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Pollination in
Aroids
As it occurs in nature and at the hand of any horticulturist
By Julius Boos
As it occurs in nature and at the hand of any horticulturist
By Julius Boos
All photos by Steve
Lucas unless otherwise noted
The pollination of aroids is dependent on
which of the two basic types of aroids may be involved. Those
"types" are based on the structure of the aroid
species' inflorescence. However, the basic
techniques are quite similar.
Aroids
are divided into two basic sections or groups. The first produces a
bisexual inflorescence with male and female flowers mixed together
throughout the entire length of the spadix. The second unisexual
group includes those which produce an inflorescence with separated
flower zones.
The separated zones are in regions from bottom
to top and include female, sterile, and finally the male flowers
which are located higher on the spadix. An overview of what
happens under natural conditions in the rain forest is in order to
be able to understand what we are trying to replicate artificially
under green house conditions.
In nature an inflorescence signals that it is about to open and begins to emit an odor known scientifically as a pheromone that is attractive to selected insects. One or several of these insect species have evolved to be the specific or "assigned" pollinator of any particular plant species. The bodies of those "assigned" pollinators have adapted to collect and hold pollen grains from one inflorescence which is shedding pollen at male anthesis and transport it to another bloom at female anthesis in order to adhere to the tiny sticky female flowers, As a result, pollination is achieved.
In nature an inflorescence signals that it is about to open and begins to emit an odor known scientifically as a pheromone that is attractive to selected insects. One or several of these insect species have evolved to be the specific or "assigned" pollinator of any particular plant species. The bodies of those "assigned" pollinators have adapted to collect and hold pollen grains from one inflorescence which is shedding pollen at male anthesis and transport it to another bloom at female anthesis in order to adhere to the tiny sticky female flowers, As a result, pollination is achieved.
That ingenious design is nature's plan and in
principal works very well as long as man has not intervened to
destructively counterbalance the natural regions where both the
plant and the pollinator were naturally selected to exist in the
environment. However, if enough of the natural area of either is
affected that outside interference may strongly disturb the natural
breeding grounds of these most important insects as well as limit or
defeat natural pollination. As becomes quickly obvious, the
destruction of any rain forest region may adversely affect the life
cycle of countless plant and animal species. Some have now either
gone completely extinct or are unusually rare as the result of
nothing more than the destruction of the habitat of the natural
insect pollinator. One example of a species that is now
incredibly rare is Philodendron spiritus-sancti from SE
Brazil. Only 6 specimens are known to exist in nature as a
result of deforestation.
The inflorescences of Bisexual Species
Both bisexual and unisexual aroid plant
species produce blooms consisting of a simple leaf-like and
sometimes attractively colored spathe that is attached to the base
of its attending spadix. The spadix is a spike or rod-like
structure along the length of which are hundreds of minute but
actual flowers.
sometimes attractively colored spathe that is attached to the base
of its attending spadix. The spadix is a spike or rod-like
structure along the length of which are hundreds of minute but
actual flowers.
Within the bisexual inflorescence each small
section which you can observe on the spadix is an individual flower
consisting of a central female structure with a stigma at its center
and with several male flowers surrounding that stigma. These male
flowers are almost impossible to observe except during male anthesis
when they are actually producing pollen. Examples of a few of the
genera of Araceae (aroids) belonging to this section include
Anthurium, Monstera and Dracontium species.
To prevent self-pollination in nature two events occur, one following the other. The first is the initiation of female anthesis. At this reproductive stage the female flower's stigmas are ready to be pollinated by the pollen from another more mature inflorescence located on a near-by plant. That second inflorescence would necessarily already have reached male anthesis. After the spathe is fully opened, a process that may take a considerable amount of time including days or even weeks, the stigmas can be observed to produce a tiny drop of liquid on their tips. This liquid serves to hold the pollen grains in place once presented by an assigned pollinator. This liquid is produced in both bisexual and unisexual reproduction.
An odor which is sometimes attractive and at others hardly noticeable by the human nose (but always attractive to the specific insect pollinator) is produced and released at this time through a process known as thermogenesis (thermo= heat, genesis= birth). That pheromone is distributed by an increase in temperature within the spadix as a result of the release of salicylic acid and other natural occurring heating of the spadix. Salicylic acid is, the same chemical compound used in the production of aspirin. After female anthesis is complete, usually in a matter of days, the stigmas dry and turn brown.
ast
up to several
weeks."
Shortly after this event occurs male anthesis initiates and pollen begins to be produced. In some species the pollen is observed as a powdery dust that is visibly shed while in others it appears like a fuzzy material on the spadix. Pollen is also produced in a similar fashion within unisexual aroid genera.
