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DAVID D. KECK
morphological gap. On the other hand, the morphological gap between
1 of these 7-chromosome species (L. phtyglossa) and 2 of the 8-chromosome species (L. septentrionalis and L. gfondulosa) is rather trivial.
The ecological characteristics of Layia species are also of considerable
interest. Many species can succeed in a considerable variety of valley
soils ranging from light to heavy. Layia Chrysanthemoides (DC.) Gray,
L. fremonta (Τ. & G.) Gray, and L. heterotricha (DC.) Η. & Α., however, are found almost exclusively in rather neutral heavy adobe soils;
L. munzii Keck occurs in alkaline adobes; L. discoidea is confined to serpentine, whereas its closest relative, L. gfondulosa, is always found in
sandy soils; and finally, L. carnosa is confined to coastal sand dunes.
Such clear-cut separations are very helpful taxonomic markers, for they
represent physiological characters as distinct as many of the morphological ones.
Were it advisable to divide the genus into sections, a course that has
not seemed necessary, the comparative morphologist would doubtless
make his choice where the most profound morphological break occurs
and set off the 5 species having scaly pappus from the other 10 having
bristly pappus. The former all have 7 pairs of chromosomes. One of the
latter group, however, also has 7 pairs. If the cytologist were to propose
sections in the genus, he would put the 7-chromosome group together
as opposed to the remainder of the species, which are in an 8-chromosome series. The geneticist when faced with the same question might be
inclined to make the sectional lines coincide with those of the 5 cenospecies. Thus the three approaches to the question would provide three
different answers. Use of the joint approach here emphasizes the undesirability of maintaining sections or subgenera in Layia. The cohesiveness of the genus is more significant than the morphologic-geneticcytologic variability. "Evidence of relationship should, therefore, not be
sought along either morphological, ecological, genetical, or cytological
lines alone, but through the employment of all four criteria considered
together, because each contributes to a more complete understanding
of it" (Clausen et al 1941).
When different lines of evidence would seem to pull in different directions, a decision has to be made as to the relative importance to be
accorded each line, and in such cases the resultant classification may be
considered temporary until further firm evidence develops. If all the
evidence is made freely available, there can be only benefit from this, as
future work can build upon these foundations.
The classification of Layia based on all the available data, including
the cytogenetic, differs from the earlier one based on classical methods of
DAVID D. KECK
morphological gap. On the other hand, the morphological gap between
1 of these 7-chromosome species (L. phtyglossa) and 2 of the 8-chromosome species (L. septentrionalis and L. gfondulosa) is rather trivial.
The ecological characteristics of Layia species are also of considerable
interest. Many species can succeed in a considerable variety of valley
soils ranging from light to heavy. Layia Chrysanthemoides (DC.) Gray,
L. fremonta (Τ. & G.) Gray, and L. heterotricha (DC.) Η. & Α., however, are found almost exclusively in rather neutral heavy adobe soils;
L. munzii Keck occurs in alkaline adobes; L. discoidea is confined to serpentine, whereas its closest relative, L. gfondulosa, is always found in
sandy soils; and finally, L. carnosa is confined to coastal sand dunes.
Such clear-cut separations are very helpful taxonomic markers, for they
represent physiological characters as distinct as many of the morphological ones.
Were it advisable to divide the genus into sections, a course that has
not seemed necessary, the comparative morphologist would doubtless
make his choice where the most profound morphological break occurs
and set off the 5 species having scaly pappus from the other 10 having
bristly pappus. The former all have 7 pairs of chromosomes. One of the
latter group, however, also has 7 pairs. If the cytologist were to propose
sections in the genus, he would put the 7-chromosome group together
as opposed to the remainder of the species, which are in an 8-chromosome series. The geneticist when faced with the same question might be
inclined to make the sectional lines coincide with those of the 5 cenospecies. Thus the three approaches to the question would provide three
different answers. Use of the joint approach here emphasizes the undesirability of maintaining sections or subgenera in Layia. The cohesiveness of the genus is more significant than the morphologic-geneticcytologic variability. "Evidence of relationship should, therefore, not be
sought along either morphological, ecological, genetical, or cytological
lines alone, but through the employment of all four criteria considered
together, because each contributes to a more complete understanding
of it" (Clausen et al 1941).
When different lines of evidence would seem to pull in different directions, a decision has to be made as to the relative importance to be
accorded each line, and in such cases the resultant classification may be
considered temporary until further firm evidence develops. If all the
evidence is made freely available, there can be only benefit from this, as
future work can build upon these foundations.
The classification of Layia based on all the available data, including
the cytogenetic, differs from the earlier one based on classical methods of