The acanthus leaf has been a favorite design element for millenia. The leaf shapes appear on many Roman and Greek capitals.
Here is an acanthus leaf motif on a Corinthian capital in the Doge's Palace in Venice. The leaf shapes are more naturalistic and profuse, wrapping over the shapes and making them organic.
William Morris used the acanthus for this 1875 wallpaper design. The leaves overlap each other in sinuous curves, and are grouped by color to suggest layers of depth.
Here's an example of how the leaf was adapted for a 19th century manual on lettering. The leaf shapes have graceful S-curves that grow in close parallel curves, and then terminate in spiral finials or flare out into lobed leaf shapes. Vestigial fragments of these leaf shapes appear on the back of the US dollar bill.
Here's what the actual leaf looks like. The two main species that inspired the designers are the Acanthus spinosus and the Acanthus mollis (above), and they grow in the Mediterranean region.
The tips of the leaf are spikier than they appear in the design configuration. Most of the design adaptations have convex lobes. There's a strong single midrib with smaller side veins and secondary leaflets branching off the main midrib.
William Morris's version retains the structure of the single midrib and side-veins, but he elongates and spirals the leaf shape. The pen-and-ink version deviates from the actual acanthus by replacing the single midrib with a series of parallel rib lines, more like a plantain leaf.
Wikipedia: Acanthus in Ornament
"It's all in the Details -- Acanthus Leaves"