DOMESTICATED PLANTS — BLAYDES 329 



40 chromosomes and is a perennial. The annual teosinte has 20 

 chromosomes. The tetraploid form has been described as a new species. 



Some nurseries list as available a hybrid horse chestnut {Aesculus 

 camea). It is a tetraploid having 80 chromosomes and will "come 

 true" from seed. It is known to have resulted from a cross between 

 the horse chestnut {Aesculus hip-pocoManum) with 40 chromosomes 

 and the red buckeye {Aesculus pavia), also with 40 chromosomes. 

 With the exception of one case, the hybrids were sterile. In the one 

 instance, doubling of the chromosomes must have occurred, resulting 

 in a complete set of horse chestnut chromosomes and a complete set 

 from the red buckeye in the hybrid zygote. The chromosomes at 

 meiosis could pair and separate in the usual way, and the usual segre- 

 gation in a hybrid would be prevented. This illustrates how a new 

 species may be synthesized through hybridization and the doubling of 

 chromosomes. 



The bread wheat, as mentioned earlier, is thought to have arisen 

 in the region of ancient Babylonia. Bread wheat has 42 chromo- 

 somes. Emmer {Triticum dicoccum) with 28 chromosomes, another 

 wheat somewhat useful as livestock feed, crossed with a noxious weed, 

 goat grass {AegUops squarrosa) ^ with 14 chromosomes, and became 

 the parents of the world's most valuable food plant ! This must have 

 occurred by the doubling of the chromosomes in the zygote (fertilized 

 Qgg) or by the fusion of two unreduced gametes, giving the same result. 

 This kind of heritable variation is by no means uncommon, as one 

 might be led to believe from the above illustrations. It is probable 

 that similar changes were involved in the origin of such domesticated 

 plants as the potato, sugarcane, brome grass, cotton, tobacco, oats, 

 apples, pears, and strawberries. 



MUTATIONS 



New, unpredictable heritable changes in plants may occur, known 

 as gene mutations. Genes are invisible units of the chromosomes, 

 each with a potentiality which may become expressed as an observable 

 characteristic of a plant or animal. Ordinarily, genes are stable, 

 remaining unchanged through many generations bridging millions of 

 years. At the other extreme, there are genes that are unstable and 

 mutate frequently in the same way. An illustration of this is found 

 in a corn variety known as Variegated Pericarp or Calico. This maize 

 variety is characterized by the pericarp of the grain being finely 

 variegated with red stripes orientated lengthwise of the grain. Each 

 red stripe or sector is the result of a mutation as shown by Emerson 

 (1914, 1917) and studied recently by Anderson and Brink (1952). 

 Occasionally grains were found which had an entirely red pericarp. 

 The gene controlling the red color is stable, since, when red grains 



