Mouse-ear Hawkweed (Hieracium pilosella)

Mouse-ear Hawkweed

The Mouse-ear Hawkweed (Hieracium pilosella) is a  perennial herb/forb in the Aster Family (Asteracceae) and order Asterales. Some authorities identify this plant with the genus and species name "Pilosella officinarum." It is common around French Hill Pond. The photographs on this page were taken to the east of the pond. The genus  name is derived from the Greek word "hierax" which means "hawk." The species name and alternative genus name, pilosella, is from the Latin for "hairy." The alternative species name comes from the Latin referring to a workshop and is generally interpreted as "ordinary" or "common."

This forb can grow to 20 inches (50 centimeters) tall, occasionally taller, but is usually shorter. The ray flowers are yellow and one inch (2.54 centimeters) wide with one flower per stem. There are no disk flowers.  Each hairy stem rises from the center of a basal rosette arrangement of leaves. The flower stem is called a "scape." Each ray flower is surrounded by green bracts with black hairs. This plant flowers from May to September. 

The basal leaves are oblong, hairy, pointed,  toothless and from one to five inches ( 2.54 to 12.5 centimeters) long.

Mouse-ear Hawkweed Leaves

The underside of a leaf has a white bloom as seen on the underside of the leaf in the middle of the following photograph. There rarely may be a few, scattered, small leaves on the scape.

Mouse-ear Hawkweed Bloom

The fruit is a small seed attached to hair allowing the seed to be blown off the plant at maturity.

 Propagation is by wind-drive seeds and leafy runners that form a matted root system.

This plant resembles a dandelion and other hawkweeds. However, it is easily identified by the shape of its basal leaves and the single flower on the scape.

The Mouse-ear Hawkweed (Hieracium pilosella) is considered a noxious weed in many states because it can be very invasive. It is used medicinally in herbal remedies as a diuretic, expectorant and antibiotic.

Microbiology of the Mouse-ear Hawkweed Leaf

The following microphotographs show characteristics of the Mouse-ear Hawkweed leaf as seen through a microscope.

The following microphotograph shows the top of a Mouse-ear Hawkweed leaf at 40X magnification.

Mouse-ear Hawkweed Leaf 40X

Note how the coarse hairs of the leaf attach to the epidermis (skin) of the leaf. The epidermis is out of focus to show the coarse hairs.

The following microphotograph shows the top of a leaf at 400X magnification. Again, the epidermis is out of focus to show the stomata.

Mouse-ear Hawkweed Leaf 400X

The underside of the leaf also has stomata as indicated by the red arrows in the following 400X microphotograph of a leaf underside. There are a large numbers of hairs on the underside of a Mouse-ear Hawkweed leaf seen as transparent or black, serpentine  or straight structures on the epidermis. These hairs appear white on the leaf but are generally transparent. The larger hairs were squished into a serpentine shape when the leaf was mounted on the microscope slide.

Mouse-ear Hawkweed Stomata on Underside of Leaf

The following microphotograph shows the underside of a Mouse-ear Hawkweed Leaf at 40X magnification. The white bloom consists of an intricate pattern of small, fine hairs scattered in spider-like units. Like the coarse hairs also seen in this microphotograph, the fine hairs are attached to the epidermis at the center of each unit. The blue arrow in the microphotograph above points to the center of a unit. Note that the bottom coarse hairs are not as big as the coarse hairs on the top of the leaf. The hairs appear black in the following microphotograph because the leaf is backlit.

Mouse-ear Hawkweed Leaf Underside Bloom 40X

The hairs on leaves and other parts of plants, called trichomes, have a variety of purposes. In general, they make the parts of the plant unpalatable to animals wishing to consume them and evaporation of the water from a leaf is reduced by the hairs as the leaf blows in the wind. They may also trap moisture and protect against too much light, particularly ultraviolet radiation. The white hairs on the bottom of a Mouse-eye Hawkweed leaf  may also reflect sunlight passing through the leaf back into the leaf increasing the efficiency of photosynthesis. The next 100X microphotograph shows parts of two larger trichomes from the underside of a Mouse-ear Hawkweed leaf. Note the barbs on the trichomes and their columnar structure.

Mouse-ear Hawkweed Trichomes Leaf Bottom

Microbiology of the Fruit

The following 20X microphotograph shows part of the head of a Mouse-ear Hawkweed flower gone to seed. This head is less than ½ inch (1.27 centimeters) in diameter. The hairs shown attach to the top of the seeds by a disk from which they radiate. These appendages are called "pappi," the singular of which is pappus. A pappus allows the seed to be blown off the stem and travel for a great distance to propagate the plant. Note how the radial hairs of the pappus resemble the spider-like trichomes on the underside of the leaves.

Mouse-ear Hawkweed (Hieracium pilosella) Pappi

As seen in the next 400X microphotograph, each pappus hair consists of cells arranged in a bundle of columns. The outside of this bundle has thorn-like protrusions. These protrusions may help stabilize the seed in flight, allow the seed to attach to hosts (like birds or human clothing) and help establish the seed when it finds a suitable place to germinate. The seed itself has thorn-like protrusions that probably have purposes like that of the protrusions on the pappi hairs. Note the similarities between the trichomes and the pappi hairs.

Mouse-ear Hawkweed (Hieracium pilosella) pappus hair 400X

Show below is a 20X microphotograph of a Mouse-ear Hawkweed seed and its pappus un-mounted on a microscope slide. There are about two dozen hairs per pappus. Some of the hairs closest to the microscope objective and the seed are out of focus to show the arrangement of the pappus. Note that the seed is only about 400 μm (0.4 millimeter or 0.1 inch) wide and 1,400 μm (1.4 millimeters or 0.35 inch) long, not including the pappus. These seeds are almost invisible once they are dispersed into the environment.

Mouse-ear Hawkweed (Hieracium pilosella) Seed

The next microphotograph shows the Mouse-ear Hawkweed seed in more detail at 40X magnification mounted on a microscope slide. The background of this microphotograph is blackened to show the seed better. The white spots on the seed are dust particles trapped in the slide mounting fluid. The pappus was flattened under the microscope cover slide. Note the ridges on the seed and the rounded lower tip where the seed was attached to the flower stem. The distance between the ridges is about 100 μm (0.1 millimeter or 0.025 inches) at the widest part of the seed.

 

Mouse-ear Hawkweed Seed

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