Rhodanthus Section
Clarkia ID Guide
The six species of the Rhodanthus clade of the genus Clarkia can be difficult to tell apart, especially between C. amoena and C. gracilis ssp. sonomensis in Sonoma county and between C. arcuata and C. gracilis ssp. albicaulis. This guide aims to point out the morphological characters that differentiate these species.
This section has five diploid species (C. amoena, C. franciscana, C. rubicunda, C. lassenensis, and C. arcuata) and one tetraploid species (C. gracilis) that is thought to have arisen from a hybridization event between C. amoena and a C. lassenensis ancestor or a closely related extinct species. In addition, C. amoena has five subspecies and C. gracilis has four subspecies which very closely resemble one another.
Key
To get down to the species level identification for species in the Rhodanthus clade, only four questions need to be answered:
1. Is the immature capsule 8-grooved? Alternatively it is 4-grooved.
2. Is there a bright red spot at the base of the petal? Alternatively it could have no spot or a spot in the center of the petal.
3. Is the inflorescence straight at the tip? Alternatively it is recurved.
4. Is the mature stigma at the same height at the anthers? Alternatively it is exerted past the anthers.
Based on the answers to the four questions above (yes = yellow, no = red), the matrix below shows the resulting species ID.
*C. gracilis ssp.
If the immature capsule is 4-grooved, the petal has a basal spot, the inflorescence is nodding at the tip, and the stigma is exerted past the anthers, the individual could either be C. gracilis ssp. tracyi or C. gracilis ssp. albicaulis. These two species aren’t really morphologically distinguishable, but they have non-overlapping ranges so they can be identified based on the location they are found.
*C. amoena ssp.
If the immature capsule is 4-grooved, the petal is unspotted or has a central spot, the inflorescence is straight at the tip, and the stigma is exerted past the anthers, the individual could be any of the four C. amoena subspecies that isn’t ssp. caurina. See the full key for differentiating these subspecies.
Fruit and Capsule
Two species, C. lassenensis and C. arcuata, have uniquely 8-ribbed fruit and capsules, while all four other species have shallowly 4-grooved fruit and capsules.
For the most part, the capsules are all slim and cylindrical, with the exception of C. amoena ssp. whitneyi which has a uniquely spindle shaped capsule that is quite enlarged at the center. The horticultural variety of Clarkia amoena that is often found in wildflower mixes is thought to derive from this subspecies, and so many garden C. amoena have this unique capsule shape.
C. arcuata is found in an overlapping range with C. gracilis ssp. albicaulis, and these two species are often misidentified for one another. C. arcuata can be distinguished by the strongly 8-ribbed ovary/fruit/capsule, long hairs present on the ovary and calyx, and the short, squat appearance of the ovary/fruit.
Flower
One main characteristic for distinguishing these species is placement of the spot on the petal, though spotless varieties are found frequently in all of these species. Two species are spotted in the center of the petal, C. amoena and C. gracilis ssp. sonomensis. These two species overlap in Sonoma county and are indistinguishable from one another based on floral characteristics alone, so other characteristics, particularly inflorescence shape, should be used to differentiate these two species instead. Both are also highly variable in their floral appearance, with the central spot being present at varying strengths and the petals sometimes shading to white at the base. The others are all spotted at the base of the petal, except for C. gracilis ssp. gracilis which is spotless.
Similarly, the basally spotted flowers of C. arcuata, C. gracilis ssp. tracyi, and C. gracilis ssp. albicaulis are all mostly indistinguishable from one another. C. arcuata can be differentiated from C. gracilis based on the characteristics of the ovary and calyx. C. arcuata also has a deeply magenta/red pigmented style, while the C. gracilis style is white or light pink, and it often times has a large magenta patch on the back of the petal which is unique to this species.
Another main distinguishing characteristic is whether the stigma (when mature) is exerted past the anthers or not, which affects the degree to which the species is self-pollinating versus dependent on animal pollinators for reproduction (outcrossing). C. franciscana, C. lassenensis, C. amoena ssp. caurina (not pictured), and C. gracilis ssp. gracilis are all thought to be highly self pollinating, and the mature stigma is at equal height to the anthers so that pollen from the same flower is deposited on the stigma. These four selfing species also have much smaller flowers and petals than their closely related outcrossing species. All four species have petals that are less than 20mm, while all of the outcrossing species (except C. arcuata) have petals larger than 20mm.
Inflorescence
One major morphological characteristic that differentiates the species of the Rhodanthus clade is the shape of the inflorescence.
Three species (C. gracilis, C. lassenensis, and C. arcuata) have an inflorescence that recurves at the tip, with immature flower buds pointing down. Shown at left above.
Three species (C. amoena, C. franciscana, and C. rubicunda) have an inflorescence that is straight at the tip, with immature flower buds pointing up. Shown at right above.
Disambiguation with C. purpurea
C. purpurea is the most common species of Clarkia, and it can often be mixed up with species from this section, due to the similar appearance of the flowers.
C. purpurea has an inflorescence that is straight at the tip (similar to C. amoena above). It is most commonly highly selfing (stigma is not exerted past the anthers), but two of the subspecies are outcrossing (stigma exerted past the anthers).
C. purpurea never has a strongly pigmented central or basal spot, like most of the species in this section, and instead it often has a wedge-type spot at the outer edge of the petal that is unique to it and C. imbricata. It can also often be unspotted, have a faint purple basal spot, or be a deeply pigmented wine color.
Unlike the slim, cylindrical fruit of most of the species in the Rhodanthus section, C. purpurea has very short and squat fruit and capsules, most similar to those seen on C. arcuata. It almost always has long spreading hairs on the ovary that are retained even when the capsules are dry.
Kimmy Stanton 