CYTOLOGY AND TAXONOMY 



243 



cross-pollination, or gametic incompatibility. Its independence may 

 be almost as complete even when hybrids are produced, for these may be 

 sterile or otherwise unable to compete with either the new type or its 

 parents. The genetic mechanisms responsible for hybrid sterility do 

 not, however, seem to be directly correlated with the genetic changes 

 producing visible specific differences. 



Another form of isolation is that based on adaptability to ecological 

 habitat. Peculiar physiological characters may not only keep the new 

 type and its relatives apart within a relatively small region, but they 

 may ♦lead to very wide differences in geographical range. Examples 



Fig. 173. ^Relationship between polyploidy and geographical distribution in the 

 American species of Tradcscantia. Outer heavy line: maximum distribution of tetraploid 

 species. Inner heavy line: maximum distribution of diploid species. Heavy cross-lined 

 area: minimum distribution of diploid species. Centered about tliis last area are the 

 known areas of four diploid species elsewhere tetraploid. {After E. Anderson.) 



of this are the following. In Tradescantia virginiana and its relatives 

 growing in the United States it is observed that the diploids grow mostl}' 

 in the south, while the autotetraploids, which grow in a greater variety 

 of habitats, have a much wider and more northern distribution (Fig. 173). 

 It is not thought, however, that autotetraploidy in this genus ranks with 

 genie differentiation and hybridization as a cause of speciation. In 

 Biscutella, a genus of cruciferous plants, the derived tetraploid types are 

 evidently spreading more rapidly in Europe than the diploids. A survey 

 of the angiosperm flora of Schleswig-Holstein has shown that the per- 

 centage of polyploid types is twice as great among northern species as 

 among southern ones, and that whereas diploids predominate on lime- 

 poor soils the polyploids constitute 95 ])or cent of the species found on 



