The summer woodland is home to shade-tolerant flowers. In Spring, before the tree canopy has unfurled, less shade tolerant plants
like the bluebell (wild hyacinth) will grow and flower and are already turning to seed when the summer leaves of the overhead trees
open out and block much of the sunlight.
In late spring and summer, the woodland transforms in several obvious ways. Some trees put-out fantastic blossoms, especially the
cherry tree and horse chestnut tree (Aesculus hippocastanum). These showy flowers are intended to attract insects for pollination.
Some insects, like bees, have good colour vision as well as a good sense of smell and are attracted to these flowers both by their
scent and also by their colours. Insects generally have excellent ultraviolet vision, being capable of seeing colours that humans
cannot see and many flowers have ultraviolet markings that are intended to attract these insects and then direct them to the pollen
and nectaries. The flowers of the horse chestnut have inner yellow markings to guide their pollinators and once a flower is pollinated
the markings change to red, signalling to the insect that the flower is pollinated and so presents no nectar reward, thus guiding
insects to flowers awaiting pollination. Beneath the canopy with its new leaves that typically are a lush bright green when newly
formed (darkening as they mature) the forest floor springs into life with rapid growth. Tracks that are not in very frequent use rapidly
become overgrown with nettles and brambles. The brambles are scrambling, thorny shrubs, otherwise known as blackberries after
their autumn fruit. These brambles put out aerial shoots that actively swing about in very slow circling movements, seeking objects
for support which they will wrap around when their touch sensors are sufficiently activated by rubbing against the support. They also
put out creeping stems that snake across the forest floor, in slow motion, colonising new ground and finding supports to climb up or
scramble over. Brambles have pretty white or pink flowers.
Lamiaceae - Deadnettle family
Among nettles are the stinging nettles (Urtica dioica) and the dead nettles (archangels). Stinging nettles have tiny hollow glass hairs
or trichomes made of silica (one of the few obvious uses of silicon in plants). These hairs easily pierce the skin and snap when
brushed, releasing the formic acid (methanoic acid) that fills their hollow shafts into the skin, like hypodermic needles, and causing a
stinging sensation on the skin. (Presumable, the acid is drawn out of the broken 'needle' by capillary action). Dead nettles are
almost indistinguishable from stinging nettles, before they flower, but they do not sting. The flowers of stinging nettles are small
whitish flowers, born in rows on stalks that radiate in whorls at intervals along the stem. In contrast, the flowers of the archangel
(Lamium album) are much larger, far fewer in number and easily seen as white zygomorphic flowers with orchid-like shapes.
Flowering plants belong to the Angiosperm group of plants. These include flowering herbs, shrubs and trees as well as grasses.
Conifers and yew trees are non-flowering and belong to the Gymnosperms (though sometimes the unfertilised cones of conifers
are referred to as 'flowers'). Gymnosperms do not completely enclose their seeds inside fruit, as the angiosperms do. Instead,
seeds are borne directly ion the scales of the female cones in conifers and partially enclosed by a fleshy aril in yew trees. Ferns,
horsetails and mosses produce neither flowers, seeds or fruit, but reproduce by naked spores.
Wind-pollinated flowers are non-scented and usually much smaller and less colourful, often greenish in colour, since they do not
need to attract insects. Wind-pollinated trees often bear clusters of flowers on catkins. Scales on the catkin seal the openings
shut when the catkin dangles motionless, but should the wind wave it about, then gaps in the tiny scale appear and the pollen
escapes to be carried away on the wind.
Wind-pollinated trees include: oak, beech, birch, ash, hornbeam, alder, elm and hazel.
Animal-pollinated trees and woody plants include: lime, hawthorn, box, cherry, apple, pear, elder, blackthorn and broom.
The distinction is not clear-cut. Some plants edge their bets, willow, for example, produces very visible catkins that can be
wind-pollinated, but are often pollinated by blue tits. The catkins produce copious amounts of nectar to feed the blue tits, an
expensive investment, which is offset by the fact that blue tits can carry a lot of pollen and travel large distances, each pollinating
Other larger animals that may pollinate trees include bats, which in the tropics pollinate kapok, balsa, durian and baobab trees.
