Strictly applied, the word cryptogams refers to all plants which reproduce by spores rather than seeds, but usually excludes the pteridophytes (i.e. ferns and their allies) that, while being spore-bearing, are included with the vascular plants. The following groups considered to be cryptogams are a diverse set of unrelated organisms. From a purely systematic perspective most cannot be considered plants, nevertheless, traditionally they have been studied by botanists and are most commonly preserved and curated in herbaria.
To assist with understanding the relative placement of the various cryptogam groups, here is an overview of the higher levels of the Tree of Life, biased towards plant phylogeny and based primarily on Judd et al. (2002), Plant Systematics: A Phylogenetic Approach, 2nd edition. The Kingdom models are summarised from Whittaker, R.H. (1969), ‘New concepts of kingdoms for organisms’, Science 163: 150–160; and Cavalier-Smith, T. (2004), ‘Only six kingdoms of life’, Proceedings of the Royal Society London B 271: 1251–1262.
Readers interested in further detail of the groups and their relationship in the Tree of Life are recommended to browse the following external sites: the Tree of Life project and the Palaeos:Eukarya pages. The following is extracted from Scott et al. (1997) and describes the circumscription and significance of the four major traditional cryptogam groups.
Nowadays generally classified in the fungi, the lichens are compound organisms, each comprising a symbiosis between a cyanobacteria (blue-green alga) and a fungus. They are therefore often referred to as the lichenised fungi. Being photosynthetic thanks to the cyanobacterial component, lichens are ecologically quite different from the other fungi, and have traditionally been studied in a quite different way; by lichenologists rather than mycologists.
Bryophytes comprise the mosses, liverworts and hornworts; that is, all plants which develop from embryos enclosed by maternal tissues but which lack lignin.
Algae is a general term that includes all photosynthetic organisms which do not have embryos, excluding lichens. They include a very wide range of unrelated groups, which in the context of this report include mainly the Green algae (Chlorophyta), a few Red algae (Rhodophyta), Yellow-green algae (Chrysophyta) and the procaryotic Blue-green algae (Cyanobacteria). These groups are currently treated as two or more kingdoms, separate from ‘plants’.
In older classification systems, fungi were included within the plant kingdom and were separated from other plant groups by the lack of photosynthesis and the production of spores. It is now generally agreed by mycologists that fungi are not plants and should be placed in one (or several) separate kingdom(s). Fungi are very diverse, including some groups closely related to algae and many others linked by life-style rather than ancestral relationship. The main groups of large (macro) fungi are the Basidiomycetes and Ascomycetes.
All the above groups lack lignin and consequently are of relatively small stature. Most algae and the microfungi are visible in detail only under the microscope. Bryophytes and lichens are slightly larger, from about 0.5 mm to 500 mm (rarely) but usually less than 100 mm. Macrofungi are the largest and tend to be much bulkier than other cryptogams.
The generally small size of cryptogams is one of the factors that have resulted in their being largely ignored in conservation. Others factors, some of which are related to size, include: failure to understand their role in ecosystems, difficulties in identification, the very great number of species (especially fungi), frequent lack of permanence, lack of attractive flowers, and the self-perpetuating problem of teaching which is biased towards angiosperm botany.
In the sequence — lichens, bryophytes, algae, fungi — there is a rough general but paradoxical relationship: relatively decreasing taxonomic knowledge and decreasing numbers of taxonomic/ecological workers, but increasing species numbers, increasing technical difficulties of study, and increasing ecological importance. Apart from some cases within the first two groups, this relationship holds both in Australia and overseas.
