Heredity — Genetics 677 



primrose (Oenothera gigas) from the common primrose (Oenothera 

 Lamar ckiana). The new species appeared suddenly and possessed a 

 double number of chromosomes in the nuclei, and this double number 

 of chromosomes remained constant afterward. De Vries had no expla- 

 nation for this phenomenon, but in recent years it has been discovered 

 that it may take place naturally or can be induced by certain chemicals, 

 by such physical influences as x-rays, high or low temperatures, or cen- 

 trifuging, or by removing the growing tip of a plant. The most striking 

 results have been obtained by using a drug called colchicine (koF ki sin) 

 which is an alkaloid poison (C22H25O6N) extracted from the seeds of 

 the plant called meadow saffron. Colchicine is almost a specific for in- 

 ducing the doubling of chromosomes without cell division in plants. If 

 a 1 per cent solution is applied to the growing buds or tips of plants, 

 cells are formed with the double number of chromosomes. Frequently 



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Y'lg. 335. — Chromosome from the salivary gland of the larva of a black fly 

 (Simulium sp.), an insect of the order Diptera (camera lucida drawing). The 

 longitudinal threadlike bands are called chromonemata, consisting of a linear 

 series of granules, the chromomeres, which have a specific arrangement of group- 

 ing. ^ is a semidiagrammatic representation of the types of chromomeres and 

 the ways in which they are connected. At a in the main figure there are two 

 rows of dotlike chromomeres associated in pairs. The band at h is composed of 

 fifteen or sixteen vesiculated chromomeres closely packed together; c-h are other 

 groupings of chromomeres along the chromonemata of the chromosome. (From 

 Painter and Griffen: Chromosomes of Simulium, Genetics 22: 616, 1937.) 



redoubling as many as three times takes place. The colchicine inhibits 

 cell division while chromosome division continues as usual. This change 

 is permanent because cells with doubled chromosome numbers continue 

 to divide to form tissues, seeds, and embryos, all with double numbers of 

 chromosomes. 



The ease with which many plants may be artificially induced to double 

 their chromosome number may explain the frequency of its occurrence 

 in Nature. Miintzing concludes that more than half of the species of 

 flowering plants, including many of our economically important plants, 

 which have been studied have doubled or redoubled chromosomes. For 

 example, different species of wheat with 14, 28, and 42 chromosomes are 



