Plant Species of San Jacinto Mountain:
Mimulus androsaceus and M. rubellus,

Fig. 1. Left: M. androsaceus. Right: M. rubellus. In each picture, on a typical computer screen, the inset at lower right displays the flowers at approximately their actual size, 3 to 7 mm wide for M. androsaceus and 3 to 4 mm wide for M. rubellus. Click on the pictures for larger versions. Photographs © Bruce Watts.

Scientific and Common Names of these Species

In the latest Jepson Manual eflora, and the Flora of North America, these species are now called Erythranthe androsacea and E. rubella. Erythranthe is the new genus name for monkeyflowers that have a pedicel generally longer than the calyx length.

The common names for Mimulus androsaceus are "androsace monkeyflower" and "rockjasmine monkeyflower". Michael Charters gives the origin of androsacea / androsaceus as "like Androsace, a small plant of the primrose family typically growing in rock gardens called rock jasmine".

The common names for M. rubellus are "little redstem monkeyflower"; "pygmy monkeyflower"; "red monkeyflower"; and, oddly, "red-stemmed himulus". You'd think "himulus" is a typo of mimulus, but it appears on a number of webpages with that spelling, so it is a widespread typo if that is what it is. Charters gives the meaning of rubellus / rubella as "pale red, becoming red". As far as we know, the stem color does not usually distinguish these two species.

Challenges in Identifying These Species

The first challenge in identifying these species is to find them. Most of our plants are so tiny that it takes sharp eyes to spot them. If you are not stationary and actively looking for tiny plants, you are unlikely to spot them.

The second challenge is that the differentiating characteristics are somewhat subtle. The killer distinction is the difference in the very-tiny markings inside the throat of their small flowers. But to even see those markings usually requires a hand lens (see the insets in Fig. 1 for what you'd see with your naked eye), and most of the time we've seen these plants there are no open flowers.

Other distinctions are how long the pedicels are relative to the calyx; the shape of the leaves; and the shape of the calyx.

Fig. 2 illustrates the challenges here. First, how many plants of these two species can you spot in the photograph? Second, note that there are no open flowers on these plants, just buds and finished flowers, so using the flowers to discriminate them is impossible without trying to pry open a tiny bud. Third, can you tell the species apart in this picture from their gestalt, using the length of the pedicel, shape of the leaves, and shape of the calyx?

Fig. 2. A standard (non-zoom) picture taken from a standing position (note Tom's shadow in pix) showing a mixed population of M. androsaceus and M. rubellus. Can you find the seven plants of these species in the pix, and separate them into two species? Click on the picture for a larger version with the seven plants outlined and two plants identified. Photograph by Tom Chester.

The rest of this page gives what we know so far about what our plants look like, and how to distinguish them. Much of this information is preliminary, since we have just begun to study these species, having found our first plants of M. androsaceus in May 2019.

Characteristics That Distinguish These Species

The Corolla Throat Markings

Our first impression is that the markings in the corolla throat are the only way to reliably determine the species from photographs. Fig. 3 shows the difference in the markings.

Fig. 2. Left: M. androsaceus. Right: M. rubellus. Click on the pictures for larger versions. Photographs © Bruce Watts.

The lower part of the corolla throat of M. androsaceus has yellow somewhat-meandering streaks interfingered with reddish blotchy streaks. In the field, with a hand lens, the yellow meandering streaks are fairly easy to pick up. In contrast, the lower part of the corolla throat of M. rubellus is mostly yellow, with small oval pink-reddish spots inside the throat, and larger pinkish linear streaks at the base of the corolla lobes.

The hairs in the lower throat are often red for M. androsaceus (but not always), and always yellow for M. rubellus.

The stigma at the top of throat reaches the base of the corolla lobes for M. androsaceus, but is a bit shorter for M. rubellus.

Pedicel Length

Abrams used the length of the pedicels, and the ratio of the pedicel to the leaf-like bracts, to distinguish the two species:

pedicels 5-20 mm long, less than twice the length of bracts, eventually deflexed-spreading ... M. rubellus
pedicels 15-27 mm long, 3 to 5 times length of bracts, permanently ascending ... M. androsaceus

However, we have plenty of plants of M. androsaceus (as determined from the corolla markings) with pedicels shorter than the length of the bracts, which would be incorrectly determined using this key. One difficulty here may be that the pedicel elongates in fruit, and/or the pedicel has different lengths between the first flowers and last flowers; see Fig. 4.

Fig. 4. A robust plant of M. androsaceus showing the variation in pedicel length in fruit from the first flowers and for the later flowers. Photograph by Tom Chester.

But the sense of the key is correct; many of our plants of M. androsaceus have pedicels that appear much longer than that of many of our plants of M. rubellus.

Show examples in Fig. 5 from Bruce or from Calphotos.

Leaf Shape

M. rubellus: linear to elliptic, proximal +- petioled, distal sessile, not clasping.

M. androsaceus: +- lanceolate to oblong or ovate, clasping.

Show examples in Fig. 6.

Calyx Shape

straight for M. androsaceus; straight to slightly curved for M. rubellus.

Show examples in Fig. 7.

Calyx Lobe Hairs

The JM key distinguishes the two species by M. rubellus having ciliate lobes, and M. androsaceus having non-hairy lobes. This can almost never be seen in pictures, since the length of the hairs is about 0.1 mm. But a hand lens should be able to see them.

Show photographs if they exist, or the line drawing from Abrams.

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Copyright © 2019 by Tom Chester and Bruce Watts.
Commercial rights reserved. All rights reserved for photographs of Bruce Watts. Permission is granted to reproduce any of the text of this page for individual or non-profit institutional internal use as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Updated 23 May 2019.