All of the information you needed to identify National Park Mystery Plant 13 as the liverwort was provided in this single clue:

Emma Thompson's Nanny McPhee meets Dan Aykroyd's Julia Child.

Actress Emma Thompson played the role of Nanny McPhee in the 2005 fantasy film of the same name and in a sequel (Nanny McPhee Returns) released this year. Dan Aykroyd, a member of the original cast of Saturday Night Live, famously parodied French Chef Julia Child in an SNL spoof that originally aired on December 9, 1978, and has since become a pop culture staple.

Many consider Aykroyd's Julia Childs skit it to be one of the greatest comedy skits in the history of television. With a studio audience looking on, Aykroyd's "Julia" begins preparing a chicken for a holiday dinner. She stresses the importance of saving the giblets, since so many good foods can be made from a chicken's liver. Suddenly, she accidentally cuts herself and begins spewing arterial blood. She tries to staunch the flow herself (while providing tips on rendering first aid), but conspicuously fails. As she grows weaker, obviously bleeding out, "Julia" repeatedly -- and hilariously -- reminds the people in her audience that they must be sure to "save the liver." As she expires, her last words are "Save the liver!" This expression has become one of the most famous gag lines in television history. An excerpt from the now-classic SNL spoof was used in the 2009 hit movie Julie and Julia.

Emma Thompson's Nanny McPhee teaches unruly children important lessons about good manners, self-discipline, and responsibility. Appearing at first to be an old witch, she sports gray hair and a face rendered grotesque by incredibly snaggly teeth, a unibrow, and huge warts (Upper Wart and Lower Wart). These features disappear one after another as the children in her care learn each of five key lessons (go to bed when you're told, etc.), and Nanny McPhee is gradually transformed from an ugly hag to a handsome woman.

The liverwort may have an unattractive name, but it's an interesting plant. Represented by an estimated 7,500 to 9,000 species, liverworts comprise the Marchantiopsida, a class of the Bryophta division that also includes mosses.

Liverworts are small, primitive, non-vascular, non-flowering land plants. They hug the ground (few grow more than an inch or so tall) and most have a flattened or prostrate appearance as well as a rather undifferentiated structure. Someone unfamiliar with the liverwort might easily mistake it for moss or lichens.

Almost all liverworts have anchoring structures called rhizoids. Although root-like in appearance, these rhyzoids lack the absorptive functions of true roots.

Liverworts need damp or moist conditions in order to reproduce, but can grow in a wide range of habitats. Their rhyzoids even allow them to grow on standing trees. Since many national parks throughout the U.S. have at least a little habitat suitable for liverworts -- such as gardens, bogs, dense forests, stream banks, fallen logs, and disturbed areas -- visitors who know what to look for can find them in parks as diverse as Maine's Acadia National Park, Colorado's Rocky Mountain National Park, Utah's Arches National Park, and Alaska's Denali National Park & Preserve.

There are two broad types of liverworts -- leafy and thallose. As the name implies, the leafy variety has flattened leaves that grow on stems. A thallose liverwort develops as a flattish, green sheet (often wrinkled or lobed) called a thalloid. Although there are far more species of leafy liverworts than thallose ones, the shape and texture of the thallose variety have most strongly influenced our perception of this plant.

The liverwort acquired its unlovely name quite honestly. The Old English word wort means "small plant," and the individuals of some thalloid liverworts are liver-shaped. Thus, the name liverwort can be construed to mean "liver-like small plant."

Herbalists in days of old believed that liverwort concoctions could be used to treat liver diseases. Since some people still believe that, liverwort extracts are included in some modern herbal remedies.

Because they have been thought (however erroneously) to act on the liver, the liverworts are collectively referred to as hepatics, a term whose meanings include "acting on [or treating diseases of] the liver." To further confuse the issue, hepatica, an unrelated flowering plant of the buttercup family, is frequently called liverwort.

Postscript: Liverworts do not differ greatly from the earliest land plants known to science. Interestingly, the findings of recent genetic research have convinced some scientists that the origins of nearly all modern plants might very well be traced to ancient liverworts.