Birds, rodents and primates may also pollinate trees and the giraffe pollinates the knobthorn acacia (Acacia nigrescens) upon
whose leaves it feeds. Bird-pollinated plants often produce red hanging flowers. Flowers pollinated by flies, like arum, are typically
pale and dull. Bee pollinated flowers are often yellow or blue and flowers pollinated by moths and butterflies can have various
colours. The cocoa tree is pollinated by biting midges that breed in decaying cocoa pods.
Separation of the Sexes
Many plants have so-called perfect flowers, which are hermaphrodite with both male and female parts. Woody plants with
perfect flowers include limes, blackthorn, broom, cherries, apple, pear, privet, elms, elder, hawthorns, horse chestnuts,
whitebeams and some maples.
When the sexes are separate, then the plant may be monoecious (exhibit monoecy) or dioecious (exhibit dioecy). Monoecy is
more common in trees and is when separate male and female flowers occur on the same plant, e.g. sweet chestnut (Castanea
sativa), alder, beech, birches, box, hazel, hornbeam and oak. Monoecious conifers have separate male and female cones, e.g.
pines, cypresses, redwoods (separate male and female cones).
Dioecy is less common in trees, and occurs when separate male and female trees are found, and is found in holly (Ilex
aquifolium, though sometimes perfect flowers are found in holly), willows (rarely monoecious), juniper, poplars (rarely
monoecious), ash (sometimes mixed) and yew. In ash and yew, male and female flowers sometimes occur on the same tree.
Click on the model of the passion
flower (left) for a biological description
of flower structure and function.
Above: The wild arum, Arum maculatum (Araceae), is a remarkable woodland flower that flowers in Spring and produces its stalked
columns of red berries in early summer. It belongs to the arum lily family (Araceae). The leaves are large and shaped like
arrow-heads and often bear dark spots. The 'flower' (actually a structure around several flowers) comprises a reddish-purple
cylindrical structure, called the spadix, borne on a stalk that descends into the basal bulb of the flower. Behind the spadix is a large
hood or spathe, formed from a modified leaf. This stately appearance probably led to their common name of Lords and Ladies. They
also have a common name of Cuckoo Pint (though they have no relationship to cuckoos!). The flowers are large, but easily
overlooked due to their greenish colour and the fact that they do not flower for long.
The size of the arum flower suggests that it is insect pollinated, but its dull colour would not be suitable for bees and butterflies.
Instead, the arum attracts flies to pollinate it. The spadix generates heat, maintaining its temperature at several degrees above
ambient. It also releases perfume that smells of decay (some say it smells of excrement, others that it smells of rotting meat and this
probably depends on the species of Arum). The heat of the spadix helps to volatilise the odour substances. These substances
consist largely of ammonia and amines. The rotting smell attracts flies that land to find the inside of the spathe very slippery and they
fall down into the bulb. The epidermal cells on the inner surface of the spathe each have a minute papilla (wart-like projection) which
is soft, springy and slopes downwards. Their are very few stomata on this inner surface, keeping the surface otherwise smooth. This
epidermis also carries numerous tiny droplets of slippery oil. The downward pointing papillae and oil droplets make it very hard for
any insect to retain a purchase. A mesh of stiff hairs filter out the largest insects, who fall no further and can escape, but those of the
right size fall through into the bulb.
