Evolvulus Trichome SEM Scans

Evolvulus Trichome SEM Scans

by Bob Harms ( email-here

)

The SEM scans shown here were provided by John Mendenhall (Institute of Cellular and Molecular Biology microscopy facility, University of Texas at Austin).

Hair Fork Asymmetry

Evolvulus trichomes are bifurcate, most commonly asymmetrical, with a 'stronger' [antrorse] (longer, often wider) and a 'weaker' fork. One of the most salient characteristics of E. arenarius trichomes is their strong asymmetry, the weak fork being significantly shorter and narrower. Since the hairs of E. arenarius are also much sparser, this is readily apparent, often at low magnification (45x). With the other taxa such short weak forks are rare and the greater density of trichomes makes viewing them more difficult. With ‘spaghettiform’ E. discolor this is extremely difficult.

E. arenarius (Winkler Co.)

The 800x scans below are 872.5 μm wide; the yellow dots/circles identify the point of trichome attachment for trichomes with a weak fork (to its right) that is visible in the scan area. With E. discolor it was nearly impossible to find any bases, and none had visible weak forks within the scan area.

E. arenarius (Winkler Co.)

E. nuttallianus (Culberson Co.)

E. discolor (Jeff Davis Co.)

Hair Width

Comparing the hair widths as viewed from above in the sections presented, E. arenarius (Winkler Co.) seems to have the widest hairs and E. discolor (Jeff Davis Co.) the narrowest, with E. nuttallianus (Culberson Co.) to E. discolor (Coahuila, Mexico) in between (from left to right)


E. arenarius (Winkler Co.)	E. nuttallianus (Culberson Co.)	E. sericeus (Hays Co.)	E. discolor (Coahuila, Mexico)	E. discolor (Jeff Davis Co.)

E. arenarius (Winkler Co.)	E. nuttallianus (Culberson Co.)	E. sericeus (Hays Co.)	E. discolor (Coahuila, Mexico)	E. discolor (Jeff Davis Co.)

Hair Base Comparison

With the dense hairs of all but E. arenarius, the size and structure of the basal cells presented a challenge to the SEM. Even so, clear differences emerged. E. arenarius and E. nuttallianus differ in having two concentric basal cells with a wider cell at the base, much wider with E. arenarius (labeled ‘b1’ & ‘b1’ below). There are differences among the E. sericeus & E. discolor base formations, especially in thickness, but the concentric 2–cell formation was not found.

E. arenarius hairs, sparse and aligned, with the strong fork to the left, weak fork ±0.25 mm. Three concentric 2–cell bases are shown in the yellow frame, where the trichomes have fallen off.

With E. arenarius Jim Mauseth has noted that "the striations in the cuticle all point toward the trichome." Although this is not as apparent with the other taxa, additional scans might determine whether this feature has distinctive value.


E. arenarius, Winkler Co.

E. nuttallianus, Culberson Co.

E. nuttallianus, Yuma Co., Colorado

E. discolor, Jeff Davis Co.

E. discolor, Chihuahua, Mexico

E. discolor, Coahuila, Mexico

For a table of somewhat smaller scans of trichome bases.

Grooves on Upper Surface of Hairs

Some hairs of all types are grooved on top, sometimes also flattened. Although it is most common and most prominent at the attachment point, it may occur along much of the hair except at the tip. With E. arenarius (Winkler Co.) (bottom image) this feature was uncommon, of limited length, very shallow and narrow. Again this feature may or may not prove distinctive once more scans are examined.

The possibility that these grooves are an artefact of air drying in the scanning process was considered, and rejected. Hairs scraped from a fresh leaf of E. sericeus and examined with a light microscope showed the same groves (images below). Drying may well have played a role — but while the hairs were on the plant in the wild. Further, all specimens scanned have been dried for herbarium storage. Whatever the origin of the grooves, they do not seem to be an artefact of the scanning process. If not all taxa show the same grooving, this may well reflect anatomical differences, even if unknown.

Focus on grooved upper surface, 2000x
E. sericeus (Hays Co.)	E. nuttallianus (Culberson Co.)
E. discolor (Jeff Davis Co.)	E. discolor (Coahuila, Mexico)
E. arenarius (Winkler Co.)

Grooved E. sericeus trichomes scraped from a freshly collected leaf and examined with a light microscope, images made at 400x.

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