Evolvulus Vestiture

Vestiture as Differentia

by Bob Harms ( email-here

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The presence or absence of pubescence has commonly been used to distinguish E. nuttallianus & E. sericeus (not earlier distinguished from E. discolor). East of the Pecos River, where the former is known to occur, this is a reliable differentia. West and south of this area, where E. discolor occurs, the presence of pubescence alone is not reliable, and other criteria are required.

A study of individual hairs has shown that hair morphology can distinguish Evolvulus taxa, but vague terms such as 'loose', 'spreading', 'appressed' are not sufficient, even misleading. The term 'sericeous,' sometimes used almost as a synonym for 'hairy' – e.g., "densely sericeous on both surfaces" for E. nuttalianus, implies a means of determining the width of hairs, but detailed examination shows that the hairs of E. nuttallianus are not fine and silky, i.e. sericeous. For a study of hair morphology.

In A Monograph of the Genus Evolvulus (1934) S.J. van Ooststroom repeatedly distinguishes two forms of E. sericeus, e.g., p. 130:

a form with leaves glabrous above.
a form with leaves not glabrous above, appressed-sericeo-villose on both sides, equally dense on both sides or less dense above than beneath; transitions to from a occur.

van Ooststroom's key (p. 103) actually notes this same distinction, but (1) vestiture is not employed to distinguish the two above taxa and (2) the key is not strictly binary. I suspect that taxonomists who cite this work as a reference have sometimes not carefully read the key or the text, or perhaps not read it at all. Note the numerous attempts to use vestiture in my review of available keys.

15. Leaves glabrous above.
... 39. E. sericeus/40. E. rotundifolius/42. E. Purpusii
15.* Leaves densely sericeo-villous on both sides
... 43. E. arenicola
(See also forms of 39. E. sericeus and 42. E. Purpusii.)

In contrast the key of L.H. Shinners treatment in D.S. Correll & M.C. Johnston, Manual of the Vascular Plants of Texas (1970) is focused on vestiture, but 'glabrous' is the only clearly defined character:

2(1). Leaf blades densely pilose on both surfaces with loosely appressed to spreading hairs … E. nuttallianus
2. Leaf blades densely pubescent beneath with closely appressed hairs and commonly some loose ones as well,
glabrous or loosely pilose above. … E. sericeus

In my survey of over 600 U.S. and Mexican Evolvulus collections I frequently found notations on Trans-Pecos collections indicating pubescence on the upper leaf surface (and these were consequently determined as E. nuttallianus). The following image illustrates a redetermination of E. nuttallianus based on pubescence (changing the original ID). Originally Tharp stopped short of giving a species epithet, and in 1960 Shinners decided it was 'sericeus.' But the plant's habit doesn't actually match either E. sericeus or E. nuttallianus. It is E. arenarius.

The following table shows that of 91 Trans-Pecos ‘E. nuttallianus’ collections I have examined from TX/LL and SRSC herbaria, I have determined 78 as E. discolor. I found no valid collections for E. nuttallianus from the Davis Mountains (Jeff Davis Co.) or from Mexico (Coahuila, Chihuahua).

	E. nuttallianus	E. discolor
Culberson	5	0
Jeff Davis	0	19
Presidio	3	17
Brewster	1	38
Pecos	1	2
Terrell	2	0
Hudspeth	1	2
total	13 (14%)	78 (87%)

Descriptions of E. nuttallianus vestiture in the literature appear to have been made at least in part on the basis of incorrectly determined collections. A further example is, in my opinion, van Ooststroom's correct determination of E. oreophilus Greene as E. sericeus var. discolor (with strong pubescence on the upper leaf surface), but this has more recently been incorrectly listed as a synonym for E. nuttallianus.

In the table below 3 taxa are represented: E. nuttallianus, E. sericeus & E. alsinoides.

Placing the cursor over an image will enlarge it; and if the cursor is held over the image, the ID will appear.