To prevent self-pollination in nature two events occur, one following the other. The first is the initiation of female anthesis. At this reproductive stage the female flower's stigmas are ready to be pollinated by the pollen from another more mature inflorescence located on a near-by plant. That second inflorescence would necessarily already have reached male anthesis. After the spathe is fully opened, a process that may take a considerable amount of time including days or even weeks, the stigmas can be observed to produce a tiny drop of liquid on their tips. This liquid serves to hold the pollen grains in place once presented by an assigned pollinator. This liquid is produced in both bisexual and unisexual reproduction.
An odor which is sometimes attractive and at others hardly noticeable by the human nose (but always attractive to the specific insect pollinator) is produced and released at this time through a process known as thermogenesis (thermo= heat, genesis= birth). That pheromone is distributed by an increase in temperature within the spadix as a result of the release of salicylic acid and other natural occurring heating of the spadix. Salicylic acid is, the same chemical compound used in the production of aspirin. After female anthesis is complete, usually in a matter of days, the stigmas dry and turn brown.
ast
up to several
weeks."Shortly after this event occurs male anthesis initiates and pollen begins to be produced. In some species the pollen is observed as a powdery dust that is visibly shed while in others it appears like a fuzzy material on the spadix. Pollen is also produced in a similar fashion within unisexual aroid genera.
Within bisexual genera
pollen is produced in rings around the spadix
over a long period of time. Some genera produce pollen in these
rings from spadice's top progressing slowly on towards the bottom.
In other genera the pollen production starts at the bottom of the
spadix and continues upward to the very tip. In genera such as
Anthurium, some species have been recorded as producing pollen
from the bottom of the spadix moving slowly upwards to the tip, and
in others, such as Anthurium regale, it was recorded that
this species produces pollen beginning at the top of the spadix then
moving slowly downwards. Aroid botanist Dr. Tom Croat of the
Missouri Botanical Garden offered this additional explanation as to
the technical terminology of either event,
"Most Anthurium produce their pollen acroscopically (toward the
apex) but rarely basioscopically as in those with thumb-shaped
spadices. I think that Dracontium produces stamens
basioscopically."
The inflorescences of Unisexual Species
The
second group of aroids are those which produce unisexual
inflorescences. Philodendron, Alocasia, Caladium and
Xanthosoma species are representatives of this group. All
produce inflorescences in which the spathe consists of two sections,
both are usually wrapped around the spadix, with a constriction
separating the two sections. The upper portion of the spathe is
called the limb or blade while the lower is a convolute tube or
chamber. On the spadix the male and female flowers occur in
separate regions or zones. Normally the zone of female flowers
occurs at the very bottom of the spadix within the lowest portion of
the spathe known as the spathe tube or chamber. Above this female
zone at the constriction is a zone of sterile male flowers. These
sterile flowers produce pheromone (odor) which attracts insect
pollinators in exactly the same fashion as bisexual species.
Normally, above these sterile flowers, and within the upper spathes
blade or limb, occurs the zone of fertile male flowers.
The
second group of aroids are those which produce unisexual
inflorescences. Philodendron, Alocasia, Caladium and
Xanthosoma species are representatives of this group. All
produce inflorescences in which the spathe consists of two sections,
both are usually wrapped around the spadix, with a constriction
separating the two sections. The upper portion of the spathe is
called the limb or blade while the lower is a convolute tube or
chamber. On the spadix the male and female flowers occur in
separate regions or zones. Normally the zone of female flowers
occurs at the very bottom of the spadix within the lowest portion of
the spathe known as the spathe tube or chamber. Above this female
zone at the constriction is a zone of sterile male flowers. These
sterile flowers produce pheromone (odor) which attracts insect
pollinators in exactly the same fashion as bisexual species.
Normally, above these sterile flowers, and within the upper spathes
blade or limb, occurs the zone of fertile male flowers.
In some genera the uppermost section of the
spadix also consists of a zone of sterile male flowers. In this
group, female anthesis occurs just at or even before the
spathe begins to show signs of opening. In Caladium
species and perhaps Xanthosoma species it has been seen the
peak period of female anthesis and receptivity actually occurs the
day before signs of opening can be observed.
The beetles that serve as pollinators in these
genera are likely attracted
by subtle odors propelled by natural
heat produced within the aroid that
is being produced by the as yet
unopened blooms. With their hairy bodies and legs covered with
pollen from a
previously visited bloom already at male anthesis,
these beetles actually force their bodies through the slightly
relaxed spathe opening and down into the chamber containing the
receptive female flowers.
Pollination
in nature is done by a species of scarab beetle assigned that task
by nature. The
beetles only stay inside the inflorescence long enough to pollinate
the spadix, eat some of the pollen, and breed.
previously visited bloom already at male anthesis,
these beetles actually force their bodies through the slightly
relaxed spathe opening and down into the chamber containing the
receptive female flowers.
