THE MUTATION THEORY 491 



essentially a cell problem. Cytological investigations have shown 

 that many of the mutations are concerned with new chromosome num- 

 bers, and the precise nature of the change which has led to the appear- 

 ance of forms with a new number is known with more or less certainty 

 in the various cases. The new number is in each case present through- 

 out the plant and will be found whether the chromosomes be counted 

 in growing petals, root tips or anthers. These discoveries led in 191 5 

 to the conception that each mutation with its new external characters 

 is the result of nuclear changes transmitted by mitosis to every cell of 

 the plant during development. A fundamental advance in the analysis 

 of mutations has thus been made. 



Mutations can now be classified into two types, (i) Mutations 

 which are inherited as Mendelian differences, and which may be looked 

 upon provisionally as the result of a chemical change in one gene or 

 locus of a chromosome. The great majority of the mutations in 

 Drosophila are of this type. On the other hand, very few Mende- 

 lian mutations are known in Oenothera. The two best known are 

 rubricalyx, which is a dominant in crosses, and brevistylis, which is 

 inherited as a recessive. (2) Mutations resulting from a visible nuclear 

 change involving a new number of chromosomes. Probably the major- 

 ity of Oenothera mutations belong here. Similar mutations have been 

 discovered in Drosophila and also in Datura. Examination of the 

 chromosome numbers in related species of wild and cultivated plants 

 shows that such changes have been of relatively frequent occurrence 

 in the evolution of many plant genera, but they appear to be less com- 

 mon in animals. 



CHROMOSOME MUTATIONS 



The mutations with new chromosome numbers, which are partic- 

 ularly characteristic of Oenothera, may be classified into three 

 types: — (i) tr is omic forms, i.e., mutations with one or sometimes two 

 extra chromosomes (2W+1); (2) triploid forms (3»); and (3) tetraploid 

 (4«) forms. Other forms with different numbers, as 20 or 30, may be 

 regarded as derivatives from these types. 



Trisomic forms are known to arise through non-disjunction, or a 

 reduction division in which both members of one pair of chromosomes 

 enter the same daughter nucleus. The best known mutation of this 

 kind is Oenothera lata with 15 chromosomes (0. lamarckiana having 

 14). But a number of others are now known, including scintillans, 

 albida, oUonga, subovata, and more recently cana, pallescens, ladiica, 

 and liquida. Several other derivatives of Oenothera lamarckiana are 

 also from their behavior almost certainly trisomic. When pollinated 



