As you move from outside the cave entrance into the cave threshold there is a change in the nature of plant life. This corresponds to changes in the habitats. There is a decrease in the amount of light and the increased shelter reduces wind and helps to maintain humidity (combined with the damp air in the cave). The proximity of an entrance means it will remain cool but relatively frost free.

FLOWERING PLANTS
Shade loving plants (sciophytes) will grow around the entrances of caves. There are three species of flowering plant commonly found in cave entrances that can cope with light levels down to 24 lux – Herb Robert (Geranium robertianum), Wood Sorrel (Oxalis acetosella) and Opposite Leaved Saxifrage (Chrysosplenium oppositifolium). (There are different methods of measuring light. Lux is one - a living room may be 60 lux, an office 400 lux and sunlight may produce 50,000 lux).

Herb Robert Geranium robertianum

Other typical flowering plants of the threshold include Ivy (Hedera helix), Cuckoo Pint or Lords and Ladies (Arum maculatum), Stinging nettle (Urtica dioica), Dog Violet (Viola riviniana) and Dog’s Mercury (Mercurialis perennis) which prefers drier conditions.
In deep shade some plants have a different form with larger leaves e.g. Dog Violet, the mosses Eucladdium vertillatum, Adoxa and Phyllitis spp.and the Asplenium ferns.
Plants have a chemical (a bit like chlorophyll) that switches its formula depending on light intensity. If light is low it exists in a form that prompts the plant to grow taller in an effort to reach more light (this is why seedlings in caves are often tall and spindly with a token leaf at the top).
Limestone soils are generally dry, well aerated and warm; waterlogging is not a problem. The soil and organic matter around entrances can produce locally acid conditions, this allows the growth of plants not normally associated with basic limestone soils e.g. Wood Sorrel.

FERNS
Fossils of ferns date back to 400 million years ago, about 300 million years before the first flowering plants appeared. Some ferns can grow in deep shade with light intensities down to 18 to 10 lux; one species, Rusty-Back Fern Ceterach officinarum, can cope down to 5 lux. Both mosses and ferns are most abundant in damp shady areas due to the need for water for the male gametes and to complete their reproductive cycle (flowering plants have evolved to overcome this problem).  A good zonation of moss and fern species can be seen in the entrance shaft to Manor Farm Swallet.
Soft Shield fern Polystichum setiferum is common around cave entrances and can be found on slopes near caves.
Nearer the entrances and growing on the walls you may see Harts Tongue Fern Phyllitis scolopendrium and Maidenhair Spleenwort (Asplenium tichomanes). These are also sometimes found near show cave lights such as these in Gough's Cave.

Hartstongue Fern Phyllitis scolopendrium, Ebbor Gorge

The Male Fern (Dryopteris filix-mas) prefers neutral conditions and can be found around cave entrances.
The British Polypody species are similar and also hybridise making identification difficult. The scarcest is Southern Polypody (Polypodium australe) which grows in limestone areas of south and west Britain. The Common Polypody (Polypodium vulgare) prefers acidic conditions throughout Britain, and the Western Polypody (Polypodium interjectum) prefers weakly acid or basic rocks and soils in western Britain.  The Western and Common Polypody species are often found growing as epiphytes on tree trunks, like this Western Polypody at the entrance to GB cave.

Western Polypody Polypodium interjectum, GB Cave

The rare Limestone Fern (Gymnocarpium robertianum), looking like a miniature Bracken, occurs on scree, rocks and natural limestone pavements and can be found growing in Cheddar Gorge. The Brittle Bladder Fern (Cystopteris fragilis) prefers damp, heavily shaded conditions and can be found on rock faces in Harptree Combe and the Mells valley. Mendip is the southern limit for both the Limestone fern and Brittle Bladder Fern.

