Apocyanaceae (Dogbane Family)

Above and below: Vinca major (Greater Periwinkle, Bigleaf Periwinkle), a member of the Dogbane Family (Apocynaceae). The Apocynaceae are an interesting and diverse family of herbs, shrubs, trees, vines and stem succulents. Many of them are exotic with unusual pollination mechanisms. Wild members of the family in the British Isles belong to the genus Vinca: Vinca major (Greater Periwinkle), Vinca minor (Lesser Periwinkle) and Vinca difformis (Intermediate Periwinkle). The hermaphrodite (androgynous) flowers have a distinctive pinwheel shape and are generally violet to blue in Periwinkles, but occasionally white. Vinca major is found in hedges and bushy scrub.

Vinca comes from the Latin verb 'vincio' ('I bind', or from 'vinculum' meaning 'bond' or 'link') due to its long and entwining shoots which were used to make wreaths. Vinca major is a slightly shrubby (subshrub) perennial. The flowers have 5 sepals, 5 petals, 5 stamens (each anther with 2 loculi or pollen-producing chambers) and one style. The large tubular flowers are reportedly pollinated by bee-flies (Bombylius), bees such as Bombus (Bumblebee) and Anthophora plumipes (the Hairy-footed Flower Bee). It has been suggested that they may also be pollinated by moths. The fruit consists of a pair of subcylindrical follicles that dry and split to release the seeds. In Vinca minor there are 3 to 5 seeds inside each follicle. Note the ovate leaves with entire margins (i.e. not divided into lobes or leaflets).

Each flower is borne on an upright stalk (peduncle) growing from a leaf axil of the erect flowering shoot. The stalk bends at 90 degrees towards its end to hold the flower horizontal. A flowering stem produces one flower at a time, the subsequent flower expanding as the previous flower withers.

The pollination mechanism has been studied in Vinca minor and Vinca major and is essentially identical in the two species, except that the slightly smaller flowers of V. minor likely cater for a different range of pollinators. In all Apocynaceae the 5 styles unite to form the column (gynostegium) and expand to produce the style head. The five short anthers (completely enclosed by the corolla or petal tube) are specially modified and positioned over a non-receptive part of the style head, which is a disc-shaped expansion. The filament of the anthers is bent inwards as a knee which abuts beneath the style head and then bends outwards to approach the inside of the corolla tube before arching over the disc-like expansion of the style head. This arrangement is important to hold the style head firmly in position within the ring of anthers. The anthers are introrse, meaning they split open inwards so the pollen falls onto the non-receptive expansion of the style head, as shown in the half-flower diagram below. The ovary consists of two carpels that alternate with two nectaries, the nectary being similar in size to the carpel. The secreted nectar fills up the corolla tube to the level of the ovary.

The anthers discharge their pollen onto this shelf (style head expansion or collar) as 10 loosely bound packets or pollinia (2 pollinia from each anther). The pollen is white, but the 'pollen shelf' style collar is orange, which is generally thought to signal the presence of pollen to bees. This 'pollen-presenting table' is enclosed by hairs: the filament of each stamen extends into a stamen cap covered in upright hairs and the style head is surmounted by a plume of silky hairs. Hairs also extend down from the stamen knees to pack the lower part of the corolla tube. These hairs create a moist chamber to protect the pollen and nectar. The pollen becomes entangled within the style plume.

The stigma produces a secretion which makes the proboscis of any feeding insect sticky, so that some of the pollen from the table and/or style plume sticks to the proboscis. Vinca minor is allogamous (requiring cross-pollination, literally requiring 'different gametes'). The fruit are 10 to 30 mm in length.

The flowers last for about 7 to 10 days and after pollination the corolla is shed along with the attached stamens and the style falls off with it. Non-pollinated flowers are shed whole. The flower stalk now reflexes to hide the fruit among the foliage leaves (possibly to prevent them becoming dehydrated).

Above: half-flower diagram of Vinca major, showing 2 stamens above the 'pollen table'.

Growth habit and vegetative spread. The vegetative stems, that is stems bearing no flowers, which are produced in autumn are prostrate (or ascending/arching and then procumbent) and root at their apex at the end of their annual growth, from where flowering stems, up to 30 cm tall, may be put out in spring. These vegetative stems may reach 2 m in length. The flowering stems are erect but after flowering they elongate and become procumbent (prostrate). The flowers are solitary and each peduncle (inflorescence stalk) grows from a leaf axil and is shorter than the length of the leaf.

Vinca minor has roots 3 to 8 cm in length and reproduces vegetatively by means of stolons (procumbent stems growing along the ground as in Vinca major). Indeed vegetative reproduction by stolons seems to be the main mode, at least in Britain. Lesser periwinkle is classed as a subshrub or vine.

Distinguishing Vinca major from Vinca minor. In Vinca major both the leaves and the sepals have ciliated (hairy) margins. The evergreen leaves are abrupt or subcordate (slightly heart-shaped, that is concave inwards) at the base. In Vinca minor the leaves have a firmer texture, are narrower, non-ciliated and attenuated at the base rather than abruptly ending. The sepals are also non-ciliated.

