Plant Species of the Bright Angel Trail:
narrow phacelia, Phacelia filiformis

See Plant Guide to Bright Angel Trail for an introduction to this page, especially the Introduction To These Species Pages.


Identification status: 100%, but there is apparently a taxonomic issue as to whether this species is distinct from P. glechomifolia.

These plants key easily using McDougall:

1' Leaves not all basal
2' Stems, leaves and capsules not prickly
3' Leaves lobed
5' Flowers not very small (not Eucrypta)
6 Lvs scalloped, not pinnately lobed or pinnate
7' Flowers lilac or purple [~20% of the flowers are white]
8 Lower flower stalks much longer than the calyx.... P. glechomifolia

McDougall states in his entry for Phacelia glechomifolia:

P. filiformis which was described as a new species from the park is probably synonymous with P. glechomifolia.

Although McDougall published his book in 1964, well after the 1951 Kearney and Peebles Arizona Flora, the 1987 Grand Canyon Checklist gave both species, and they are both given in the current online Grand Canyon Checklist.

The Kearney and Peebles key to separate these two species is:

11. Corolla 5 to 9 mm long; ovules not more than 25 .... P. filiformis
11'. Corolla 10 to 15 (exceptionally only 9) mm long; ovules 35 or more ... P. glechomifolia

I didn't measure any corollas in the field, but I have three photographs with my fingers for scale that can be used to measure the corollas. Measurements on close-up photographs require care, since the scale is usually not at the same distance from the camera lens as the object. Fortunately, in two of the three photographs, I have a finger both in front of and behind the corolla, and hence can get a fairly good measurement. In the third photograph, a portion of my finger is about at the same distance.

The three measurements are: 8.2, 8.6, and 8.8 mm, from photographs number 4, 12 and 3 shown below. This places them in the P. filiformis camp, although it is a bit uncomfortable that these corollas are close to the magic dividing line of 9 mm.

The question here is, of course: are these two species actually separate?

The claim that there are two extremely similar beautiful little phacelia species endemic to the Grand Canyon area, P. filiformis and P. glechomifolia, is a little hard to believe at face value. These two species have essentially the same geographic range (see below), look identical in overall habit, leaf, flower and fruit, and flower at essentially the same time. They appear to differ only in the size of their flower, and in their full elevation range (they have considerable overlap in elevation; see below).

I'm always skeptical when the difference between two species is only on a size, especially when there seems to be a magic dividing line (9 mm in this case) in what might be a continuous distribution in size from one species to the other. (Both the number of ovules and the corolla size might be correlated with plant size, and thus may not be independent characteristics.)

However, oftentimes species described in such keys are truly different, with other differentiating characteristics and distinct geographic ranges, and it just appears from the artificial key that they might not be. But in the case of these two species, I am not aware of any other differentiating characteristics, and they do not have distinct geographic ranges.

The common geographic distribution, and the significant overlap in elevation, make it hard to understand why these two species do not form a common breeding population. Some other barrier to crossbreeding must occur if these are separate species, and it must have happened so recently that the species have not had time to evolve significant differences in morphology, other than the size of the flower, or in geographic range.

However, there is a much simpler explanation that can account for the claimed differences between these two species. Many species that thrive at lower elevations become smaller at higher elevation, with smaller flowers, simply because growing conditions are not as optimal at higher elevation. Such species germinate later at higher elevation, and run out of soil moisture sooner in their lifetime after germination. For example, in California Mimulus guttatus produces long racemes of many large flowers at lower elevation, but produces only a small number of small flowers at its upper elevation range. (Note that not all species follow this pattern, especially those that grow only at higher elevations.)

An arbitrary division of a single such species on the corolla length will produce a higher elevation species with smaller flowers, and a lower elevation species with larger flowers. Both species will have the same geographic range, and the same overall morphology, and their elevation ranges will partially overlap.

The evolution of these plants is also much easier to understand if they are a single species. These plants are clearly associated with the Canyon, and the existence of the Canyon for 5 to 60 million years is a very plausible cause for the evolution of a single species adapted to the Canyon occupying the ecological niche of these plants. It is less clear why two species endemic to the Canyon area, with the same geographic range, inhabiting the same ecological niche, would evolve.

However, the above arguments are only hand-waving and circumstantial evidence; only the morphology (and DNA) of the plants can tell us whether there are one or two species here. Perhaps someone has measured a number of vouchers and performed a Principal Component Analysis to verify that there are indeed two species here. If so, please let me know!

