E. arizonicus, Miller Canyon, AZ	E. alsinoides, Jim Hogg Co.	E. sericeus var. cymosus, San Patricio Co.	E. sericeus var. cymosus, Hays Co.

Selected Pedunculate Evolvulus Taxa [click for large images]

by Bob Harms ()

The following key presents a different approach to identification of U.S. Evolvulus taxa (including those same taxa in Mexico and Central America). It is based on examination of over 750 U.S., Mexican and Central American Evolvulus collections from TX/LL, GH, US, NMC, NMCR, UNM, SRSC herbaria, my own field observations and collections, and online specimens at US, JSTOR and TROPICOS. [For a relevant selection from van Ooststroom's key.]

Apart from a glabrous/pubescent distinction, I have concluded that vague vestiture terminology (e.g., 'loose', 'spreading') and corolla color are not useful differentiae. Sepal form has limited validity, in that E. sericeus sepals are never "lanceolate or narrow-lanceolate" (van Ooststroom p. 101) which suffices to rule out E. sericeus in all areas, but is not sufficient for the full range of E. nuttallianus variation.

This key focuses foremost on venation type, noted by van Ooststroom for most taxa, although his ability to observe the venation in our taxa was limited. Leaf–clearing and transmitted–light techniques (some low tech) have shown that all specimens with clear E. sericeus and E. discolor habit and phyllotaxis characteristics have basal acrodomous or palmatipinnate leaf venation, and all specimens lacking these characteristics have pinnate vennation.

One consequence of this key is that many collections from the Trans-Pecos and western states that border Mexico which in 2012 were placed with E. nuttallianus will now be correctly determined as E. discolor. (van Ooststroom's E. sericeus var. discolor). To the extent that these collections have influenced existing keys and descriptions, such work will need to be reevaluated.

1. Inflorescence pedunculate and upper leaf surface pubescent … E. alsinoides/(E. arizonicus).¹
1. Inflorescence either not pedunculate or pedunculate with upper leaf surface glabrous² … (2)

E. nuttallianus, Utah	E. discolor, Brewster Co.	E. sericeus, Brazos Co.	E. sericeus var. cymosus, Hays Co.		E. sericeus var. holosericeus, Chiapas, Mexico	E. discolor, Chihuahua, Mexico

Distinctive Evolvulus Venation — left 3 have been cleared; 3^rd from right is backlit with sunlight (camera picture);
rightmost 2, densely pubescent, have been rehydrated and scanned 'as a transparency'
[click for large images, including the untreated leaves]

2(1). Leaves with prominent midvein and weakly defined pinnate venation (secondaries visible only with leaf clearing); phyllotaxis pentastichous 2/5, upper leaves flat or slightly u-shaped and erect (rarely falcate); if upper leaf surface glabrous, then lower surface is also.

… (3)

3(2). Foliage sparse with internodes commonly > 4 mm; mid leaves linear, length/width ratio > 8:1; trichomes strongly asymmetrical with weak forks$nbsp;0.25 mm long; hairs denser on upper surface; corolla yellowing with age.

… E. arenarius

3 Foliage dense with internodes rarely > 4 mm; mid leaves elliptical, length/width ratio < 8:1; trichomes weakly asymmetrical with weak forks > 0.25 mm long; hairs denser on lower surface; corolla not turning yellow with age.

… E. nuttallianus

2. Leaves with basal acrodromous or palmatipinnate venation (generally visible on some leaves without clearing); upper leaves distichous, conduplicate, and often falcate; if upper leaf surface glabrous, then lower surface is sericeus.

… (4)

E. nuttallianus, glabrous form, Hays Co.	E. sericeus var. cymosus, Hays Co.	E. sericeus var. sericeus, Kleberg Co.	E. discolor, Central New Mexico	E. discolor, Jeff Davis Co.

Glabrous upper leaf surfaces [click for larger images]

4(2). Leaves with broad clasping base, with 1 or more pairs of strong secondary veins from the point of attachment, basal acrodromous; upper stem hairs rising from the base of the trichome fork and curving back down as they lengthen; multiple stems arising from a branched crown above an underground vertical subligneous rhizome > 2 mm wide that extends as far as 8 cm down to the taproot.

… E. discolor

E. nuttallianus, Hays Co.	E. nuttallianus with E. sericeus var. cymosus, Hays Co.	E. sericeus var. cymosus, Hays Co.	E. discolor, Jeff Davis Co.

Habit (cf. also discussion of habit in Felger et al. 2012) [click for larger images] 4 Leaves with narrow subsessile base, with 1 pair of weak secondary veins from just above the point of attachment, often opposite, suprabasal acrodromous; upper stem hairs straight and pressed flat close to the stem; multiple stems, < 2 mm wide at the base, becoming lignescent only above ground, often branched below ground, arising as narrow vertical rhizomes from thick horizontal rhizomes.

… E. sericeus

E. sericeus var. sericeus, Tamaulipas, Mexico	E. sericeus var. cymosus, Hays Co.	E. discolor, Jeff Davis Co.	E. discolor, Presidio Co.

Vertical & horizontal stem/rhizomes. Taproot is visible only on the bottom left of the Jeff Davis plant. All branching for E. discolor is above or immediately below ground level.
[click for larger images — with these 'S' is stem/rhizome; 'R', root]

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¹In my opinion E. alsinoides and E. arizonicus are best treated as varieties of a single taxon. van Ooststroom 1934 (75-76) seems to have reservations about the status of E. arizonicus, noting "both the Arizona speicmens and the Argenina ones are rather variable in habit, size, form of the leaves and indumentum." Differentiae presented by Austin 1990, Felger et al. 2012. — i.e., sepal length, corolla size, vestiture, leaf shape – are partially overlapping or nearly overlapping (ranges) (as in the following table):

	E. alsinoides	E. arizonicus
Sepal length	2–2.5 mm	3–3.5 mm
Corolla width	(5–) 7–10 mm	(10–)12–22 mm
Leaves	elliptic, ovate or oblong to lanceolate	lanceolate to linear-lanceolate
Stem vestiture	long spreading trichomes	appressed pilose to tomentose, rarely with spreading trichomes
Sepal vestiture	densely pilose	pilose to tomentose

Measurements of collections in TEX/LL and SRSC show even greater variability than indicated by Felger et al. 2012, especially stem vestiture, sepal length and corolla width.

²An alternative venation–based key would eliminate any need for pubescence, since long–peduncled taxa, with the exclusion of E. sericeus, do not have palmatipinnate venation; i.e.,

1. Leaves with basal acrodromous or palmatipinnate venation; upper leaves distichous, conduplicate, and often falcate … E. sericeus, E. discolor
1. Leaves with pinnate venation; upper leaves not as above … 2 (peduncle length distinction alone).

A more serious problem with pubescence is that without additional qualifications, it limits the above key to northern Mexico, since the long–peduncled E. filipes, which, like E. sericeus, has forms with the upper leaf surface glabrous and is known from Sinaloa, Durango, and Vera Cruz (van Ooststroom, p. 68).

References

Austin, D.F. 1990. Comments on southwestern United States Evolvulus and Ipomoea (Convolvulaceae). Madroño 37:124–132.
Felger et al. 2012. Convolvulaceae of Sonora, Mexico. I. J. Bot. Res. Inst. Texas 6: 459–527.
van Ooststroom, S.J. 1934. A monograph of the genus Evolvulus. Meded. Bot. Mus. Herb. Rijks Univ. Utrecht. 14:1–267.

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