56 THE PLANT SPECIES 



The fourth difficulty of the species which will be mentioned 

 here is the reversion in many plant groups to asexual reproduc- 

 tion. The biological species is a community of cross-fertilizing 

 individuals linked together by bonds of mating and isolated re- 

 productively from other species by barriers to mating. Both the 

 internal organization and the external boundaries of species are 

 defined in terms of gene exchange. When sexuality is absent, the 

 biological species concept breaks down. Since the individual 

 members of asexual assemblages do not share in a common gene 

 pool, the group is not integrated into true species. The taxonomic 

 difficulties in apomictic groups like Crepis sect. Psilochaenia and 

 the clonally reproducing Opuntki, to cite only two examples listed 

 in Table I, are due to the development of swarms of variants, 

 including various hybrid types, which propagate themselves 

 asexually. 



Hermaphroditism, which prevails in the flowering plants, opens 

 up the possibility of another deviation from sexuality and cross 

 fertilization as it is known in the higher animals. Many her- 

 maphroditic plants set seeds by a mixture of outcrossing and 

 selfing, and many others, particularly among the annual herbs, 

 are autogamous or automatically self-pollinating. It might be 

 supposed that these deviations would contribute to partial or 

 complete breakdown of species integrity in partially or com- 

 pletely self-pollinating plants. Only under certain circumstances 

 (as in the autogamous but highly heterozygous Oenothera bien- 

 nis group) does this appear to be true. Regular self-pollination, 

 although it modifies the intraspecific variation pattern in im- 

 portant respects, does not prevent the integration of the separate 

 populations into species units in most autogamous groups. 



The explanation of the compatibility of self-pollination with 

 species integration seems to be that autogamy is rarely if ever 

 absolute. The self-pollination, although predominating for a 

 while, is supplemented by enough rare outcrossing to tie the 

 individuals together into populations and the populations into 

 species. 



To summarize, natural hybridization ranks high as a source of 

 a species problem in the higher plants. Sibling species and asexual 



