Visible leaf venation with E. sericeus, apart from the mid vein is

1. often not immediately apparent with mounted specimens having strong pubescence on both leaf surfaces, but generally can be found at the base of lower leaves on the lower surface;
2. highly variable, often barely visible or not visible on the glabrous upper surface of leaves (unless backlit).

A survey of E. sericeus holdings at the Univ. of Texas TEX/LL showed that this variation in venation is not determined by leaf position on the stem, a first hunch, since visible venation is often more apparent on the somewhat broader lower leaves. It is also not directly correlated with leaf width or length, or relative age (apart from being mature).

In his A Monograph of the Genus Evolvulus (1934) van Ooststroom describes venation for E. sericeus):

E. sericeus

Midrib impressed above, more or less prominent beneath; lateral nerves in the narrow leaves absent [for evidence that van Ooststroom is not correct here], in the broader ones 1 or 2 pairs from the leaf-base, long ascending.

E. sericeus var. discolor (our E. discolor)

midrib and 2-3 pairs of lateral nerves, rising from near the base, impressed above, slightly visible beneath;

The venation of E. sericeus (which differs from that of other Texas Evolvulus taxa) is palmatipinnate, and although the secondary venation is low-rank and highly variable there is a clear tendency toward a pattern of paired secondaries from the base (as noted by van Ooststroom) parallel to the leaf margin to the mid point or higher, with pinnate secondaries, if present, limited to the upper half of the leaf.

This would appear to match E. P. Klucking's ‘intermediate’ venation pattern (The classification of leaf venation patterns, 1995 (p. 39):

The Intermediate system consists mostly of venation which is a combination of apically directed palmate secondary veins and laterally directed pinnate ones. The apically directed secondaries occur in the lower part of the leaf and the pinnate veins in the upper part.

In working with pressed herbarium specimens determining venation was relatively difficult for several reasons. First, the narrower leaves of those forms with glabrous surfaces generally fold when collected, and unless care had been taken prior to drying, the glabrous upper surface is mostly not visible. A number of collections had no visible upper surface or only a small corner of one. Note how this natural folding occurs in the freshly collected stem 'A' in the image below.

Second, with fresh leaves the upper-surface vein impression of the lateral veins was often quite faint, apparent only by virtue of slight color differences. It probably would not maintain its visibility with drying and aging of the leaf surface.

Finally, for unknown reasons many, if not most, leaves that did not fold had been mounted face down.

Consequently I made a short study with fresh stems collected 26 September 2012 from N. Hays Co., from two somewhat different environments: A, from a xeric environment with shallow soil; B, from a grassy area with deep soil. The two stems are shown below with leaves enumerated from bottom to top. Each leaf was forced flat and inspected for visible venation with a dissecting microscope. A number of different types were noted and images were made of each type. The results and images are presented below. [For a survey of 99 leaves, including the two shown here.]