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Pollination Ecology of the Araceae by Dr. Danny Beath 2 PICTURES OF AROIDS IN RAINFOREST Contents Introduction
The
aroid "flower" is characterised by a compound inflorescence composed
of numerous tightly packed florets on a rod shaped "spadix" which
is often surrounded by or sub-tended by a leaf like bract or "spathe".
The florets can be perfect flowers on a hermaphrodite inflorescence
or in separate female and male zones on a monoecious inflorescence.
Specialised sterile zones and sterile processes are present on
the spadices of some aroid species and these have specific roles
in scent production and insect rewards. Anthesis in Araceae is
always protogynous with the female florets receptive before the
male florets release pollen. Aroids are pollinated by insects,with
the visitors being mainly bees, flies and beetle species. A typical
hermaphrodite inflorescence A typical
monoecious inflorescence Pollination ecology of the Araceae GHANA (West Africa) Amorphophallus johnsonii N.E. Brown This
species has a huge monoecious inflorescence, which rises up from
the forest floor to a height of over two metres at the start of
the rainy season in April. Anthesis starts with a vague fishy
smell in the late afternoon (figure 1. amorjon4.jpg)
which rapidly gets stronger as dusk falls around 6:30pm. Soon
after dusk the upper sterile portion of the spadix (osmophore)
begins to exude an evil smelling slime which eventually covers
the entire surface of the osmophore . This is accompanied by a
powerful stench of rotting fish and the arrival of large numbers
of black carrion beetles (Phaeocrous amplus Arrow), which
circle around the bloom before landing on the wet spadix and subsequently
slipping into the bottom of the spathe or kettle (figure 2. amjohn1.jpg).
More than a hundred beetles can end up inside the kettle, where
they are trapped by the overhanging lip. Any pollen they are carrying
from previously visited blooms is deposited on the receptive female
florets. (figure 3. amorjon1.jpg).
At the base of the kettle there are numerous micro-verrucae on
the inner spathe surface (figure 4. amorjon3.jpg).
The exact function of these processes is unknown but they may
facilitate the beetles movements as they move about on the first
night. Visiting beetles often end up mating while trapped inside
the bloom overnight. By 10:00pm the osmophore has dried up and
the vile odour ceased. figure
1. amorjon4.jpg (bloom
at pre-anthesis) (These images were taken in Jachie Sacred Grove in Ghana during April 1991) The beetles remain quiescent inside the bloom throughout the following day until they begin to stir at around 4:30pm. Just as this is happening, there is an abrupt and simultaneous ejaculation of pollen from all the male florets, which exude long strings of sticky yellow pollen (figure 4. amorjon2.jpg). As the beetles ascend the now dry and non slippery spadix they crawl past the male florets and become covered in pollen. The beetles climb to the top of the osmophore and fly off as dusk falls (figure 5. amjohn2.jpg). figure
4. amorjon2.jpg (male
florets exuding pollen) (These
images were taken in Jachie Sacred Grove in Ghana during April
1991) Culcasia angolensis Welw. Ex Schott This aroid is a rampant vine found in most of Ghanaç¿s rainforests and it flowers on the ends of mature shoots in the forest canopy. The 4 cm long monoecious blooms open early in the morning to reveal a widely flared spathe and a stubby spadix, with receptive female florets at the base. The bloom produces a sweet perfume that attracts large numbers of tiny fruit flies (Drosophila sp.) which then feed on sugary droplets exuded on the inner spathe surface (figure1. culang1.jpg). As many as 140 flies can accumulate on a single bloom by the end of the day. During the afternoon the upper male spadix zone begins to heat up and by dusk it can be several degrees celcius above ambient air temperature. The heating attracts fruit flies onto the spadix and it is crowded with flies by dusk. Soon after dusk the spathe begins to dry out and bend backwards and by 8:00pm the entire spathe has disintegrated in a powdery mass which eventually falls to the ground (figure 2. culang2.jpg). The fruit flies remain on the warm spadix overnight. By dawn the following day the upper male zone has cooled down and it produces large quantities of powdery pollen as the first rays of the sun hit the forest canopy. The fruit flies are covered in pollen which is carried to the next newly opened bloom as soon as they fly off. figure1.
