Article written by Graham Paul.
Liverworts will grow in containers and on the surface of flower borders and rockeries where they are usually associated with compacted soil or poor drainage. They do not harm plants but they can look unsightly and may inhibit growth of weak specimens or young seedlings. On paving and other hard surfaces they can spoil the general appearance and cause a slip hazard in wet weather.
The name ‘liverwort’ originates from an ancient belief that if the shape of a plant or other physical characteristic resembled a part of the human body, then it could be used to cure sickness in that area. Liverwort was used to cure liver disease, since the shape and texture of the thallus resembled human liver tissue. Likewise, Lungwort was used for respiratory problems, the leaf shape having provided the common name for the plant and suggested its usefulness as a herbal remedy. This ancient form of medicine was known as ‘The Doctorine of Signatures’ and dates back to the rule of Roman Emperor Nero. Pedanius Dioscorides (40 to 90AD), a Roman citizen of Greek origin, had a life-long interest in botany and worked as a physician attached to the Roman army. He practised herbal pharmacology, collecting medicinally useful materials (plants and minerals) from all over the Roman Empire, using them to heal wounds and cure sickness. Dioscorides recorded his extensive knowledge of herbal remedies in a five volume encyclopedia titled ‘De Materia Medica’ – a work that became the main reference for pharmacology in Europe and the Middle East for over 1500 years.
1/6 - Thallose liverworts growing between paving blocks
2/6 - Alternate generations diagram
3/6 - Male sexual organs (antheridia) silvery-brown platforms raised on stalks
4/6 - Female archegonia of Marchantia spa
5/6 - Female archegonium with yellow capsule - containing spores
6/6 - Gemma cups containing small green disc-shaped gemmae
Liverworts were one of the first plants to inhabit dry land on earth. They are thought to have been present for about 407 million years. However, in 2010, some fossils were found in Argentina that indicates the existence of five different types of liverwort spores dating from 470 million years ago. There are estimated to be 9,000 species of liverwort in existence today and these fall into two types; the more familiar ones grow in the form of a fleshy, flattened, leafless body known as a ‘thallus’ and account for about 15% of the total species. The remaining 85% are the leafy liverworts that have flattened stems with two or more ranks of overlapping leaves.
In the Amenity industry we are less likely to encounter problems with leafy liverworts, so I shall concentrate my emphasis on the ‘thallose’ liverworts that we might see on paths, between slabs or paving blocks, growing in the open ground or invading plant pots in nurseries.
Liverworts are classified in the Plant kingdom division Marchantiophyta, one of three divisions in the group of lower plant forms known as the Bryophytes. This group also contains the mosses and the hornworts. (The Bryophytes do not have true vascular tissues for transport within the plant).
Liverworts have almost global distribution; occurring in most habitat types but more commonly in those with high levels of moisture or humidity, although a very few species have adapted to the extreme environments of desert and arctic regions. Probably the main reason that liverworts are unfamiliar to many people is because of their small size, which can range from between 2 and 20mm in width and usually much less than 10cm long.
The Generation Game
In order to better understand the complexities of these bryophytes, it may help to look at the concept of alternate generations, a term primarily used to describe the life cycles of plants. In the cell, genetic information is carried on chromosomes and these can either be paired (diploid – referred to as 2n) or single (haploid – referred to as n).
During the lifecycle of a plant, the chromosome state alternates between the single haploid and the paired diploid state. In higher plants the majority of the lifecycle is in a diploid state with only a brief period in the haploid (gametophyte) state where chromosomes are unpaired. However, in the liverwort lifecycle the reverse is true; the majority of time is spent in the gametophyte (haploid) phase. Liverworts are therefore said to have a gametophyte dominant life cycle.
Species of liverwort may be either monoecious (having male and female reproductive organs on the same plant) or dioecious (with separate male and female plants) This phenomenon is not unusual in higher plants; Common Nettle (Urtica dioica) is a good example of a dioecious plant.
The life cycle of a liverwort begins with the germination of a haploid spore into a temporary stage called a protonema, which develops into a mature gametophore – the body that supports the sex organs. The male organs are known as antheridia and in the Marchantia species I photographed, they are raised on stalks.
The female sexual organs are known as archegonia and consist of a ‘spoked’ archegonial head that is also elevated on a stalk.
The male antheridia produce sperm cells that have two flagellae to propel them towards the egg cell produced in the female archegonia. Part of the journey between the male and female organs may be assisted by rain splashes, the male antheridia in some cases acting like a trampoline. Japanese researchers found that some liverworts could project sperm up to 15cm into the air in rain drops, which allowed them to fertilise egg cells more than a metre away. After fertilisation occurs, a short-lived diploid stage is reached with the formation of a sporophyte. As the sporophyte matures it develops a capsule, which will produce the haploid spores that disperse to germinate and complete the lifecycle.
Some species of liverwort produce small, green disc-shaped objects called ‘gemmae’ in shallow cup-like structures located on the upper surface of the thallus.
These tiny gemmae are a means of vegetative (asexual) reproduction; when displaced from the cups by raindrops, they will germinate, upon landing on a suitable substrate, and develop into a new plant that is genetically identical to the parent.
Unlike the higher plants, liverworts have no stomata for gaseous exchange and to regulate water in the cells, instead they have permanently open pores on the surface. Another difference is the absence of roots for anchoring the plant and supplying water and nutrients. Part of this function is performed by unicellular rhizoids located on the underside of the gametophyte (thallus), which serve to anchor the plant to the substrate it grows on. Although these rhizoids resemble root hairs of higher plants, they do not provide water or nutrients to the liverwort.
The presence of liverworts in ornamental borders is an indication of poor growing conditions allowing these lower plant forms to compete, as is the case with the other common bryophyte problem, moss. Compaction, excessive moisture resulting from poor drainage, low light levels, acidic and impoverished soils are all likely to encourage the spread of liverworts.
In nurseries, the use of good quality compost, thoroughly clean containers and routine inspection and removal of invading liverwort plants will help to prevent major problems. Where plants are to remain in containers for long periods, the application of a layer of grit to the surface will prevent unwanted liverwort growth.
On paths and other hard surfaces, maintain good drainage and keep light obstructions to a minimum by pruning overhanging trees and shrubs.
Liverworts have survived on this planet for well over 400 million years so it is understandable that they are not the easiest weed problem to deal with. However, some products available today are worthy of note:
- Hard surface cleaners will remove liverworts from paths, patios and other porous hard surfaces. ‘Safor’, marketed by Amega Science, contains benzalkonium chloride and urea hydrochloride and claims to ‘deter’ liverworts. There are many other products containing benzalkonium chloride that make claims along the whole spectrum between good control and deterrent – depending on the co-formulants in each product and the confidence of the manufacturer.
- Products based on fatty acids, such as ‘Bayer Moss Killer’ (from their garden chemicals range) are reported to be fairly effective; the Bayer product label states that it ‘kills moss, algae and liverworts’.
- Products containing acetic acid such as ‘New Way Weed Spray’ from Headland Amenity are worth trying provided that the surface can stand up to acidic sprays. Acetic acid works in a similar fashion to the old non-translocated total weedkillers such as paraquat (now withdrawn), killing surface weed growth by corrosive action.
- ‘Armillatox’ a hard surface cleaner will remove liverworts and has a useful added effect in reducing the spores that serve to spread the problem and re-infest treated areas.
The difficulty with liverwort control is that they produce copious amounts of spores, so unless all of these are eradicated by an application of a product, the problem is likely to recur.