TRENDS IN SYSTEMATIC BOTANY
89
The sexual forms occur mostly west of the Cascades and show a moderate
amount of variation. The Sierra Nevada representatives are apomictic
and very uniform. They differ slightly but perceptibly from the Rocky
Mountain forms, which are also apomictic, indicating that the apomicts
cannot all belong to one clone. Even though in general the species is
morphologically uniform, apomixis has possibly arisen in it several times
or mutations have entered into its history, or both. In Poa pratensis
the apomictic strains produce measurably more uniform offspring than
the sexual strains. The remarkable feature of such very widespread
species as these two Poas is that they retain their specific identities so
clearly even with this mixture of reproductive methods.
Taxonomists have treated apomictic genera variously. Unfortunate
examples of attempts to recognize taxonomically all the morphologically
distinguishable apomictic clones in some large variable genera are afforded by works on Hieracium (Zahn, 1921-1923; Pugsley, 1948), Taraxacum (Dahlstedt, Haglund, Marklund, M. P. Christiansen, and other
Scandinavian authors in many papers), and Crataegus (Murrill, 1940).
These workers were not experimentalists and were scarcely aware of the
biological background of the overwhelming number of variants that
faced them in the herbarium.
A much more practical classification has been proposed for the American species of Crepis (Babcock and Stebbins, 1938), in which careful
cytological analyses, pollen examinations, and castration experiments,
combined with thorough field work, disclosed the presence and extent
of the diploid species, the polyploids, and the apomicts. Twelve species
are recognized, of which three are wholly sexual and nine have both
sexual and apomictic components. In the latter nine the sexual components are very restricted both in distribution and in the degree of
morphological variation. They are linked to a considerable extent by
hybridization on the polyploid level, where the great bulk of their representatives are found. Primarily as a result of apomixis these polyploids
break up into ' microspecies," but in this treatment these are not accorded
formal taxonomic status but are listed as apomicts with a Latin name.
Accordingly, the biological picture in American Crepis is faithfully
depicted within the bounds of a utilitarian taxonomic classification.
Before this experimental work disclosed their true biological nature, the
classification of American Crepis forms was wholly inadequate.
Hybrids between sexual and apomictic species may reproduce apomictically as in Calamagrostis
purpurea crosses (Nygren, 1946), or, as is
more commonly the case, they may be sexual, as in PotentiUa (A. Müntzing and G. Müntzing, 1945), Poa (A. Müntzing, 1940; Akerberg, 1941)
89
The sexual forms occur mostly west of the Cascades and show a moderate
amount of variation. The Sierra Nevada representatives are apomictic
and very uniform. They differ slightly but perceptibly from the Rocky
Mountain forms, which are also apomictic, indicating that the apomicts
cannot all belong to one clone. Even though in general the species is
morphologically uniform, apomixis has possibly arisen in it several times
or mutations have entered into its history, or both. In Poa pratensis
the apomictic strains produce measurably more uniform offspring than
the sexual strains. The remarkable feature of such very widespread
species as these two Poas is that they retain their specific identities so
clearly even with this mixture of reproductive methods.
Taxonomists have treated apomictic genera variously. Unfortunate
examples of attempts to recognize taxonomically all the morphologically
distinguishable apomictic clones in some large variable genera are afforded by works on Hieracium (Zahn, 1921-1923; Pugsley, 1948), Taraxacum (Dahlstedt, Haglund, Marklund, M. P. Christiansen, and other
Scandinavian authors in many papers), and Crataegus (Murrill, 1940).
These workers were not experimentalists and were scarcely aware of the
biological background of the overwhelming number of variants that
faced them in the herbarium.
A much more practical classification has been proposed for the American species of Crepis (Babcock and Stebbins, 1938), in which careful
cytological analyses, pollen examinations, and castration experiments,
combined with thorough field work, disclosed the presence and extent
of the diploid species, the polyploids, and the apomicts. Twelve species
are recognized, of which three are wholly sexual and nine have both
sexual and apomictic components. In the latter nine the sexual components are very restricted both in distribution and in the degree of
morphological variation. They are linked to a considerable extent by
hybridization on the polyploid level, where the great bulk of their representatives are found. Primarily as a result of apomixis these polyploids
break up into ' microspecies," but in this treatment these are not accorded
formal taxonomic status but are listed as apomicts with a Latin name.
Accordingly, the biological picture in American Crepis is faithfully
depicted within the bounds of a utilitarian taxonomic classification.
Before this experimental work disclosed their true biological nature, the
classification of American Crepis forms was wholly inadequate.
Hybrids between sexual and apomictic species may reproduce apomictically as in Calamagrostis
purpurea crosses (Nygren, 1946), or, as is
more commonly the case, they may be sexual, as in PotentiUa (A. Müntzing and G. Müntzing, 1945), Poa (A. Müntzing, 1940; Akerberg, 1941)