Plants of Southern California: Mentzelia albicaulis and M. montana
Plants of Southern California: Mentzelia albicaulis and M. montana Fig. 1. Flower Bract of Mentzelia albicaulis. Note there is no white area at base; only the different color of the midvein. The bract is narrowly lanceolate and entire. Fig. 2. Flower Bract of Mentzelia montana, from Lily Spring in the San Gabriel Mountains at an elevation of about 7000 feet. Note the white area at the base of the bract, the almost round shape of the bract, and the five teeth / lobes. Click on the pictures for larger versions.
Mentzelia albicaulis and M. montana are two very similar species. The only significant differences between them given in the Second Edition Jepson Manual treatment are:
- The color of the base of the flower bracts (the leaf at the base of the flowers and fruit) is different. The bract of M. albicaulis is green, occasionally ± white base, faint, small, whereas the bract of M. montana has base ± white. Although this characteristic is used in the key to separate them, the key ignores the occasional white base for M. albicaulis, making the separation seem clearer than it actually is. Many specimens may be determined cleanly from this characteristic, but some specimens may be problematic since both species are described as occasionally having the same ± white base.
- The shape of the flower bracts differs. The bracts for M. albicaulis are lanceolate to ovate, 3-toothed to entire, whereas the bracts of M. montana are lanceolate to obovate, 3–7-toothed. This characteristic can also readily determine some specimens, but specimens with lanceolate, 3-toothed bracts are ambiguous. (See an important note on separating a bract from a leaf.)
- The curvature of the fruit can sometimes differ. M. albicaulis occasionally has fruit curved up to 180°, whereas the fruit of M. montana is said never to curve more than 45°.
And that's it for the significant distinguishing characteristics from the Jepson Manual treatment! The numeric ranges for every other morphological quantity are essentially identical.
The geographic distribution is said to be different for these two species, with M. albicaulis in the desert, Great Basin, Modoc Plateau, extending into Teh, SnGb and SnBr, and with M. montana in the desert mountains and in the higher elevations to the west of M. albicaulis. This translates into a difference in elevation range. The elevation range for M. albicaulis is given as 0-2300 m (0-7500 feet), and the range for M. montana is 600-3400 m (2000-11,150 feet).
However, voucher plots show a much-less-distinct separation of the range of these species, with the primary difference being that M. albicaulis is very widespread in the desert, with only a very few vouchers of M. montana present in a few mountains in the desert regions.
From plots of the geographic distribution and elevations of vouchers, one might reasonably ask whether M. montana is just the higher-elevation form of M. albicaulis. Both plots for each species are here: M. albicaulis and M. montana. Open the links in separate browser windows and compare them, noting that the elevation plots have different blue-shaded regions since the Jepson Manual elevation ranges are different for the two species. You may have to resize the geographic plot to put them on the same scale.
Furthermore, the Jepson Manual says that M. albicaulis intergrades with M. montana, implying that at mid-elevations, plants are intermediate between the two species and the species do not retain their separate identities there. This adds to the speculation that M. montana is just what M. albicaulis looks like at high elevations.
Remember, this is just speculation. The chromosome numbers differ between these species, with M. montana having 2n=36, and M. albicaulis having 2n=54 and 72, so there are some possibly-significant differences between the species. However, one wonders why 2n=36 was separated out, and 2n=54 was not. Also, creosote has three different chromosome numbers, 2n = 26, 52 and 78, with each chromosome number population residing in different areas, but these populations are all considered the same species. It would be very interesting to see what DNA has to say about the differences between M. montana and M. albicaulis.
Of course, these species might be in the process of speciation, with the end points at low and high elevation having distinctly different plants, but with them still able to hybridize where they overlap. The flower bracts show in Fig. 1 and Fig. 2 above certainly look markedly different, and flower bracts are important species disciminants in Mentzelia.
This page primarily exists to hold some photographs of the bracts of the two species, so I and others can see how they vary from place to place.
Important note: Care must be taken to distinguish a leaf in the inflorescence from a flower bract! For example, in this picture, it would be easy to mistake the leaf at lower left as the bract, since it appears to be right next to the flower. But note that the stem continues up from that leaf, and the flower bract is on the right, touching the base of the fruit. All the bracts in this pix are entire to 3 lobed, whereas the leaf is at least 6 lobed, and maybe 7 lobed.
Fig. 3. Pictures of the flower bract for M. montana
From Lily Spring Area, San Gabriel Mountains, near 8000 feet elevation
Fig. 4. Pictures of the flower bract (and one fruit recurved 180°) for M. albicaulis
From Arroyo Salado, Borrego Badlands, near 800 feet elevation From "Canyon 41", the canyon just south of Moonlight Canyon in Agua Caliente County Park, Tierra Blanca Mountains, near 1400 feet elevation From Glorietta Canyon, just southwest of Borrego Springs, near 1400 feet elevation From the Whale Peak Trail, Vallecito Mountains, north of Agua Caliente County Park, from five separate locations along the trail, from 4200 feet elevation to 5300 feet elevation. Location 1, at 4200 feet elevation, near the Schad Trailhead Location 2, at 4250 feet elevation. Location 3, at ~4320 feet elevation. Location 4, at ~4550 feet elevation Location 5, at 4800 feet elevation
Copyright © 2012 by Tom Chester
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Last update: 26 April 2012