Liverworts



For inquiries contact Livija Kent , Associate Professor, Biology Department, University of Massachusetts Amherst

The name "liverwort" derives from the Anglo-Saxon word "lifer, meaning liver and "wyrt", the Anglo-Saxon word for plant. During the 16th century, it was commonly applied to the genus Marchantia, a flat, branching, ribbon-shaped plant the margins of which were claimed to resemble the lobes of a liver.

MARCHANTIA GAMETOPHYTE

Since the Doctrine of Signatures, a popular concept at the time, proclaimed that God had bestowed upon each plant He had created a mark or sign that pointed to its medicinal value, the liverwort Marchantia with its imagined liver-shaped lobes was believed to be useful for the treatment of liver ailments.

There are approximately 8,500 species of liverworts. They are widely distributed, occurring from the arctic to the tropics. Although some grow in relatively dry places and a few are submerged aquatics, most liverworts occur in places where moisture is generally available, e.g., on damp soil or moist rotting logs, along shaded stream banks, on rocks in streams, or on wet rock outcroppings; a few even grow under saline conditions.

The dominant generation in the liverworts is the gametophyte; it is the larger, long-lived plant, the plant you are most likely to see in the field. The gametophytes, which range from approximately 0.15 mm to 2.5 cm in width and 2 mm to 25 cm in length, are mostly prostrate thallose or leafy forms. The thallose gametophytes are flat, membranous forms with even, slightly wavy, lobed or leafy margins.

THE THALLOSE LIVERWORT Blasia

The leafy gametophytes are distinguished into stems and leaves.

THE LEAFY LIVERWORT Bazzania

Often, the leaves are arranged in two lateral rows, but, not uncommonly, a third row of smaller leaves is present on the ventral side of the stem, the side appressed to the substrate. The gametophytes, whether thallose or leafy, are anchored to the substratum by microscopic hair-like, colorless rhizoids. The latter are unicellular entities and should not be confused with the macroscopic, multicellular roots of other plants.

There is a great diversity in the shape of liverwort leaves; they may be undivided, variously lobed or divided into hair-like segments, or they may be folded into two lobes of unequal size with the smaller lobe situated atop or below the larger.

Vegetative (asexual) reproduction in the liverworts may be accomplished by branching and the dying off of the older parts of the plant so that the branches become separated, by specialized whip-like branches or by leaves that drop off the plant. It is often effected by one- to several-celled or sometimes multicellular propagules called gemmae that are produced on the margins and surfaces or at the tips of leaves, or on modified branches. Sometimes they are formed in specialized crescent-, cup-, or flask-shaped containers that are borne on the upper surface of the gametophyte.

Liverwort gametophytes, and indeed the gametophytes of all land plants, produce eggs and sperm. When a sperm fuses with an egg a new cell called a zygote is formed. In the liverworts, this cell eventually develops into the short-lived, small and compact sporophyte which usually consists of a terminal globose to cylindrical spore case called a CAPSULE, a stalk called the seta and a basal mass of cells called a foot which is embedded in and, therefore, obscured by the tissues of the gametophyte.

LIVERWORT SPOROPHYTE

When the sporophyte is mature, the seta elongates carrying the capsule 2-3 inches into the air. The capsule then splits open, usually into four segments or valves, and the unicellular thick-walled spores within are released. Under suitable conditions, each spore can germinate and give rise to another gametophyte.

The liverwort commonly shown in introductory texts is Marchantia. This is unfortunate because Marchantia is not a typical liverwort. In contrast to the vast majority of liverworts which are internally simple leafy forms, Marchantia is a highly specialized, structurally complex thalloid plant.

There are probably two reasons why Marchantia was initially the liverwort of choice for general textbooks. Because Marchantia is very common in northern Europe, it was, in the latter half of the 19th century, the most studied liverwort and, consequently, was described in all the books, including American botany texts which were patterned after those published in Europe. Furthermore, in the late 1800s, a German botanist, Leopold Kny, made exceptional illustrations of Marchantia that most printers preferred to use. Here is Kny's wall chart of the male gametophyte of Marchantia.

KNY'S WALL CHART SHOWING MALE GAMETOPHYTE OF Marchantia

Regrettably, the practice of using Marchantia has persisted to the present time; it is still the most, and not uncommonly the only, illustrated liverwort in introductory texts. It seems the time has come to break with tradition and to stop focusing on Marchantia. The liverworts would be far better represented by leafy forms such as Calypogeia or Lophocolea, plants with two lateral and one ventral row of leaves.

THE LEAFY LIVERWORT Lophocolea

REFERENCES on Liverworts

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