658 PHARMACEUTICAL BOTANY 
conclusions have been verified by many subsequent investigators 
and his methods extended to the discovery of mutants among 
many species of plants and animals. 
Curomosome Murations.—Since De Vries’ discovery, cyto- 
logical studies have been applied to the study of mutations. 
These have shown that many mutants possess new chromosome 
numbers throughout their structure. For instance, Oe¢nothera 
lamarckiana possesses 14 chromosomes while one of its mutants, 
Oenothera lata, has 15. . 
Po.yPpLoipy.—The normal somatic cells of the bodies of higher 
plants and animals have two sets of chromosomes (diploid). 
Polyploids are forms having multiples higher than two, e.g. three 
or more chromosome sets. Polyploids have been found to 
originate by hybridization followed by chromosome doubling. 
Extreme temperatures are claimed by Sax and others to pro- 
duce doubling of the chromosomes. Polyploidy has been found 
to account for many mutants in wild plants as the chrysanthe- 
mums, maples, “roses, apples, cherries, hawthorns, erigeron and 
rumex and in cultivated plants including wheat, oats, tobacco, 
sugar cane, dahlias, etc. 
Tue Mopern THrory or tHe Gene.—The current theory 
had its origin in the work of Thomas H. Morgan, an American 
geneticist, who discovered that the fruit fly, Drosophila melanogas- 
fer, was in a mutating condition. About 400 mutant characters 
have been identified in this insect. Morgan showed that the 
genes can be linked into four groups that correspond to four 
pairs of homologous chromosomes in the germ cells. The loca- 
tion of the characters or their determiners or genes on the 
chromosomes has been worked out by Morgan and his associates, 
and their experiments have shown that genes may cross over 
from one homologous chromosome to another. They have 
further shown that genes displaying linkage with each other are 
located in the same pair of chromosomes, that the substance of 
the chromosomes binds the genes to each other and causes one 
factor of a specific character to be inherited along with another 
gene on the same chromosome. 
Several explanations have been advanced concerning the 
causes of gene mutations or the appearance of new hereditary 
