Aristolochia floral structure

Legend:
li - limb
fa - fauces
an - annulus
tu - tube
sy - syrinx
ut - utricle
gy - gynostemium
ov - ovary

My first post for this blog, this post is about the genus of plants to which I have the most interest and fascination towards at the present moment, and thus would like to write about first.

The name Aristolochia can be seen to have been derived from Greek, aristos "best" + locheia, "childbirth" (wiki), which probably refers to the shape of the flower, which somewhat resembles a birth canal. However I shall not expound on the history of the genus but rather on the flower itself.

Firstly, the flower doesn't seem to contain the structures found in that of stereotypical flowers. Visually, the flower seems to lack sepals, petals, styles and filaments. Instead, it resembles a length of tube with an inflated base to house the reproductive organs, which consists of stigmatic lobes and stamens fused to form a structure called the gynostemium. In Aristolochias, in the absence of petals, the sepals are seen to be fused to form what can be seen as the rest of the flower, which consists of the limb, tube, and utricle(refer to the photos and legend above). Another structure found in the flower neither mentioned in the photo or legend is that of the hair-like structures found lining the limb and tube, known as trichomes, which are of vital importance in the pollination mechanism of the flowers of this genus.

The flower displays protogyny, in that the flower goes through the female stage first, then the male stage. These two stages take place over 2 days, and henceforth will be referred to as Day 1 and Day 2. The separation of the maturity of the female and male parts of the flower constitutes a temporal separation, effectively preventing self pollination within the same flower. However, some studies have suggested that Aristolochia are capable of self pollination between different flowers on the same plant.

With regards to the floral syndrome of brood-site mimicry, some insects that are attracted to the flower do deposit eggs within the flower, and while the general Aristolochia species do not function as a brood-site and the larvae that hatch end up dying, some species have been found to be a food source for the larvae after the flower has detached off the plant.

Day 1:

On the first day the flower is open, osmophores that are located on the surface of the limb produce chemical attractants, perhaps resembling that which is produced by oviposition substrates such as carrion, decomposing plant matter etc, or pheromones of the opposite sex of the target pollinators. Pollinators consist of mainly flies, such as those from Phoridae.(As a side note, I am inclined to believe that scent plays a primary role in attracting pollinators from a distance and floral appearance takes on a secondary role, not matter how much the flower resembles an oviposition substrate in terms of visual appearance.) The attracted insect lands on the limb of the flower, and then proceeds to enter the flower via the fauces, going past the annulus into the tube.

The insects have to push past inward/downward pointing trichomes in order to reach the utricle. It is the recursive nature of the trichomes which does not allow the insects to backtrack out of the flower once they have begun crawling along the tube. These trichomes flex inward easily, but are unable to flex outwards in the opposite direction, and thus while insects may have to exert relatively little energy to push past the trichomes in the direction of the utricle, they will have to exert a lot more energy to push past the trichomes in the direction of the exit, providing an effective trapping mechanism. Sure, there are insects which are strong enough to push past the trichomes in the wrong direction, but generally there is a size to strength ratio, meaning the bigger the insect the stronger it can be. This means that one way the flower can limit or even eliminate the chances of an insect being strong enough to damage the trichomes is to limit the size of insect allowed to enter the flower. This is done through the diameter of the tube and the trichomes lining the inner surface of the tube, which effectively reduce the inner diameter of the tube. Together with the restrictive opening at the bottleneck, the syrinx, only insects up to a maximum diameter are able to reach the utricle. Thus the flower excludes any excessively large pollinators which are capable to damaging the gynostemium or the trichomes.

Assuming that the initial attraction to the flower is smell produced by the osmorphores located in the limb, what is it that draws insects into the flower? Is it another chemical produced by the inner walls of the tube or utricle which serves as the attractant or is it other factors such as humidity, darkness, carbon dioxide etc. Perhaps for cases where the flower mimics carrion, insects that are attracted to the flower would perceive darkness in the flower as a signal that the 'carrion' is thick and damp enough to function as a broodsite. However, due to the presence of a circular 'light window' comprised of a thin ring of transluscent cells located at the basal end of the utricle in a large number of Aristolochia species, regardless of whether the flower mimics carrion, I am inclined to think that darkness in the flower is not a sufficiently valid attractant for pollinators in a sizeable proportion of species in the genus. This is due to the fact that these insects display phototaxis and thus are attracted to the light and not the dark which is how the light ring works in the flower, amidst a dark interior in the utricle. Hence the other factors, namely chemicals and to a minor extent, humidity, may play a bigger role of attracting the insects into the flower. After the initial push past the trichomes in the front end of the tube, the insect senses light further down in the tube thanks to the light window in the utricle, and thus thinking that the light is an indication of an exit, is motivated to head deeper into the tube. Thus in this case the insect, once trapped, is held in the flower overnight. In return, some Aristolochia species have flowers that contain nectariferous trichomes lining the inner wall of the utricle, which are able to sustain the insects overnight, providing sufficient nourishment to sustain them on the flight to another flower the following day.

Day 2:

On the second day of their incarceration, changes in the flower as compared to a flower from day 1 include the stigmatic lobes no longer being receptive to pollen and closing in upon themselves, putting the ripened anthers in greater prominence, and thus putting the trapped insects in greater exposure to the pollen. Another change in the structure of the flower is that of the trichomes. On day 2, the trichomes have all shriveled up, thus they no longer effectively restrict the movement of the insects when moving through the tube. In conjunction to the shriveling of the trichomes, the flowers of some species have tubes that tend to straighten in relation to the utricle on the second day of flowering, allowing the insects even easier access to the exit, on account that the bend in the tube before the syrinx is of a hindrance to the movement of the insect. One other change is that the osmorphores on the limb no longer emit a smell. The trapped insects are now able to climb out of the flower through the now enlarged diameter of the tube, and hopefully for the plant, get trapped in another Day 1 flower of the same species, or the same plant.