If, however, no one has ever done a rigorous study as to whether these two species are separate, it is a simple matter to measure the extant vouchers and possibly resolve this issue. Even a simple plot of corolla size versus elevation might immediately tell the tale.

Here is an example of how easily the measurement of corolla size versus elevation might answer this question. The following two plots are from a very simple simulation, using the elevations of vouchers used in the analysis given below, assuming in the first case two separate species and in the second case a single species.

For the simulation of two separate species, I did a Monte Carlo simulation drawing the corolla length for each specimen randomly from a uniform population of 5-9 mm for vouchers determined as P. filiformis, and 10-15 mm for vouchers determined as P. glechomifolia. For the simulation of a single species, I drew the corolla length for each specimen solely as a function of elevation, ignoring the voucher determination, as specified in the following table:

Simulated Corolla Length Ranges Versus Elevation For One Species Model

Elevation (feet)Corolla min (mm)Corolla max (mm)

The results of the simulation are shown below:

Simulated Corolla Length Measurements Versus Elevation For One Species Model

Simulated Corolla Length Measurements Versus Elevation For Two Species Model

In the one species model, everything above the black line at a corolla length of 9.5 mm would be determined as P. glechomifolia, and everything below that length would be determined as P. filiformis. This simple model (without any tuning!) reproduces the observed elevation ranges for the two "species" well, including the overlap of their ranges between 3000 and 4000 feet.

There is a dramatic difference in the plots, so this issue might be easily resolvable.

Pictures of the two species

You can judge for yourself whether these two species show any visible difference by comparing pictures of vouchers at SEINet: P. glechomifolia; P. filiformis

Pictures of the plants from the Bright Angel Trail are below. These plants had corolla lengths of ~8.2-8.6 mm, placing them within the P. filiformis part of the key. Note that the plants in my photographs, from elevations of 6630 feet and 5450 feet, appear nearly identical to the plants of P. glechomifolia in the voucher ASU217024 taken at 1400 feet shown in the SEINet picture, just a bit younger.

Analysis of Geographic and Elevation Ranges

In order to examine whether these two species actually have different geographic ranges, I obtained voucher information for the two species from a SEINet search on 13 May 2008.

The following plot shows the geographic distribution of the vouchers of these two species which have geographic coordinates given:

The position of one voucher was corrected for a typo in its latitude.

There is no evidence that the two species have distinct geographic ranges. The eastern, northern and western limits of the two species are very close; only the southern limit in the west Canyon appears different. As shown in the plot below, that difference is almost surely an artifact of elevation.

In fact, the geographic distribution of these two species is almost exactly that predicted by elevation, under the hypothesis that this is a single species split into two by elevation.

The following plot shows the elevation versus longitude for vouchers that have both values:

Kearney and Peebles give the elevation range for P. filiformis as 3000 to 7500 feet; for P. glechomifolia as 4000 or lower, which is a good summary of most of the points in the above plot.

The only anomalous point in the above plot is the P. glechomifolia voucher from Cliff Spring on the North Rim at 8200 feet elevation. That determination might be in error, or the vouchered plant might simply have grown robustly due to its favorable location at a spring, producing flowers large enough to pass beyond the 9 mm threshold for P. glechomifolia.

The plot above is precisely what one would expect if one made an arbitrary cut in corolla length for a single species whose corolla became smaller with elevation. All low elevation plants would be one of those species; all high elevation plants would be the other. The two "species" would interfinger at intermediate elevations depending on growing conditions for each plant.

Other Characteristics

From a SEINet search on 13 May 2008, there are 28 vouchers of this species from the Coconino County portion of the Grand Canyon, including two from this trail at Redwall and Supai Formations; and along the "Devil's Corkscrew.". Note that there may be additional vouchers at other herbaria not available through SEINet.

First occurrence on Bright Angel Trail: just off-trail on the use trail at the switchback at mile 0.40, elevation 6630 feet (2021 m); on-trail at mile 2.00, elevation 5450 feet (1661 m).

Number of plants along Trail: at least 20 plants on-trail were found in at least 3 different locations in May 2008.


From 5 May 2008, mile 0.40:









From 5 May 2008, mile 1.95:




See Resources for Grand Canyon Flora for further information on most of these references. Entries in the second column are either the name used in that source or a page reference. The name is linked to online pages when available. If a given reference does not contain this taxon, the entry is either left blank or contains a hyphen.

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Copyright © 2008 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last Update: 15 May 2008