Pollination
in nature is done by a species of scarab beetle assigned that task
by nature. The
beetles only stay inside the inflorescence long enough to pollinate
the spadix, eat some of the pollen, and breed. The inflorescence continues to open and
experience female anthesis for only one day, then on the
second, perhaps also a third day, the female flowers are no
longer receptive and the upper male flowers in the now fully opened
spathe produce large amounts of pollen to be picked up on the bodies
and legs of the departing beetles.
Male anthesis in unisexual genera is
usually over in 24-36 hours. This serves for that newly
collected pollen to be transported to another opening bloom nearing
female anthesis. However, this male anthesis lasts only one night.
French aroid pollination researcher Marc Gibernau added this
explanation,
"one reason for the
beetles to leave the inflorescence which is a great place to stay is
that the spathes close and force the beetles up along the spadix. Once
above the male zone, they will eat some of the pollen. If they
don't go up, they can finish by crashing the spathe against the spadix,
I observed it once in French Guiana. So the plants "decide" when
the pollinators arrive and depart."
The Cyclocephala beetles in
Marc's photo can be seen near the bottom of a Philodendron spadix as they are
eating pollen.
Artificial pollination
To achieve hand-pollination, the pollen from
an older bloom of the same or a related species which is already at
male anthesis must be transported. In nature the event is completed
by an insect pollinator that has been attracted by the bloom's
odor. But within cultivation it must be done manually. The hand of
man must perform the tasks of collecting and transferring pollen
from one inflorescence which is experiencing male anthesis to
another younger infloresence which is at the stage of female
anthesis.
To accomplish this task within any group of
aroids an artist's brush wetted with sterile water is used to
collect and hold the pollen from the spadix on one plant and
transfer it to the wet stigmas on the spadix of the second
inflorescence of another plant which is now undergoing female
anthesis. I
f the pollination is successful the inflorescence will
"hold" on the plant and the fruit will begin to develop. If the
artificial pollination is truly a success this will become obvious
as the female flowers enlarge to produce berries. Each berry
ranging in color from red to orange through yellow, purple or white
may contain a single large seed or several medium sized seeds which
are surrounded by a pulp and skin which is often tasty depending on
the genus and species involved. The
development time may be long and in some
species may take more than a
year from pollination to ripeness and seed maturity.
f the pollination is successful the inflorescence will
"hold" on the plant and the fruit will begin to develop. If the
artificial pollination is truly a success this will become obvious
as the female flowers enlarge to produce berries. Each berry
ranging in color from red to orange through yellow, purple or white
may contain a single large seed or several medium sized seeds which
are surrounded by a pulp and skin which is often tasty depending on
the genus and species involved. The
development time may be long and in some
species may take more than a
year from pollination to ripeness and seed maturity. 
Hand-pollination of any aroid species can be
accomplished relatively simply by collecting pollen from one bloom
using a small brush such as a camel hair artist brush wetted with
sterile water and transferring this pollen to a bloom as soon as it
is observed the second is beginning to open. Be careful to observe
for the sticky liquid exuded by the female flowers once ready to
accept the pollen. Coating the female flowers with as much pollen
as possible will act to ensure the pollen "holds" and the
reproductive cycle is complete.
It is possible to collect and
store pollen for periods of up to one year by simply sweeping any
excess pollen into a small glass tube that can be tightly capped.
The tube should contain a small amount of desiccant to prevent
moisture from reaching the stored pollen. If you only have a
single specimen producing pollen and none to which the pollen can be
transferred simply sweep the spadix and brush the pollen with a
totally dry brush pushing the pollen grains into the test tube.
If the pollen comes in contact with moisture it will not remain
viable. Cap the tube tightly and store the labeled tube in a
freezer. Once another spadix develops and enters female
anthesis take the tube from the freezer a few hours before use and
warm it slowly to room temperature. If you use it within one
year the chances are good it will still be viable.
Watch pollen develop! 78 Days of the
Anthurium regale
reproductive cycle.
http://www.exoticrainforest.com/Anthurium%20regale%20pc.html
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Terms you may encourter in further research:
Anther - (AN-ther)
The part of the stamen where the pollen is produced and contained
Anthesis
- (an-THE-sis)
The period during which a flower is fully open and functional. The time of pollination or bloom.
Inflorescence - (in-flor-ES-ence)
A group or cluster of flowers on a branch of a plant, in botany used to describe the structure of a spathe and spadix
Infructescence - (in-fruc-TES-sence)
Nectar
- (NEC-tar)
Perigone - (PERU-gown)
Pheromones
(fer-UH-mones)
A scent produced by the spadix during anthesis to attract insect pollinators and distribute pollen. Pistil - (PIS-til)
The female, ovule-bearing organ of a flower,
including the stigma, style, and ovary
Pollen - (POL-in)
sometimes incorrectly called flower sperm, is a fine to coarse powder consisting of pollen grains which produce the male gametes (sperm cells) of seed plants
Stamen - (STA-men)
Male reproductive parts of a flower
Stigma - (STIG-ma)
Stigmatic Hairs
(stig-MAT-ic)
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