culang1.jpg (bloom at the
beginning of the first day with fruit flies) (These
images were taken 15-20 metres up a tree in Jachie Sacred Grove
in Ghana during June 1991) CENTRAL AMERICA Anthurium formosum Schott This species grows as a large herb in pre-montane rain forest and produces 15cm long hermaphroditic blooms on 1 metre long stems. Flowering lasts between 8-15 days and begins with the female receptive phase during the first 2-3 days, followed by a rest for a few days which is then ensued by pollen production for 3-4 days. Florets ripen progressively from the base to the tip of the spadix during anthesis. The bloom is scented between 6:30 am and 10:00am during the female and male (pollen) phases and during these times swarms of noisy euglossine bees are attracted by the blooms spearmint gum flavoured perfume (figure1. Aform1.jpg). Two species of bee visit A. formosum with typically 3-8 small irredescent orange bees (Euglossa flammula) and 1-3 larger black and yellow species (Eulaema merriana). Only male bees visit the blooms and they come to scrape off a scented wax on the surface of the spadix, which is used to produce a bee pheromone for attracting female bees. The bees alternate between mopping up the wax on the spadix and transferring it to storage organs in their back legs while hovering motionless by the bloom. Frequent territorial disputes occur between individual bees on crowded blooms. In this way the bees will be pollinating these plants by transporting pollen from male phase blooms to newly active female phase blooms. figure1. Aform1.jpg (inflorescence during anthesis with euglossine bees) (this
photograph was taken in the Wilson Gardens in Costa Rica during
July 1995) Anthurium hacumense Engler This has a similar habit to the previous species with 6 cm long hermaphroditic inflorescences on 50 cm long stems, but it grows at a higher altitude in montane cloud forests. The flowering cycle is similar to A. formosum with the first 2-3 days being female receptive, followed immediately by pollen emission over a 7-9 day period. Florets become active at the centre of the spadix initially with subsequent activity spreading out to both ends of the spadix . The bloom is scented between 10:00 am and 1:00pm during the female and male (pollen) phases and during these times between 4 and 9 irredescent blue-green euglossine bees (Euglossa hyacintha) are attracted by the blooms intensely sweet fragrance (figure1. ahacum1.jpg). Male bees visit the blooms and scrape off a scented wax on the surface of the spadix, which is used to produce a bee pheromone in a similar way to the previous species. In the photograph it can be seen that one of the bees has a yellow "pollinia" attached to its back. This is proof that this bee has visited an orchid species recently and may be involved in orchid pollination as well. figure1. ahacum1.jpg (inflorescence during anthesis with euglossine bees) (This
photograph was taken in the Wilson Gardens in Costa Rica in August
1995) Dieffenbachia longispatha Engl. K. Krause This is a common aroid of most lowland rainforests and is found growing as a large erect herb in in the forest understorey, with inflorescences appearing in terminal clusters. I observed this species in two localities in Costa Rica and Panama and I found significant variations in floral morphology, fragrance and type of pollinator. The populations on Barro Colorado Island in Panama exhibited narrow dark green inflorescences (figure1. diflong3.jpg) and large orange female florets (figure 2. diflong4.jpg), which produced a spicey peppery odour and attracted Cyclocephala gravis (Bates) and Cyclocephala sexpunctata (Cast.) mainly. Populations at La Selva in Costa Rica showed broader yellowish spathes (figure 3. diflong1.jpg) and small straw yellow female florets (figure 4. diflong2.jpg) producing a foul rancid odour and attracting Cyclocephala amblyopsis (Bates) and Cyclocephala gravis mainly, as well as several species of tiny nitidulid beetles. The La selva form has recently been named as a new sub species called Dieffenbachia longispatha creberapistila Croat & Grayum. figure1.
