Increase in Number of Species 201 



three fertile levels of ploidy occur, diploid, tetraploid, and hexaploid 

 with gametic numbers of 37, 74, and 111. Sterile perennial triploids 

 and pentaploids (3/i and 5n) also occur in nature where the parent 

 species overlap. More complex examples are found in the section 

 Clmmaemehnium of the genus Viola (Fig. 87) in which occur 



(Se) O 



'<6, 



sempervirens Doughs i i=== — ^==^=^ Baker 



O O CF — -^) O- 



nquaefolia 



crassQ maior 



n=i8^ O 



canadensis, , .,, 



sempervirens ruqulosa ^^^— ^ ' querxetorum NuUallii 



praemorsa 



5ervirens ruqulosa ^.^^ ^ ' Quercetorum 



qloheWa^^ Uouqlasn ,■• \ utahfen 



/~\ bifbra— iiif"'' ocellato lobata / purpurea-^ vcllicolo 



^ — rotundiiolio pubescens, oheltonu pedunculata, tomentosa 



qlabnusculo charlestonensis 



^^=5-^ Natural ». Suqocstcd ^ ^^ Ar|ificial 



^ hybrids ^ -*' ompniploids hybrids 



Fig. 87. Polyploid levels in the section CJiamaemelanium of the genus Viola. 

 (After Clausen, Stages in the Evolution of Plant Species, courtesy of Cornell 

 University Press.) 



four levels of polyploids above the diploid level (Clausen, 1951). 

 Several levels occur also in the genus Clarkia (Fig. 34) but with 

 irregular chromosome numbers because of the different numbers 

 in the original parent species. 



There is an apparent limit to the extent of polyploidy compatible 

 with viability and fertility, probably associated with increasing 

 problems of physiological ontogenetic adjustment with an increase 

 in genes affecting the same processes. The great bulk of plant poly- 

 ploids are tetraploids or hexaploids, but occasionally higher levels 

 occur. In the higher plants natural species seldom exceed 12-ploids 

 (Stebbins, 1950; Clausen, 1951). In mosses possible decaploids are 

 the highest natural level so far found (Steere, 1954); in ferns the 

 highest reported gametic chromosome numbers of 250 to 260 may 

 represent as high as a 20-ploid (Manton, 1950). 



