Fig. 1. Typical Evolvulus Trichome types: A: E. arenarius; B: E. nuttallianus; C: E. discolor & E. sericeus; D: E. alsinoides. Vertical line indicates point of attachment. See also Figs. 2 & 3 below.
[Click section to enlarge]

by Bob Harms ( )

Evolvulus trichomes are bifurcate, most commonly asymmetrically, with a 'weak' branch and a 'strong' branch. The strong branch is often longer (cf. Fig. 1 above), directed toward the leaf tip, and with E. arenarius &E. nuttallianus regularly thicker than the weak branch at the base (cf. Figs. 2, 3). The weak branch is often significantly reduced. Each branch is widest at the base and gradually reduced to an acuminate tip. The density of hairs may differ on the two leaf surfaces. Of the 4 taxa discussed here only E. arenarius has greater density on the upper surface. The longest trichomes were found on the lower surface of E. discolor &E. alsinoides.

1. Trichome width at the base differs for the two forks — strong fork with max. width 19.6 μm, max. width at the base of the weak fork ca. 70% of that of the adjacent strong fork; trichomes not different on the two leaf surfaces; strong fork rarely¹ exceeding 2 mm.
¹ E. nuttallianus collections from Nebraska and Colorado had hairs ±3 mm.

… 2

2(1) Weak fork rarely > 0.25 mm long, hairs denser on upper surface

… E. arenarius

2. Weak fork rarely < 0.3 mm long, hairs denser on lower surface

… E. nuttallianus

E. nuttallianus (Culberson Co.) hair forks at base, 6000x. SEM scan by John Mendenhall. (Click to enlarge)

1. Trichome width at the base not consistently different for the two forks; hairs different on the two leaf surfaces; strong fork may exceed 2 mm;

… 3

E. discolor (Jeff Davis Co.) hair forks at base, 2000x. SEM scan by John Mendenhall. (Without the full trichome the strong/weak forks can't be determined.)

3(1). Trichome branches never symmetrical

… E. discolor & E. sericeus

3. Trichome branches often symmetrical, but may also be strongly asymmetrical in length

… E. alsinoides

Fig. 2. Key: A: E. arenarius; B-D E. nuttallianus; E.: E. discolor; F.: E. sericeus; G-H: E. alsinoides

[click on image for enlarged view]
Fig.3. Detail of 0.5 mm of weak & strong forks from point of attachment (vertical line). (The hairs shown in full length view in Fig. 1 are included here.)

For larger sets of hair photos.