Friday, 18 May 2012

Liverworts - The Unknown Plants

There is a common group of plants of which many knowledgeable naturalists are completely ignorant.  They are distantly related to mosses, and are referred to as liverworts, or in more technical jargon, hepatics. There is indeed a superficial resemblance to mosses, and both are classified together in a larger grouping called the bryophytes. This resemblance, however, is deceiving. Identifying small organisms such as these, is somewhat comparable to naming trees on the opposite side of a valley. With a telescope the task becomes considerably easier. In our case the task becomes considerably easier by using a hand lens, which is also considerably easier to transport through the bush.

According to the latest research findings, the liverworts have a very special geological history. Both genetics and the fossil record indicate that they are the oldest land plants. Their ancestors, 400 million years ago were the first little plants to colonize the land, and the liverworts have been here ever since.

Careful scrutiny with a hand lens or magnifying glass will show some of the differences between mosses and liverworts. In fact the differences are so great that the two groups are probably no more closely related to each other than grasses are to pine trees. 

There are two groups of hepatics - the marchantioids, and jungermannioids. The first group is named for Marchantia polymorpha, the classic liverwort, so called because its surface looks like that of the liver and according to the Medieval Doctrine of Signatures was a cure for conditions believed to be liver diseases. Unfortunately there does not appear to be any evidence to support this belief. The marchantioids form flat, fairly tough, green plates, usually on soil, and usually about 3 to 5 cm. long. Marchantia polymorpha is common on shaded soil in gardens and greenhouses. The larger, but similar Conocephalum conicum, the Great Scented Liverwort, is frequently seen on seepage sites in the coastal forest. When scratched it produces a perfume-like odor, hence the common name. Both these plants have a snakeskin-like upper surface of small polygons. 

The jungermannioids are often called the leafy liverworts because most of them have small leaves, and look very much like mosses. In fact the differences between the two groups are not readily apparent without a little experience. Although the distinctions are subtle, mosses and liverworts can soon be distinguished from each other, even when the actual species identifications cannot be ascertained.  Leafy liverworts tend to have a softer, more translucent, flatter appearance. The leaves are usually lobed, and this can sometimes be discerned with a lens. Except for a few rare exceptions, moss leaves are unlobed. But the most obvious dissimilarity from mosses involves the capsules - the cases in which the spores are produced, and the stems on which they are borne. Moss capsules are tough and remain on the plants for many months. They look like little jars with an opening at the top, an opening which is usually covered by a row of teeth. These teeth open and close with changes in humidity, thereby controlling spore release. The capsule is green and photosynthetic when young, but brown when old. The stem, or seta, below the capsule is strong and wiry. When young it is also green. Leafy liverwort capsules are entirely different, and look like small fungi. They are usually spherical, black and shiny, and when mature open out into four petal-like valves. The seta is a translucent white, and very evanescent. For this reason spore cases are only observable for a few weeks, and for most species this period occurs in the spring. Moss capsules last for many months.

Liverworts are more dependent on humid or moist environments than many of the mosses - sites such as stream banks or tree trunks in shaded coastal forests. Although mosses grow luxuriantly in such habitats, some moss species also flourish in dry areas where liverworts are almost always absent. 

Probably the primary reason these little plants are almost unknown revolves around their lack of economic importance. Liverworts do not destroy timber or compete with crops, and at the present time they do not produce commercial products. They may, however, offer the potential for future medicines. Within hepatic cells are small structures known as oil bodies. Dissolved within these structures are a number of complex chemicals that may deter parasites or predators. Such compounds could conceivably offer potential for new antibiotics.

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