diflong3.jpg (Bloom of
Dieffenbachia longispatha, BCI form) (These pictures were taken on Barro Colorado Island Panama and at La Selva Biological Station in Costa Rica in July and September 1997) Apart from these differences, both forms had a similar pattern of flowering with inflorescences often opening on the day before anthesis proper. Anthesis begins soon after dusk (day 1) with odour emissions peaking 30 minutes after sunset and persisting for 30-45 minutes before fading away. During this time most of the scarab beetles arrive. Newly arrived beetles crawl inside the lower spathe chamber and begin to feed on the sterile florets interspersed between the fertile female florets (figure 5. diflong6.jpg). On Barro Colorado Island between 1-4 beetles visited each bloom while at La Selva up to 12 individual beetles could be found in a single bloom. Small numbers of tiny nitidulid and staphylinid beetles appeared during the afternoon immediately before anthesis (day 1) inside the lower spathes of blooms at La Selva. The beetles remain inside the spathes until the following evening (day 2) when they emerge on to the male spadix region as dusk falls. The beetles feed on the pollen as it begins to appear around 20 minutes after sunset (figure 6. diflong5.jpg). Beetles stay on the spadix for up to an hour before leaving the bloom. Soon after pollen emission the lower spathe chamber starts to constrict and the upper spathe limb begins closing. By the following morning (day 3) the upper spathe limb remains half open and it does not close completely until the following night. figure
5. diflong6.jpg (scarab
beetles feeding on sterile florets) (These
pictures were taken on Barro Colorado Island Panama and at La
Selva Biological Station in Costa Rica in July and September 1997)
Philodendron fragrantissimum G. Don This species is a common vine in lowland rainforests and it flowers on the ends of ascending stems between 2-15 metres up in the lower forest canopy. Inflorescences open gradually during the first day and by 4:30pm the spathe is flared wide open with a potruding spadix (figure 1. philfrg1.jpg). As dusk falls the bloom begins to emit a sweet spicey perfume and the spadix begins to warm up. 30 minutes after sunset, the whole spadix is very warm (up to 37 c) with a strong spicey smell and brown scarab beetles (Cyclocephala sexpunctata) begin to arrive at the bloom. Between 1 and 4 beetles will visit a single inflorescence and upon arrival they immediately crawl down into the enclosed lower spathe chamber and begin to feed on a zone of sterile florets just above the receptive female florets (figure 2. philfrg3.jpg). By 9:00pm the spadix has cooled down and the beetles become quiescent and they remain inside the bloom through the next day. figure
1. philfrg1.jpg (bloom
on first evening during female phase) (These photos were taken 4 metres up a tree on Barro Colorado Island in the Panama canal during July 1997). By dawn on the second day the spadix has retracted back inside the spathe and the spathe kettle has constricted a little. As dusk approaches the bloom emits a weak spicey smell and the trapped beetles begin to crawl out of the kettle and onto the spadix (figure 3. philfrg4.jpg). Just after sunset, brown resinous beadlets begin to ooze out off the male spadix zone. Soon after this pollen is released all over the male spadix zone and the beetles begin to feed on it as it emerges, becoming covered in the sticky resin and pollen mixture as they crawl around feeding on the spadix (figure 4. philfrg2.jpg). The pollen clad beetles leave the bloom between 30-45 minutes after sunset and will often fly off into a newly opened bloom nearby. The spathe closes slowly during the night and is completely shut by the third morning. figure
3. philfrg4.jpg (scarab
beetles emerging on second evening during male phase) (These
photos were taken 4 metres up a tree on Barro Colorado Island
in the Panama canal during July 1997). Philodendron platypetiolatum Madison This species is a common vine in lowland rainforest habitats at La Selva and flowers are produced from leaf axils on ascending stems between 1-3 metres up in the lower shrub layer. Inflorescences open early on during the first day and with the spadix canted well forward my midday (figure1. philpla1.jpg). Several species of nitidulid beetles arrive in the late afternoon between 3:30-5:00 and they accumulate in the lower spathe chamber (figure 2. philpla2.jpg). Half an hour after sunset at around 6:00pm the the spadix begins to warm up and the bloom begins to emit a sweet spicey perfume. Peak heating and scent production are between 6:15 and 7:30pm, with the spadix reaching 37 c. The upper male spadix zone cools down by 8:15pm, but the medial sterile zone remains warm until after 9:00pm. Orange resinous beedlets begin to appear from prominent resin canals in the lower spathe tube after 7:00pm and these droplets enlarge and slowly creep higher up the spathe limb overnight. Two species of scarab beetles (Cyclocephala amblyopsis and Cyclocephala sexpunctata) visit P. platypetiolatum and they arrive between 5:30-7:45pm. Between 1 and 4 beetles will visit a single bloom and upon arrival they immediately crawl down into lower spathe chamber and begin to feed on a narrow zone of sterile florets just above the receptive female florets. By 9:00pm the beetles become quiescent and they remain inside the bloom through the next day. figure
1. philpla1.jpg (inflorescence
