EXTRA CHROMOSOMES — BLAKESLEE 443 



The three examj^les tlmt have been jjiven involve compensation 

 l)etween two tertiary (.hroniosonies. A tertiary and a secondary 

 chromosome may also compensate. An example is the compensating 

 type involvini? the vrhite and the stippled chromosomes. Models of 

 the type are shown in Plate 13, Figure 2. Its formula is: 



1777— 17-18— /.S-l— 1-2— 2 1. 



Compensating types involving a secondary and a tertiary chromo- 

 some form a chain of five attached chromosomes at reduction 

 division. 



Another compensating type, similar to the one just given but of 

 different appearance, has the following formula: 



3-5— 3-4— 4-22— 22-21— 21-22. 



In addition to compensations between two tertiary chromosomes 

 and between a secondary and a tertiary chromosome, it should be 

 possible to obtain compensations between a fragment and a tertiary, 

 and between a fragment and a secondary chromosome. Such types 

 have not yet been obtained but crosses have been made from which 

 it will probably be possible to secure them. 



"With the examples given in mind, it w^ill be possible to define a 

 compensating type as one in which parts of two different composite 

 chromosomes compensate to form the equivalent in chromosomal 

 material of a whole normal chromosome, leaving the remaining non- 

 compensating parts of the two composite chromosomes to bring about 

 changes in structure in the plant affected. 



ORIGIN OF CHROMOSOMAL TYPES 



Of the balanced types, tlie In or haploid comes from the develop- 

 ment of an unfertilized egg cell and hence has the same reduced num- 

 ber of chromosomes as the sex cells. Ilaploids have arisen spontan 

 feously over a hundred times in our cultures of the Jimson Weed, bui. 

 what external stimulus if any is responsible for their occurrence is 

 not known. Haploids have been found in several other species. 



Tetraploids (4n) have been relatively abundant in Datura. They 

 frequently appear first as a 4n branch on an otherwise 2n plant. 

 This fact and other evidence leads to the belief that the doubling of 

 chromosomes takes place after fertilization has occurred. High 

 temperature at time of seed germination seems to stimulate the pro- 

 duction of 4n individuals. 



Triploids (3n), as earlier shown, are produced by crossing a 4n 

 individual with pollen from a 2n plant. 



Of the (2n-f 1) types, the primaries arise spontaneously from time 

 to time in our cultures but can be obtained in great numbers among 

 the offspring of 3n parents. 



