Major leaf venation patterns can sometimes be revealed by using the transparency facility of modern scanners, such as the Epson Perfection 4800 flatbed scanner, at 2400 dpi resolution. I initially discovered this as I was scanning cleared leaves mounted on glass slides. A fresh leaf was placed on the scanner plate on the position for a slide and a clear glass slide was placed on top of it. With the leaves of E. sericeus the major secondary veins were strikingly apparent.

Dried pressed leaves from herbarium specimens did not give quite such clear results, but when these were first soaked in hot water for several hours, the venation patters became clearer. This was true with western specimens of E. sericeus with heavy pubescence on both surfaces (often incorrectly assumed to be E. nuttallianus). For example:

E. sericeus — Chihuahua. Rightmost is a reflective light scan; next to it, a transparency scan of the dry leaf; the left 2, both surfaces after rehydrating.
[Click to enlarge.]

The leaves of E. nuttallianus do not give as clear an indication of major venation patterns. The midrib is always strong, but secondary veins are difficult to see when rehydrated.

E. nuttallianus — Bell Co. Lower leaf half, reflective scan on right; C. L. & Gertrude York 54360 [TEX] 5/25/1954
[Click to enlarge.]

For fresh transparency scans of E. nuttallianus.

For transparency scans of E. alsinoides.