roots arise from the internal "thorny" 

 tissue. 



Further, a plant chimaera may con- 

 sist of a mixture of diploid and poly- 

 ploid tissues, in which the t}pical 

 number of chromosomes (diploid) 

 may be covered or mixed with a higher 

 (polyploid) or lower (haploid) number 

 of chromosomes. Since polyploid tis- 

 sues are frequently coarser than tissues 

 which contain a smaller number of 

 chromosomes, the result may be apple 

 fruits with uneven polyploid sectors or 

 ribs, flowers with large and small petals 

 in the same flower, and anthers which 

 contain pollen with varying chromo- 

 some number. When such anomalous 

 flowers are used as parents in breeding, 

 the resulting progenies are in conse- 

 quence confusing and unpredictable. 



But the plant breeder is no longer 

 ignorant of these situations. By means 

 of a drug (colchicine) derived from a 

 species of Colchicum, he is able to in- 

 duce an artificial change in one season 

 so as to provide him with just the plant 

 material he needs for breeding pur- 

 poses. 



Thus, the southern muscadine 

 grape is a diploid with 40 chromo- 

 somes. It does not cross readily with 

 the northern bunch grape which has 38 

 chromosomes. By treating both species 

 with colchicine, the chromosome num- 

 ber of each has been doubled (tetra- 

 ploid) . These new forms will now com- 

 bine to blend the characters from both 

 species. Again, a variety of apple may 

 have three sets of chromosomes (trip- 

 loid), and may not be useful as a 

 parent in breeding because of the ab- 

 normalities which arise during reduc- 



tion division. Happily, the plant 

 breeder can induce a doubling up of 

 the chromosome number to form a 

 fertile hexaploid which can now be 

 successfully used in breeding. 



TTie plant breeder has been given 

 additional new tools by the geneticist, 

 cytologist, chemist and plant physiolo- 

 gists. By performing a Caesarian section 

 on immature fruits, removing the par- 

 tially developed embryos, and culturing 

 them like incubator babies as with 

 certain species of Prunus, he has suc- 

 ceeded in making heretofore impossible 

 crosses. 



The contributions of hybrid vigor 

 have been put to valuable use. In the 

 case of those plants which carry the 

 male flowers and the female flowers 

 in separate parts, as in com, it has 

 been possible to remove the male parts 

 (tassels) easily and so effect the cross- 

 fertilization from selected inbred lines 

 to produce seed stocks with hybrid 

 vigor and other desired characters. But 

 with many other plants which carry 

 both male and female parts in the same 

 flowers, as the onion, snapdragon, to- 

 mato, petunia and carrot, the tedious 

 separation of male and female parts has 

 made hybrid seed expensive and diffi- 

 cult. Here again, however, the plant 

 breedei has used his scientific skills 

 eflPectively. He has located individual 

 plants which are by nature male sterile, 

 as with male-sterile onions. He has 

 found ways of producing lines of male- 

 sterile onions, which when planted 

 with selected male-fertile lines produce 

 an abundant supply of hybrid onion 

 seed. Other vegetable and flower 

 plants are responding to this approach, 

 promising to revolutionize the seed 

 trade. 



Pollination and fruit set. — As re- 

 cently as the 1890's pollination and 

 fruit set were little understood by com- 

 mercial fruit growers, and their impor- 

 tance was poorly appreciated. Com- 

 patabilities and incompatabilities are 