on first night during female phase) (These photos were taken 2 metres up in low shrubby vegetation at La Selva biological station, Costa Rica, in September 1997). By dawn on the second day the spathe has deflated, with a constricted neck and the spadix is retracted back inside the spathe. The beetles remain trapped inside the shrunken lower spathe chamber (figure 3. philpla3.jpg). As dusk approaches between 5:00-5:30pm pollen begins to emerge on the male spadix zone (figure 4. philpla4.jpg). Trapped beetles begin to crawl out and feed on the pollen. The pollen clad beetles leave the bloom soon after sunset and will often fly off into a newly opened bloom nearby. The spathe closes slowly during the night and is completely shut by the third morning. figure
3. philpla3.jpg (scarab
beetle inside lower spathe chambrer on second day) (These
photos were taken 2 metres up in low shrubby vegetation at La
Selva biological station, Costa Rica, in September 1997). Philodendron radiatum Schott This aroid is a large arboreal vine, commonly found in lowland rainforests. This species produces big pinnatisect leaves and massive climbing stems which reach up into the upper canopy up to 20-25 metres. Clusters of 30cm long inflorescences appear on terminal shoots. Anthesis starts with the spathe opening around midday, which is followed by steady inflation of the kettle and forward protrusion of the spadix during the afternoon (figure 1. philrad1.jpg). The sterile and male zones of the spadix are already several degrees celcius above ambient air temperature when the spathe opens and they begin to heat up in earnest after 5:00pm, half an hour before dusk. Soon after dusk the spadix heats up very quickly to over 40 c and an hour after dusk at 6:30 it reaches a peak of 43 c. This heating is accompanied by a powerful sickly sweet smell which surges out of the inflorescence in successive waves. A smattering of large red viscous droplets appears around the sterile spadix zone an hour after dusk (figure 2. philrad2.jpg). Large brown scarab beetles (Cyclocephala ampliota Bates) arrive at the active blooms between 6:30-8:00 pm and these crawl down into the kettle and begin to feed voraciously on the sterile spadix zone. The sterile spadix zone remains very warm for longer than the male spadix zone which cools down to below 35 c by 9:30pm. The sterile zone does not cool down to the same temperature as the male zone until after 4:30am. figure
1. philrad1.jpg (inflorescence
during female phase on first day) (These pictures were taken in La Selva Biological Station in Costa Rica during September 1997) By dawn on the second day the whole spathe has constricted considerably and the spadix is retracted right back inside the spathe (figure 3. philrad3.jpg). During the day the beetles remain trapped and quiescent inside the kettle, but the inflorescence is often visited by several large trigona bees which crawl inside the spathe and mop up any remaining residue from the previous nights resinous secretions (figure 4. philrad4.jpg). As dusk approaches numerous tiny red resinous beadlets emerge all over the male spadix zone (figure 5. philrad5.jpg). Just after dusk the beetles begin to emerge onto the male spadix zone and start feeding on the pollen which emerges suddenly around 30 minutes after sunset (figure 6. philrad6.jpg). The spathe commences to close fairly rapidly soon after dark and it is usually completely shut an hour after sunset, forcing the pollen clad beetles to fly off in search of a freshly opened bloom nearby. figure
3. philrad3.jpg (inflorescence
on second morning) (These
pictures were taken in La Selva Biological Station in Costa Rica
during September 1997) Spathiphyllum phryniifolium Schott This species is found growing as a small herb in damp flushes on the forest floor in lowland rain forest habitats. The hermaphrodite blooms emerge on slender stems and are 10-12 cm long. Anthesis lasts 9-12 days, with the female phase lasting 4-5 days, followed immediately by the male phase of 4-6 days. Pollen is produced simultaneously all over the spadix continuously during the male phase. The inflorescence is active between 6:30am and 11:00am when a weak scent of cheap soap is evident. The fragrance attracts 1-5 trigona bees (Trigona fulviventris Gueren Melville) at a time as well as several chrysomelid beetles (figure 1. spathphr.jpg). The beetles arrive sporadically during anthesis and are more commonly seen during pollen production in the male phase, when they feed on the pollen. The beetles tend to stay on the bloom over the whole period of anthesis. The bees came and go on regular cycles during the mornings. During the female phase the bees collect wax from the conical styles on the spadix and typically spend between 1-2 minutes scraping off the wax with their mandibles, followed by 10-25 seconds of hovering flight while they transfer the wax to their back leg baskets. Each bee normally spends 30-40 minutes collecting the wax, followed by a trip to a nearby nest to deposit their wax cargo and returning to the bloom in 10-15 minutes. This cycle repeats itself until scent production stops around 11am. During the male phase the bees repeat the same behaviour, but concentrate on pollen collection instead. Experiments using marked bees revealed that each bloom was visited by the same consort of 1-3 bees each day during flowering, with brief visits from other unmarked bees. figure 1. spathphr.jpg (Inflorescence during anthesis, with visiting trigona bees) (This
