212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 4 



particularly their end-to-end union in synapsis, forming rings or 

 chains, that cannot be dealt with here. 



Many such cases of supernumerary chromosomes have now been 

 discovered in various plants. The reduced number of chromosomes 

 is known as haploid (In)? ^^^^ usual condition resulting from the 

 union of two haploid sex cells is known as diploid {^n), that in 

 which there is one additional chromosome is 2,n+l, etc., that in 

 which a diploid unites with a haploid is known as triploid {^n), 

 that in which two diploid cells unite is a tetraploid (4n), and cells 

 with still larger numbers of chromosomes are called in general poly- 

 ploids. One of the most notable of these cases of supernumerary 

 chromosomes has been found by Blakeslee and his associates in the 

 numerous mutants of the common jimpson (or Jamestown) weed, 

 Datura stramoniwm. Here the typical diploid number is 24, but the 

 addition of one or another chromosome (2n.+ l) has given rise to 

 12 different mutants, while many other types are produced by the 

 further addition or subtraction of chromosomes, as well as by the 

 breaking in two of certain chromosomes and their recombinations, 

 a phenomenon known as segmental interchange, translocation, or 

 '' crossing over." 



Haploid, diploid, triploid, and tetraploid plants of one species 

 often differ markedly in appearance, and they breed true if the chrom- 

 osomes from the two parents are sufficiently alike so that they can 

 unite in synaptic pairs before the formation of the sex cells. Many 

 true Linnaean species are known that have their chromosomes in 

 multiples of some basic number common to all of them and these 

 species have probably arisen by multiplication of this basic number. 

 For examj^le, many species of roses, and indeed many genera of the 

 large family Rosaceae, have chromosomes in multiples of 7, and in 

 those genera where the basal number is 8 as in plums and cherries, 

 or 17 as in apples, hawthorns, and quinces, Darlington and Moffett 

 have shown that this unusual number has arisen from ancestral 

 species with 7, through nondisjunction of chromosomes at the time 

 of cell division. In wheat, oats, and barley the basal number of 

 chromosomes is 7, while different species have multiples of this 

 number. Different species of Chrysanthemum have chromosomes in 

 multiples of 9; more than 40 species of groundsel {Seneclo) have 

 chromosomes in multiples of 10; seven species of docks and sorrels 

 {Rumex) also have chromosomes in multiples of 10. Many other 

 similar cases of wild species with chromosomes in multiples of some 

 basic number could be cit^d. In other native species as in the genera 

 Viola and Crepis, chromosomes may be in multiples of some basic 

 number, or they may be that basic number plus one or two, as in some 

 mutants of Oenothera and Datura. 



