446 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



somes in a circle the greater the opi^ortunity for the occurrence of 

 compensating types in the offspring. 



SUMMARY OF CHROMOSOMAL TYPES IN DATURA 



In the preceding section the manner of origin of the most impor- 

 tant chromosomal types in the Jimson Weed has been discussed. In 

 the present section a summary will be given of the types so far 

 identified in this species. 



In Table 2 (p. 450) the main chromosomal types are listed together 

 with the number of different forms actually identified and the number 

 theoretically possible for each type. The calculations are based on 

 the assumption that the two parts into which a chromosome breaks 

 are always equal. This assumption is Imown to be contrary to fact 

 and in consequence the number of forms theoretically possible is 

 much larger than the calculated figures. Formulae are given in the 

 last column which enable one to apply the calculations to species 

 with different numbers of chromosomes. Primary, secondary, and 

 tertiary chromosomes are represented by the Roman numerals I, II, 

 and III. It should be pointed out that these three kinds of extra 

 chromosomes are defined in terms of our standard line 1. The class 

 numbers in the column at the left may be of convenience for refer- 

 ence. 



Of the unmodified balanced types, In, 2n, 3n, and 4n forms are 

 the only ones so far obtained in Datura. Doubling of the 3n and of 

 the 4n types should give 6n and 8n forms but these theoretically 

 possible forms have not yet been found in Datura. 



Class 5 are the primary (2n + l) types. Of these types, each with 

 an unmodified chromosome extra, there can be only as many as 

 there are different kinds of chromosomes. All the 12 possible to 

 Datura have been identified. 



Class 6 are the secondary (2n + 2/2) types in which the extra 

 chromosome consists of a doubled half chromosome. Since there 

 are 24 halves that can be doubled, there are 24 secondary types that 

 are theoretically possible. Of these, 14 have been identified and 

 kept under cultivation. Two of the primaries (El and Sp) have 

 very poor viability but each has a single secondary with good via- 

 bility. Since a primary is intermediate in character between its 

 two secondaries, it is likely that the missing secondaries of El and 

 Sp would have too poor viability to permit them to survive even if 

 they were actually formed. Others of the missing secondaries may 

 also be nonviable. For convenience in explanation, we have spoken 

 of the secondary chromosomes as having been formed by a doubling 

 of one-half of a normal chromosome. There is evidence, however, 

 that the parts into which a chromosome breaks in the formation of 

 a secondary are not always equal. 