Some species prefer more acidic conditions but can still be found around cave entrances where there is moist, peaty woodland soils or where the lime has been leached from the soil. Conditions may then be too acidic for the Shield Ferns or the Harts Tongue Fern, but are more suited to the Mountain or Lemon-scented Fern (Oreopteris limbosperma), Hard Fern (Blechnum spicant), Lady Fern (Athyrium filix-femina), Broad Buckler Fern (Dryopteris dilatata), Bracken (Pteridium aquilinum) and Wilson’s Filmy Fern (Hymenophyllum wilsonii). Overall they are relatively uncommon on much of the Mendips due to the prevalent calcareous soils.

Many ferns are green year round but those that die back in winter include the Brittle Bladder Fern, Broad Buckler fern, Male Fern, Limestone Fern, Bracken, Lady Fern and Mountain Fern.

MOSSES
Mosses and liverworts are Bryophytes - simple green plants that have the ability to suspend their metabolism when desiccated (unlike a flowering plant that shrivels up and dies). Because they have no roots most of them absorb water, nutrients and gases over their whole surface. Like lichens they can be used as indicators of air and water pollution.
As you approach a cave entrance the shade increases and the ferns are replaced by mosses and liverworts - mosses are commonest at 10 lux, liverworts at 10 to 5 lux. As the light decreases only green algae and blue green algae (cyanobacteria) are found.
Typical mosses in the more shaded parts of limestone (i.e. in the gulleys on rock faces) are the yellow-green Comb Moss Ctenidium molluscum with its curved leaves, Maidenhair Pocket-moss Fissidens adianthoides and Crisped Neckera Neckera crispa.

Fissidens sp., Bos Swallet

The genus Eurhynchium includes several species that grow on boulders and walls around entrances.  They are tolerant of low light and can survive by show cave lights.
Some mosses form dense cushions e.g. Whorled Tufa-moss Eucladium verticillatum; this one along with the golden green pinnate Fern-leaved Hook-moss Cratoneuron filicinum, prefers wetter habitats such as springs, where it forms the basis of tufa dams. Eucladium sp. and Curled Hook-moss Palustriella commutata are tufa forming species, the mosses actively promoting the deposition of carbonate from seepage water. Tufa eventually hardens into a porous brown limestone called travertine.   
Anomalous Bristle-moss Orthotrichum anomalum forms neat dark-green cushions on base-rich rocks (such as limestone) and artificial walls, buttresses etc., usually in exposed situations.
Hart's Tongue Thyme-moss Plagiomnium undulatum is common on rock walls by damp entrances.   

Hart's Tongue Thyme-moss Plagiomnium undulatum, Read's Cavern

LIVERWORTS
Liverworts are not quite as desiccation tolerant as mosses and so prefer moister habitats. The Great Scented Liverwort Conocephalum conicum is very common in cave entrances and becomes more stunted the further into the cave it grows.
Another common liverwort of cave entrances is Endive Pellia Pellia endiviifolia. Another liverwort found in limestone gulleys is the Star-headed Liverwort Marchantia polymorpha.

ALGAE
This can be seen growing on the walls and roof inside cave entrances where it faces the outside. The green colour, marking the presence of algae can be seen on parts of the roof and walls inside the larger cave entrances on the Mendips (e.g.  Brownes’ Hole and the impressive entrance to Wookey Hole) and quite some way down the Manor Farm Swallet entrance shaft.
On the nutrient deficient surface of limestone rock faces a thin grey/black film of cyanophilic algae may be present - these can fix their own nitrogen from the air.
The orange patches seen on limestone are actually a green algae called Trentepohlia. It contains carotenoid pigments that mask the green chlorophyll. It sometimes combines with certain fungi to form a lichen.

LICHEN
Lichens are usually associations between fungi and photosynthetic partners. The definition of a lichen is constantly changing as scientists discover new associations. Many 'species' live on limestone, and their hypae may actually extend into it. Those on limestone are often small and difficult to identify - as a result they have been neglected by biologists and there is scope for new discoveries in this area. They are able to survive extremes of conditions and can be found on the limestone pavements of Wales, Yorkshire and Derbyshire, on exposed clints and in the damp shaded depths of the grykes.