image was taken on Barro Colorado Island in the Panama Canal during
July 1997) Syngonium schottianum H. Wendl. ex Schott This species is found growing in lowland rainforests as an elegant vine with 1 metre long leaves on stems ascending to 10-15 metres up trees. Inflorescences appear in terminal bunches on mature stems. Blooms begin opening on the day before anthesis and by dawn on day one the spathe is fully inflated and displaying a vertically erect spadix (figure 1. synscot1.jpg). As dusk falls at 5:30pm there is a faint aroma and the female florets become moist and receptive (figure 2. synscot2.jpg). 30 minutes later there is a strong tangy perfume resembling oranges and pineapples and the sterile and male spadix zones begin to heat up. An hour after sunset the whole spadix is very warm at 35 c with an intense fragrance. Black scarab beetles (Erioscelis columbica) arrive between 6:00-7:00pm and they accumulate in the lower spathe chamber to feed on the sterile floret zone. The male spadix zone cools down by 9:30 pm, but the sterile florets remain very warm until after 10:00pm, with beetles actively feeding until 9:00pm. By the following morning the spathe neck has constricted a little and the female florets are brown and unreceptive. As dusk falls at 5:30pm pollen begins to emerge all over the male spadix zone and during the next half hour a fluffy mass of pollen is extruded and beetles emerge to feed on it (figure 3. synscot3.jpg). Beetles usually fly off by 6:30 pm. Throughout anthesis the upper spathe is covered in large numbers of small black mirid bugs (Miridae) which play little role in pollination, but they disfigure the spathe by sucking sap. After anthesis the spathe does not shut as in Philodendron, but slowly rots away over the next few days. figure
1. synscot1.jpg (inflorescence
during first day) (Photos
taken at La Selva Biological Station, Costa Rica, 10 m up a tree,
during September 1997) Xanthosoma pilosum K. Koch and Xanthosoma helleborifolium Schott These
two species exhibit very similar patterns of anthesis and they
are both small seasonal herbs which emerge and flower during the
months of July and August, in forest clearings and light gaps
on Barro Colorado Island. In X. pilosum, 1-4 blooms emerge from
the bases of velvety cordate leaves while X. helleborifolium
produces 1-3 blooms from the long petioles of elegant palmately
compound leaves. In X.. helleborifolium anthesis begins
on the first day with the spathe opening around midday to reveal
a wide open bloom by mid afternoon (figure 1. xhell1.jpg).
This contrasts with X. pilosum, where the spathe does not open
until 5:00-5:30pm on the first day of anthesis (figure 2. xpilum4.jpg).
In both species the female florets are moist and receptive on
the first evening (figure 3. xpilum3.jpg
and figure 4. xhell4.jpg).
In both species the sterile and male spadix zones begin to heat
up soon after dusk at 6:30pm and by 7:15pm the spadix is very
warm and a sweet peppery perfume is evident. Soon after this the
active blooms are visited by several species of scarab beetle,
mainly Cyclocephala gravis and Cyclocephala sexpunctata,
but also by the occasional individual of Cyclocephala carbonaria.
In X. pilosum between 2-6 beetles arrive at any one bloom,
but more than 10 beetles is not uncommon and once I counted 19
beetles on a particularly overcrowded bloom! (figure 5. xpilum1.jpg).
X. helleborifolium was much less successful at attracting beetles
during my observations in 1997 and typically only one, or occasionally
two beetles, were found in the blooms. In both species most beetles
arrive between 7:20-7:40pm during peak heating and scent production,
when the male spadix zones reach 37 c. Observations on marked
beetles confirmed that they approach active blooms from downwind
and fly up a "scent" gradient with a zig zag flight pattern. Once
the beetles are within a metre of the inflorescence, they fly
around in decreasing circles until they hit the upper spathe limb.
Newly arrived beetles crawl straight down into the spathe kettle
by squeezing past the spathe neck constriction and then immediately
start to feed on the enlarged sterile florets just above the female
floret zone. On overcrowded blooms in X. pilosum I often
observed later arriving beetles being evicted by the feeding occupants
inside the bloom and they were forced to fly away to another bloom.
The spadix cools down by 8:00pm and beetles stop feeding by 8:30pm.
figure
1. xhell1.jpg (Inflorescence
of X.. helleborifolium on first day) (These
images were taken on Barro Colorado Island in the Panama Canal
during August 1997) By the second morning the female florets have gone brown and unreceptive, with the captive beetles remaining quiescent in the bottom of the spathe and in most cases the sterile florets are chewed away (figure 6. xhell3.jpg). Just after dusk, between 6:45-6:55pm the beetles crawl out onto the male spadix zone (figure 7. xpilum2.jpg and figure 8. xhell5.jpg) This coincides with a transitory mild heating episode in the male spadix zone, which triggers off a sudden ejaculation of fluffy white pollen between 6:55-7:05pm (figure 9. xhell2.jpg). The beetles feed voraciously on emerging pollen until flying off at anytime between 7pm and 8:00pm to another newly opened bloom up wind. figure
6. xhell3.jpg (Bloom on
2nd day, showing quiescent beetles and chewed sterile zone) (These images were taken on Barro Colorado Island in the Panama Canal during August 1997)
References |
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