E. sericeus narrow-leaf plant, 18 November 2012 [Hays Co.]. [click to enlarge.]

Evolvulus sericeus Narrow-leaf Venation
by Bob Harms (email-here)

In his A Monograph of the Genus Evolvulus (1934) van Ooststroom describes venation for E. sericeus as:
Midrib impressed above, more or less prominent beneath; lateral nerves in the narrow leaves absent, in the broader ones 1 or 2 pairs from the leaf-base, long ascending.
In Texas populations of E. sericeus with narrow leaves the palmatipinnate venation pattern (i.e., pairs ascending from the base) is not absent even for the narrowest leaves — although these nerves may not be apparent in dried, pressed herbarium specimens or even in fresh specimens.

Two E. sericeus var. cymosus plants with mature leaves much narrower than usual were collected and scanned (one shown above). (Note that if these plants were simply pressed the conduplicate leaves would fold and reveal only a midrib.)

Plant 1:

Two leaves were selected and forced flat for closer inspection: (1) the longest leaf [yellow arrow] and (2) an immature leaf at the top [red arrow] — shown below. Both leaves were scanned at 2400 dpi, the long leaf with reflective light

  1. The longest leaf (26 mm long, 2.5 mm at the widest point). Both surfaces were scanned with reflective light, the upper surface barely showing one secondary vein, and with transparency lighting through the leaf — revealing the expected palmatipinnate venation, clear to the midpoint. [click image for enlarged view of the leaf base]

  2. The immature leaf (13 mm long, 1.5 mm at the widest point), which showed no venation in normal light, was scanned with transparency lighting. This revealed a very faint secondary vein at the base [click image for enlarged view of the leaf base].

Plant 2:

Two weeks later, at the end of November, I found an even tinier plant, perhaps dwarfed by the fall drought, and brought it to test the notion that narrow leaves lack visible secondary venation (although I knew what the answer would be). Here is a branch from that plant (click on it for an enlargement of the portion in the yellow box):
The long narrow leaves are ±1 cm, but only ±1.5 mm wide. And all mature leaves showed the expected venation pattern, although one leaf (to the immediate left of the scale) had suprabasal secondaries.
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