- A study was made to clarify the relationships of Mentzelia mollis
Peck and two poorly understood taxa of Mentzelia (Loasaceae) to the
other members of the genus, its sections, and its subsections, and
to describe the general ecology of each taxon.
Within the genus, the section Trachyphytum is characterized by
pendulous seeds, an annual habit, and placentae that do not expand
as the seeds mature. The section is further divided into three subsections
on the basis of width of filaments and shape and surface
texture of the seeds.
Seven populations of M. mollis were found, all growing in a
floristically depauperate community on greenish clay derived from the
Sucker Creek Formation and all within the Succor Creek drainage of
Malheur County, Oregon and Owyhee County, Idaho. The Leslie
Gulch Mentzelia occurs only in Leslie Gulch, west of Succor Creek,
at the base of talus slopes of the same Sucker Creek Formation.
The Colorado Mentzelia has been found only in Colorado and Utah,
near the border between the two states.
Morphological and cytological comparisons of M. mollis and
the Leslie Gulch Mentzelia show differences in floral structure,
branching pattern, leaf shape, seed size and shape, and ploidy level.
Mentzelia mollis is a tetraploid (n=18) and the Leslie Gulch Mentzelia
is an octoploid (n=36). Both have narrow filaments, pendulous seeds,
and placentae which do not expand as the seeds mature. Comparison
of the Colorado Mentzelia with M. mollis shows similar branching
pattern and seed size, but differences in the seed surface detail and
in floral structure. The scanning electron microscope was utilized
to compare seed shape and surface detail of these three taxa with
that of three other species of section Trachyphytum.
Both M. mollis and the Leslie Gulch Mentzelia have flowers
which open about 2:30 p.m. and close after dark, and both are visited
by the same Bombyliid-type insects in early evening. Both tolerate
extraordinarily high concentrations of potassium, sodium, and
calcium, as well as high soil surface and air temperatures.
Placement of all three taxa in section Trachyphytum was made
on the basis of relative placental growth and annual habit. Seed shape
and surface detail justified their placement in subsection Trachyphyta.
Morphological considerations and ploidy level seemed sufficient evidence to describe the Leslie Gulch Mentzelia as a new species,
Mentzelia packardiae Glad; and morphological differences and geographical
separation indicated that the Colorado Mentzelia should be
similarly handled. It was described as Mentzelia thompsonii Glad.
An existing key was adapted to include all three species.
Speculation concerning possible origins of M. mollis and M.
packardiae leads to the hypothesis that M. mollis is a relic which
survived the climatic and geological fluctuations of the past. Its
origin is considered in relation to its being a member of a mature
polyploid complex. It is also considered possible that there may be
some relationship between M. mollis and the possibly younger M
packardiae. Both species are edaphic endemics, and their evolution
is briefly considered in that respect.