Woodland_Flowers

Woodland Flowers

Click on the model of the passion flower (left) for a biological description of flower structure and function.

Meadow flowers.

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.

Liliaceae - 'Lily family' (sensu lato - in the broad sense)

Above: the bluebell, Hyacinthoides non-scripta (Lily family, Liliaceae sensu lato). This species is characteristic of ancient woodland in northwestern Europe.
Learn more about bluebells.

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. The lily family (Liliaceae) has undergone major revision in recent years. Hyacinthoides non-scripta is now usually placed in the family Asparagaceae (Asparagus family) based on molecular work examining evolutionary closeness.

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 color vision as well as a good sense of smell and are attracted to these flowers both by their scent and also by their colors. Insects generally have excellent ultraviolet vision, being capable of seeing colors 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, colonizing new ground and finding supports to climb up or scramble over. Brambles have pretty white or pink flowers.

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.

Lamiaceae - Deadnettle family

Among nettles are the stinging nettles (Urtica dioica) and the dead nettles (archangels). Stinging nettles (Urticeae, Nettle Family) 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 hard to distinguish 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.

The Yellow Archangel , Lamiastrum galeobdolon (Lamium galeobdolon), is a member of the fascinating family Lamiaceae. It is found in woodland and on hedgebanks across most of europe, and is also introduced in parts of the USA. Other members of this family that may be found in woodland include the bugle, Ajuga reptans.

Learn more about the Lamiaceae.

Below: Bugle, Ajuga reptans (Lamiaceae).

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 (that is they are terrestrial).

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.

Araceae - Arum Family - pollination in Arum, 'lord of the flies'

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). Common names include: Lords-and-Ladies, Devils and Angels, Parson in the Pulpit and Cuckoo-pint. 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 color 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 color 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 odor 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 odor 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), odor 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 hummingbird!

Above: the brightly colored fruit are fairly easy to see in the undergrowth, but are poisonous to humans and many animals, but birds can eat them and disperse the seeds in doing so.

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.

Ranunculaceae - Buttercup family

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, but then we have to account for the odd appendage when present. The many stamens are clearly visible with their yellow-orange anthers.

Below: Lesser Celandine (Ranunculus ficaria) is also a member of the buttercup family (Ranunculaceae) found in woodland in spring, from March.

Euphorbiaceae - Spurge 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: Dog's Mercury (Mercuralis perennis) is 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.Each tiny seed contains an oil gland that is nourishing to ants and so ants carry away the seeds to feed on the oils, buring and dipsersing the seeds, helping them to germinate.

Geraniaceae - geranium family

Above: Herb Robert, Geranium robertianum, growing against an ancient oak. (Geranium family, Geraniaceae).

Primulaceae - Primrose family

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.

Above: Primrose (Primula vulgaris).

Pin-eyed primrose.

Thrum-eyed flower of primrose

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.

The photos below show one pin-eyed and one thrum-eyed flower, but can you tell which is which? (click images for full size).

Caryophyllaceae - Campion family

Above: Greater Stitchwort (Stellaria holostea) is found in hedgerows and along woodland borders in the British Isles, and has been introduced into parts of the US. It is a member of the Campion / Pink family (Caryophyllaceae).

Flowering Trees

Above and below: the catkins of Hazel (Corylus avellana) are one of the first flowers to be seen in early spring. Not all woodland flowers are showy! Many trees and shrubs rely on wind for pollination and so do not need to attract insects. The yellow catkins are male (a catkin is a pendulous spike, a spike being a central axis bearing stalkless flowers and accompanying bracts). In addition to scale-like bracts, the male anthers can be clearly seen. The female flowers are solitary, small, greenish and budlike but with red protruding stigmas, ready to capture pollen blowing past on the wind. A couple of female flowers can also be seen in the image above.

Above: 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.

Wind-pollinated flowers are non-scented and usually much smaller and less colorful, often greenish in color, 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 many trees.

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 colors. The Cocoa tree is pollinated by biting midges that breed in decaying cocoa pods.