E. sericeus venation via light transmittance. [Hays Co.].	E. nuttallianus venation via light transmittance. [Hays Co.].

by Bob Harms ()

A serendipitous result of the use of a slide scanner to image cleared leaves was the discovery that light transmitted through the untreated (fresh) leaves of E. sericeus reveal that taxon's distinctive venation pattern. With E. nuttallianus only the midrib is clearly visible. Leaves from dried herbarium specimens when rehydrated with boiling hot water show the same results, with E. asinoides intermediate. This difference is consistant for these taxa, even when E. sericeus is heavily pubescent on both surfaces.

In short, when rehydrated:

• The secondary veins of E. sericeus, paired from the base have a high degree of light transmittance, and have the same color as the midrib, typically pale yellow with leaves that have not turned brown, and contrast with the darker surrounding mesophyll (& higher order veins). Leaves with heavy pubescence on both surfaces tend to be slightly less translucent.
• The pinnate secondary veins of E. nuttallianus only barely transmit light, may not have the same color as the midvein. If visible at all, it is because they are darker (i.e., transmit less light) than the surrounding mesophyll.
• The pinnate secondary and higher order veins of E. alsinoides have the same color as the midrib and the mesophyll (or slightly darker than the mesophyll). They are visible by virtue of their shape — a continuous smooth line, often with a dark border, surrounded by textured mesophyll.

The anatomical features that account for these differences in light transmittance, which are not correlated with environmental factors, is currently unknown.

E. sericeus east of the Trans-Pecos [click for full leaf view]

Brazos Co.	Hays Co.	Kleberg Co.	Nueces Co.

E. sericeus in the Trans-Pecos & Mexico [click for full leaf view]

Jeff Davis Co.	Jeff Davis Co.	Brewster Co.	Chihuahua	Coahuila

E. nuttallianus [click for full leaf view]

Missouri	Colorado	Dallas Co.	Missouri Co.	Winkler Co.

E. alsinoides [click for full leaf view]

Willacy Co.	Hidalgo Co.	Cameron Co.	Val Verde Co.	Presidio Co.