Vinca difformis is similar to V. minor but more robust with flowering stems reaching up to 30 cm tall and larger flowers.

Distribution. Vinca major occurs naturally in the Mediterranean (subspecies major) and North Turkey and the Caucasus (subspecies hirsuta). It has been introduced into the British isles, where it has naturalized, and to the southern and western USA. Vinca minor is native to southern Europe but has been introduced into the eastern USA, China, Russia and northern Europe. In the British Isles V. minor is generally considered to be an archaeophyte (an ancient introduction that became naturalized) but is possibly native to southern Britain. It has been cultivated in the Mediterranean since at least Roman times.

The family Asclepiadaceae (Milkweed family) is now generally considered a subgroup of the Dogbane Family (Apocynaceae). It includes some exotic forms with interesting pollination mechanisms.

Ceropegia

Ceropegia is a genus of herbs, shrubs or climbers whose remarkable lantern-like flowers act as fly traps to effect pollination. It occurs in Africa, Asia and Australia.

The pollination mechanism is as follows:

1. A fly lands on the odor secreting zone near the mouth of the corolla (petal) tube. the odor attracts specific pollinators by species and even gender of the insect.

2. The fly explores the dark zone near the entrance of the tube.

3. Downward-directed conical tubercles covered in secreted oil and downward-directed hairs cause the fly to slide in and prevent its escape. In some species the hairs are hinged to deflect downwards only.

4. The fly is drawn to light coming through translucent windows in a ring around the sexual organs, where the fly may pick-up or deposit pollen.

5. Inside the corolla is a structure called the corona (or outer corona), which is derived from the stamen filaments and consists of five nectar cups that are situated around the column (gynostegium) formed from the five fused stigma/styles and surrounded by the five anthers (the anthers form the inner corona). The nectar cups are positioned opposite stigmatic grooves in the column and when the fly withdraws its head, after feeding on the nectar, the broad end of its proboscis (the labellum) catches in the groove and withdraws a clip with a pair of pollinia attached. Pollinia are packets of pollen, each anther producing two pollinia. The two pollinia plus the connecting clip is called a pollinarium. Since the five grooves are in between the five anthers and the clip straddles the groove, it picks up one pollinium from each anther in a pair of adjacent anthers. Any pollinia that the fly is already carrying are pulled off further down in the groove and deposited on the stigma.This mechanism is explained in more detail in the related Asclepias below.

5. One to two days after anthesis (flower opening) the corolla tube becomes horizontal and the hairs shrivel, allowing the pollen-laden flies to escape.

The pollinarium is illustrated below: the connector consists of a hard hook-like corpusculum clip and a pair of retinacula, each retinaculum connects the corpusculum to a pollinium. A corpusculum and its pair of retinacula form a structure called a translator. Each pollinium comes from an adjacent anther. Each anther produces two pollinia, one in each locule and each connected to a different translator. The pollinarium of Asclepias is illustrated below:

Asclepias (Milkweeds)

Unlike Ceropegia, the Milkweeds are not trap flowers, but the mechanism of pollen removal is similar. The style column is surrounded by the five anthers (the inner corona) and the (outer) corona of five nectar cups. One pollinium from each of an adjacent pair of anthers is joined by a translator into a pollinarium (corpusculum clip, pair of retinacula and a pair of pollinia). There are ample beautiful photographs of these flowers on the Web.

Pollination mechanism. After feeding on nectar, as the pollinator (a bee, wasp or butterfly) leaves one of its legs slips into one of the stigmatic slits positioned between each adjacent pair of anthers. Guided by the 'guide rails' at the sides of the slit the leg is pulled upwards and the tarsal claws catch hold of the corpusculum: a hard rigid and non-sticky clip. As they rapidly dry the retinacula move and bend through 90 degrees bringing the pollinia together, holding the pollinarium firmly to the insect's leg. When entering another flower, the pollinia slide into one of the slits in the new flower and break off, depositing the pollinia in inside the stigmatic chamber. Asclepias speciosa is the preferred host for monarch butterfly (Danaus plexippus) adults and larvae.

Concatenation. Sometimes when the insect delivers the pollinarium and must break the retinaculua to escape when the pollinarium gets caught in a stigmatic slit, the broken end may easily snag a new pollinarium and insects may end up carrying chains of pollinaria. Insects too small or weak to escape may get trapped and die.

There are many variations on these pollination mechanisms within the Apocynaceae and the interested reader has plenty of choice for carrying out further research! As a further example, Stapelia is a cactus-like plant that produces flesh-colored or dark purple-red flowers that smell like rotten meat and are often hairy and up to 16 inches (40 cm) in diameter. These flowers mimic rotten corpses so effectively that pollinating flies may even lay eggs within them.

At first sight it might seem odd that the Periwinkle should be in the same family as Ceropegia or  Asclepias but it does have the style column characteristic of all Apocyanaceae though it lacks the corona of nectar cups and sheds its pollen as pollinia, even though they are loosely bound in Vinca.