448 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1930 



Class 18 is a compensating type like those in class 17 which, in 

 addition to two tertiary chromosomes, contains an extra primary 

 chromosome. 



Class 19 includes types with a single fragment of a chromosome 

 extra. By inbreeding, types may ..be obtained with two of some 

 of these fragments. These are included in class 20. Similarly an 

 attached (or translocated) fragment may be present once or twice 

 to form the types represented by classes 21 and 22. A considerable 

 number of primary and secondary (2n + l) types have been com- 

 bined Avith the fragment types of class 19 and with the translo- 

 cated type of class 21. These compound types are distinct but have 

 not been included in the tabulations. 



The modified diploids (2n) listed in classes 5 to 22 are the most 

 distinct of the extra chromosomal types. As pointed out in the 

 discussion of the different " Globe " types shown in Plate 9, the addi- 

 tion of an extra chromosome to a 3n or to a 4n type brings about 

 relatively little change in the appearance of the plant affected. 

 However, 4n types may be strongly modified by the addition of a 

 larger number of chromosomes than would be possible in the dip- 

 loid (2n) series. Furthermore, deficiencies may be transmitted by 

 the sex cells of 4n parents since the normal sex cells of 4n plants are 

 2n. A (2n— 1) sex cell would be a deficiency but might be viable 

 since it would have at least one chromosome of each kind. The most 

 extreme modification of a 4n is that given in type 32. In two dif- 

 ferent sets. Doctor Belling found an extra chromosome and in 

 two other sets he found a single chromosome missing. The 3n and 

 4n types modified by deficiencies were determined by study of their 

 chromosomes. The plants were obviously abnormal but the ab- 

 normalities have not been connected up with the particular chromo- 

 somes which were missing. It is difficult to evaluate the unbalance 

 due to the addition or loss of single chromosomes in the 3n and 4n 

 series. 



By means of extra chromosomes and parts of chromosomes it has 

 been found possible to secure a wide range of variations in Datura 

 affecting the structure and physiology of all parts of the plant 

 that have been studied. In fact, chromosomal variations in this 

 species have been much more frequent than apparent changes in 

 the ultimate factors or genes, as may be seen by comparing the 

 genes listed in Table 1 with the chromosomal types in Tables 1 

 and 2. 



It is not believed that Datura is an exception among plants in 

 that it is capable of producing visible variations in large numbers 

 by means of major changes in amount of chromosomal material. 



