The above schemata show the two distinctive alternate leaf arrangements found with Texas Evolvulus: distichous (2–ranked): E. sericeus, E. discolor, E. alsinoides and pentastichous 2/5 (5–ranked): E. nuttallianus, E. arenarius. The columns represent stems and the small red circles, leaf nodes; the green boxes, two spiral cycles. When distichous, successive leaf nodes are arranged in two vertical rows (here, even–numbered or odd–numbered), the nodes 180° apart around the stem.
With the pentastichous 2/5 pattern 5 nodes occur in 2 spirals around the stem (indicated by the fraction 2/5) on 5 vertical rows; thus nodes 1, 6, 11 etc. are aligned above each other on the same row (similarly nodes 2 & 7, 3 & 8, 4 & 9, 5 & 10).
Leaf Arrangement (Phyllotaxis) in Texas Evolvulus
by Bob Harms (
)
Discerning the leaf arrangement on pressed specimens is often difficult, especially with short internodes, but it can be done.
[click to enlarge]
The distichous arrangement of E. sericeus, E. discolor & E. alsinoides:
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Left: E. sericeus, Hays Co.; Right: E. discolor, Scan of plant with strong pubescence on both leaf surfaces, Presidio Co.
This specimen had been identified as E. nuttallianus, presumably based on the dense pubescence on the upper leaf surface. | E. alsinoides, Jeff Davis Co. |
The pentastichous 2/5 arrangement of E. nuttallianus & E. arenarius:
Photos of E. nuttallianus from Hays Co.
Scan of E. arenarius with longer internodes, Winkler Co.
Plant Resources Center Home Page — Flora of Texas
— Evolvulus