Inside the bulb, the inner wall has numerous tiny channels or air spaces, that connect to stomata in the outer surface. This provides
the inside of the bulb with air for the trapped insects. The lower part of the bulb has no papillae or oil droplets and the trapped
insects are free to walk around and over the female flowers clustered at the base of the bulb. Thus, any pollen they carry from
another flower of the same species may pollinate the female flowers. The male flowers are not yet shedding pollen, so self-pollination
is avoided. When the flower is ready, it releases its odour, from the tip of the spadix, most strongly at sunrise, as soon as the air
begins to warm. The smaller dung-feeding insects that slip inside the hood cannot get airborne quickly enough to escape (larger flies
may succeed in flying away and are in any case prevented from entering the bulb by the hairs). The hairs over the bulb entrance are
oily and slippery at this stage and those insects small enough slip through. Contrary to some claims, the insects are generally never
observed to deflect the hairs or walk into the bulb. The spacing between the hairs varies from plant to plants, so that the maximum
size of insect allowed to enter also varies. No insect is released before nightfall. The wall and central column are too slippery to allow
insects to escape. Insects may attempt to fly away from a dark chamber, such as a darkened room or cave, when they can see a
daylight opening and have enough space to deploy their wings. The bulb, with its various internal organs has insufficient room and
the hood blocks the sight of direct daylight above. Indeed, illumination of the bulb, at least in some arums, comes predominantly from
below, tempting the insects to seek an exit below, where there is none.
The insect guests are looked after to some degree, the female flowers secrete water and some organic foodstuffs, and humidity
within the bulb is high and the bulb is also ventilated. However, some insects do die inside the bulb and the insects seek to escape. In
at least one arum lily studied (Sauromatum) heat production occurred in the floral bulb at night. This presumably keeps the
pollinators warm, and active as they effect pollination. On the morning of the second day, the odour is much reduced. The bulb and
hood remain unclimbable, but the axis bearing the spathe has become climbable, as its surface cells have crumpled. The insects
climb the spadix and escape. On their way out, however, they must climb past the male flowers, which are now ripe, picking up pollen
on their way out. Should they become trapped by another Arum, then they may pollinate it. As arums set seed quite readily, clearly
many hapless insects get caught more than once!
The development of the spadix in arum lilies has been shown to be under the control of the phytohormone salicylic acid. This
hormone is produced by the stamen-bearing flowers as they develop on the spadix and triggers heat production (thermogenesis),
odour secretion and unfolding of the spathe. Thermogenesis occurs when the mitochondria inside the cells (organelles that oxidise
foodstuff fuels to release energy) switch from producing ATP (an 'energy storage' molecule) as they do in normal aerobic respiration,
to generating heat instead. They consume starch fuel reserves inside the spadix in order to generate this heat and the starch
content of the spadix decreases from about 32% of the dry mass of the spadix to only 6% after 8-10 hours of thermogenesis. The
starch is broken down into glucose and the glucose oxidised to liberate the heat. Oxygen is needed to oxidise the glucose and during
thermogenesis the rate of oxygen consumption by the spadix reaches a rate equivalent to the oxygen consumption of a flying
Below: bindweed (Calystegia) with its large
white trumpet-shaped flowers (family:
Convolvulaceae, Bindweed family). Click
all images for full size.
Pollination in Arum
The Arum fruit are easy to spot and there are
rather a lot of Arum near here now in fruit.
Above: Wild arum growing by a roadside, click images to enlarge.
White Archangel, click images to enlarge.
The White Archangel , Lamium album, is a member of the fascinating family Lamiaceae. Other members of this family that may be
found in woodland include the yellow Archangel, Lamium galeobdolon and the bugle, Ajuga reptans.
Learn more about the Lamiaceae.
Above: the bluebell, Hyacinthoides non-scripta (Lily family, Liliaceae). This species is characteristic
of ancient woodland.
Learn more about bluebells.
Above and below: a wild arum (Arum maculatum) growing in woodland on calcareous soil.
Left: the wood anemone, Anemone nemorosa
(Buttercup family, Ranunculacea), is a common flower
of early spring in temperate woodland. These flowers
are quite large, 2 cm or more (about an inch) in
diameter. The 6-7 'petals' are actually sepals and/or
petals, as these appendages are essentially identical
and are referred to as tepals. In the middle flower,
however, there do appear to be two separate whorls
of three appendages, in which case the lower whorl
would correspond to sepals, the higher to the petals.
The many stamens are clearly visible with their
Above: Herb Robert, Geranium robertianum, growing against an ancient oak. (Geranium family,
Above and below: wood spurge, Euphorbia amygdaloides (Euphorbiaceae), is an indication of
ancient woodland. Note the four C-shaped nectaries in each flower. These flowers are petal-less,
the petal-like structures are actually bracts. If you look carefully then you can see that female
flowers have a three lobed stigma, each lobe of which is often forked.