Ecologically, these are probably most important as primary colonisers and stabilisers. On bare rock they are the most common initiators of colonisation (except in rain-tracks) and often remain the sole colonisers, with no real successors. But it is on soil crusts in semi-arid areas that lichens, together with other cryptogams, have perhaps their major role, holding the soil in place, preventing erosion, both stabilising the surface and building up humus to form more fertile soil. This very important function has been too little appreciated (Scott in Smith 1982) although soil crusts have begun to receive more attention in recent years. In wet forests, lichens may be abundant epiphytes, especially in areas of higher light intensity such as the canopy where they form part of the water-absorbing and cation-exchanging mantle through which much of the run-off into the forest is filtered. The significance of this function is still being investigated. The relationship between the mycobiont (fungal component) of the lichen and the host, which appears to span the range from epiphyte to saprophyte to parasite, is also much in need of modern critical investigation. It is quite likely that lichens are of much greater significance in many forest ecosystems than has yet been rigorously established.
More than most other plants, lichens have proved to be exceedingly difficult to grow in culture so that conservation ex situ or by transplantation is not possible and conservation in situ is essential.
The ecology of bryophytes is akin to that of lichens. Both have large numbers of poikilohydric species, i.e. resurrection plants, capable of withstanding prolonged periods of drought in the desiccated state and resuming growth and metabolism rapidly on being moistened again (Scott in Smith 1982; Moore & Scott 1979). Major components of the soil crusts, and also of wet forests, bryophytes differ from lichens ecologically in that they generally prefer darker and often moister micro-climates, form denser clusters of greater water-holding capacity as epiphytes, and in rainforest may form a high proportion of the ground flora cover so that they act as a carpet through which all rainfall is filtered. In the stabilisation of sandy soils, bryophytes (mainly mosses) establish shortly after the initial colonisation by angiosperms, and are the major force in stabilising the soil by building up humus and capturing and consolidating blown sand.
Bryophytes are an ancient group and hence tend to have fragmented distributions. Many of them, if not endemic, are found only in small scattered and isolated populations of very small numbers of plants with little ecological resilience and apparently not capable of spreading to other uncolonised but seemingly suitable areas. This presents particular conservation problems.
Although algae are, together with lichens and bryophytes, important components of the soil crust flora and, if blue-green algae (Cyanobacteria) are included, are probably the major nitrogen-fixing components of the crusts, it is the planktonic forms that are probably of greatest significance in freshwater as in salt water. Phytoplankton is the basis of the food chains and hence is of crucial significance in the biosphere as the principal carbon-fixing and oxygenating agent. Without algae, all freshwater bodies would be effectively dead.
The alge of streams, in particular diatoms, are potentially powerful organisms for monitoring and assessing aquatic health. Filamentous algae also respond dramatically to natural and regulated flood regimes, and their importance in the dynamics and viability of stream ecosystems has so far received only cursory attention worldwide. In Australia, algae have been overshadowed by fish, invertebrates, chemistry and hydrology in all scientific studies of freshwater ecology.
The potential importance of algae in industrial biochemistry is very great. It seems certain that algal cultures, already under experimental trial, will become industrially significant in the relatively near future. There is a considerable, almost untapped resource of chemical compounds (including oils, some of which may have been the source of most petroleum deposits) and also of protein and carbohydrates for foodstuffs. It is important to conserve the available genetic diversity for future exploitation, if for no other reason.
A few fungi are well known to lay people although the overall importance of the group is generally not recognised outside scientific circles.
Although of undoubted importance for their very widespread symbiotic relationships with higher plants, the most striking significance of fungi is in the decomposition of dead plant material. Whereas some ecosystems might be able to function in the absence of lichens, bryophytes and even algae, and certainly without angiosperms, it is difficult to conceive of any bar the simplest ecosystem surviving in the complete absence of fungi. Especially in forests, a large part of the nutrient flux is mediated by fungi. The healthy functioning of forest fungi is hence of paramount importance to all the forests of the world, including those in Australia.
As a source of antibiotics, edible foods, yeasts and, conversely, of plant and animal pathogens, the economic significance of fungi is very great and is still scarcely tapped. As with all cryptogams, the proportion of fungal species that have been chemically investigated is minuscule.