Plant Species of the Borrego Desert: Nyctaginaceae: Mirabilis bigelovii and Mirabilis tenuiloba species, Four O'Clocks

Bigelow's desert four-o'clock, Mirabilis bigelovii. Plant from Rockhouse Canyon, 1 February 2009; inset flower from Box Canyon, 2/11/09.
slender-lobed four o'clock, Mirabilis tenuiloba, from Alma Canyon, 15 December 2009

We have two Mirabilis species in the Borrego Desert that are quite similar at a glance: Mirabilis bigelovii (=M. laevis varieties retrorsa and villosa) and Mirabilis tenuiloba. Richard W. Spellenberg, in the Flora of North America treatment of Mirabilis, comments that it is difficult to separate many Mirabilis species because the flowers look similar, even though the species are morphologically and ecologically different.

M. bigelovii is widespread in the Sonoran Desert, whereas M. tenuiloba only grows on the extreme western edge of the Sonoran Desert. In the Borrego Desert, there are vouchers of M. bigelovii from at least ten locations. In contrast, there are vouchers of M. tenuiloba from just three locations: Borrego Palm Canyon; Elephant Trees Discovery Trail; Starfish Cove. The following maps give the distribution of vouchers with coordinates for each of these two species, obtained from a Consortium of California Herbaria search on 17 December 2009:

Click on the map for a larger version.

I have not yet had much experience in distinguishing these species, since I have seen so few specimens that might be M. tenuiloba. I have examined literally on the order of one hundred specimens in the Borrego Desert for the sparsely retrorse-pubescent stem hairs of M. bigelovii var. retrorsa, and all of the plants I have examined have had them. Fortunately, this apparently eliminates the possibility that any of them could be M. tenuiloba, as shown in the table below.

The floras report that these two species differ on the following characteristics:

CharacteristicM. bigeloviiM. tenuiloba
Stem hairsvar. retrorsa: often glabrous or glabrate basally, sparsely to densely retrorse-puberulent distally
var. bigelovii: moderately to densely villous or viscid-villous, increasingly dense and viscid distally
usually glandular-viscid
Leafspreadingspreading to ascending
Leaf blade width0.5-3.5 (5) cm1.7-7 (12) cm
Involucre length3-7 mm7-16 mm
Involucre lobestriangular to triangular-lanceolatenarrowly lance-oblong
Involucre lobes base width ratio to heightoften 70-100%30-50%
Perianth length8-16 mm12-18 mm

Other differences are given in Munz, such as the length of the fruit and the habit of the plant (decumbent to erect), but the Flora of North America gives essentially the same range for both species for those differences.

These are not many differences! Worse, the characteristics either overlap significantly, or there is a magic dividing line that separates the species, such as an involucre length of 7 mm.

As I always am in such cases, I was a bit skeptical about whether these two species were actually different. However, after studying the plants in the field in the Alma Canyon and Wash area on 15 December 2009, it did look like there were two separate species here.

The rest of this page reports my findings from that trip. It will be revised as I learn more about the differences between these two species. In particular, I may find that some of the differences seen here are not consistently seen in other places, so do not treat these differences as gospel. Also, some of these observed differences may be due to the specific environments of these plants (sun vs. shade; sandy soil vs. rocky soil; etc.).

First, there is a distinct gestalt difference in the overall appearance of the two species here, as well as some geographic separation. The plants of M. tenuiloba were found only in the Canyon itself and just below it; they had noticeably larger leaves, and they were more decumbent.

The pictures at the top of the page show a difference in the habit of the plant, with the stems of M. bigelovii being fairly erect whereas the stems of M. tenuiloba are almost prostrate. This difference was clear in Alma Canyon and Wash, but may not be always be the case, according to the Flora of North America. (Note that the picture above of M. bigelovii is from elsewhere, since I failed to take a picture of the overall plant in Alma Wash. All pictures below are from Alma Canyon and Wash.)

The leaf size was strikingly different in Alma Canyon and Wash. Specimens of M. bigelovii had leaf widths 10-20 mm for one plant, and 10-27 mm for another plant. Specimens of M. tenuiloba had leaves twice as wide; 22-43 mm for one plant, 30-45 mm for another plant, with a third plant having leaves up to 54 mm wide:

Also, many of the leaves of M. tenuiloba were ascending, whereas all of the leaves of M. bigelovii were spreading.

Second, the involucre lobes had the shape differences given in the floras. The lobes of M. bigelovii were indeed triangular, whereas the lobes of M. tenuiloba were oblong-lanceolate. No specimens of M. tenuiloba were in bloom, so the comparison below is for involucres that had finished flowering.

Mirabilis bigelovii
Mirabilis tenuiloba

However, note that the ratio of the lobes to the fused portion of the involucre is roughly the same for each species in this case, so that characteristic does not seem to be a good one to separate the species here.

Third, the length of the flowering involucre was indeed quite short for the M. bigelovii plants. I measured a range of 5.5 to 6.0 mm on the flowers of one plant, and another plant had every involucre being 5.0 mm. This is solidly in the 3-7 mm range for M. bigelovii, and solidly outside the range of 7-16 mm for M. tenuiloba.

However, the length of the fruiting involucre is not noticeably different. I measured 6-9 mm, with most being 8 mm, for one specimen of M. bigelovii, and 8-10 mm for one specimen of M. tenuiloba.

I'll continue to collect data on these two species and see what I can learn about them.

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Copyright © 2009 by Tom Chester.
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Updated 18 December 2009.