The Autoploids 327 



and Ixlow the hajjloid number. Another conuuon jMaetice is dou- 

 1)1 ing the triploid to make hexaploids. Such a bridge is regidarly fol- 

 lowed in Gassy piuni, where the hybrid between American tetraploid 

 and a species becomes a sterile triploid. 



Certain advantages may be gained from triploids thai are not 

 possible otherwise, if the optimum chromosomal number is closer 

 to tri})loid than tetraploid, production may i)e increased over either 

 diploid or tetraploid. If rij^ened seeds can be eliminated or reduced, 

 as in the trij^loid watermelon, a new type fruit is obtained. These 

 features in triploids are limited but seem important. 



Finally trijjloidy raises problems of seed production: an extra 

 propagation of parental stocks to preserve the two types, as well as 

 a specific hybridization to produce the seed for each generation. Suc- 

 cess may depend upon solving these problenrs. Triploid seeds do not 

 germinate as well as those of other polyploids. Finthermore, the 

 cross between tetraploids and dij)loids cannot be readily made for all 

 autoploids. 



i^.2-i: Triploids i)i xixitcrniclons. Reasoning from the lact that 

 seedless fruits in nature are due to certain reproductive failures, the 

 idea was conceived that seedless watermelons woidd result if triploids 

 were made. The female sterility notable among trijjloids would lead 

 to this achievement. Such work was initiated in japan in 19.H9. Ten 

 years later the first triploid watermelon fruits appeared on the market 

 in lai^an.-'"- '""• '"^ This may be regarded by practical breeders as a 

 very short time for the production of a new variety. Triploid water- 

 melons were a new concejjt in\olving hybridization and polyploidy 

 breedi ng procedmes. 



The tetraploid parents are produced by colchicine applied at the 

 seedling stage. These plants have 44 chromosomes and are easily dis- 

 tinguished from the diploid by seed size, ))ollen size increase, and 

 other characteristics. After the tetraj^loids are produced, these varieties 

 become the seed parent with the tliploids as jjollinators to make the 

 triploid.""- '""• ^•^•''' 



Seeds obtained from a tetraploid fruit and pollinated b\ the dip- 

 loid are triploid. Upon planting such triploid seed, fruits without 

 seeds may be had. Early in the season, and late, the ovides develop 

 hard coats that resemble seeds. These are emjjty. but the term seed- 

 less becomes meaningless when Iruits show these cores or empty seeds. 

 Therefore, the term trij^loid is far more desirable. To avoid these 

 difficidties, the fnst pistillate llowers are removed to elimiiiaic ihe 

 fruits with seed shells."' 



When triploid plants are growing, pollinations must be made by 

 diploids because the pollen of triploids (fio\vers) is not sufficient to 

 induce fruit development. 1 hus, interplanting diploids with trip- 