Above: the Early Purple Orchid, Orchis mascula, is found in a wide range of habitats, including
woodland. It frequently occurs where Bugle (Ajuga reptans) occurs nearby. The plant apparently
offers no reward to the bees that pollinate it (though it possibly produces a small amount of
sugary sap in the wall of the spur, the spur is essentially empty). It relies on naive bees emerging
from hibernation and the proximity of Bugle. Bees feeding on nectar from Ajuga may visit the
nearby Orchis mascula, until they apparently eventually learn that it isn't worth it.
Learn more about orchids.
Left: a dehisced catkin of sycamore, Acer
pseudoplatanus (Maple family, Aceraceae). Usually
up to 60%, sometimes 100%, of the flowers on such a
catkin are male, the rest being hermaphroditic.
Learn more about sycamore trees.
Left: Bugle, Ajuga reptans (Lamiaceae).
Primulaceae - Primrose family
Primrose (Primula vulgaris).
Primroses (family Primulaceae) have an interesting mechanism to reduce self-pollination and enhance cross-pollination, meaning
that they are designed to be pollinated most easily by pollen from a different plant. Cross-pollination prevents 'in-breeding' and
maintains genetic diversity in a population, which also helps to counteract the effects of detrimental mutations in genes, since with
cross-breeding, an organism is more likely to inherit a healthy copy of a gene if its parent population contains faulty copies.
Primrose flowers, even on the same plant, come in two morphological varieties: pine-eyed and thrum-eyed. pin-eyed are so-called
because the carpel has a long style that raises the stigma to the top of the floral tube, where it is visible from above as a 'pin-eye'
like structure. In a pin-eyed flower the stamens are lower down than the stigma in the floral tube. In thrum-eyed flowers, the style
is shorter and the stamens higher, so that the stigma is far beneath the stamens. Pollen must travel from a stamen to a stigma,
and attach to the stigma, to effect pollination.
If an insect with a long proboscis visits a pin-eye flower and reaches for the nectar at the base of the floral tube, then pollen
will rub onto its proboscis. If this same insect then visits a thrum-eye flower, then the pollen is at the right height to rub off
onto the short stigma of the thrum-eye flower. The insect's head will also become covered with pollen from the thrum-eye
flower. If this insect now returns to a pin-eye flower then the pollen on its head will be at the correct height to rub off onto the
stigma of the pin-eye flower.
Above: Greater Stitchwort (Stellaria holostea) is found in hedgerows and along woodland borders in
the British Isles. It is a member of the Campion / Pink family (Caryophyllaceae).
21st April 2014
28th March 2015
8th June 2015
This article focuses on British woodlands, which are typical temperate woodlands which have been subject to varying degrees of
management or interference. However, most of the research for this article is based on ancient and semi-ancient woodland where
interference has been relatively modest.
Left and below: Lesser Celandine (Ranunculus ficaria)
is also a member of the buttercup family
(Ranunculaceae) found in woodland in spring, from
Orchidaceae - Orchid family
Orchids are important epiphytes in tropical forests, but in temperate woodlands they have adapted to live in the soil and
leaf-litter of the woodland floor.
Below, Dog's Mercury (Mercuralis perennis) another member of the Euphorbiaceae (Spurge
family) frequently found on the floor of temperate woodland in late winter and early spring. This
plant is dioecious (separate male and female plants). Female flowers have 3 sepals and a hairy
ovary and no functional stamens and are borne in groups of 1-3 on stalks. Male flowers have 3
sepals and 8-15 stamens. This plant is extremely posionous.
Ranunculaceae - Buttercup family
Geraniaceae - geranium family
Euphorbiaceae - Spurge family
Caryophyllaceae - Campion family
This could be the Hedge bindweed (Calystegia sepium) or the Large Bindweed (Calystegia
silvatica). The way to distinguish these two is to look at the two bracteoles of the epicalyx
surrounding the green calyx beneath the petals: these overlap one-another in C. silvatica.
Liliaceae - Lily family
Araceae - Arum family