Philodendron
Sexual

When philodendrons are ready to reproduce, they will produce an inflorescence which consists of a leaf-like hood called a spathe within which is enclosed a tube-like structure called a spadix. Depending on the species, a single inflorescence can be produced or a cluster of up to 11 inflorescences can be produced at a single time on short peduncles. The spathe tends to be waxy and is usually bicolored. In some philodendrons, the color of the base of the spathe contrasts in color with the upper part, and in others, the inner and outer surfaces of the spathe differ in coloration. The paler color tends to be either white or green, and the darker usually red or crimson. Pelargonidin is the predominant pigment causing the red coloration in the spathes. The upper portion of the spathe is called the limb or blade, while the lower portion is called the convolute tube or chamber due to its tubular structure at the base. The spadix is more often than not white and shorter than the spathe. On the spadix are found fertile female, fertile male, and sterile male flowers. The fertile male and female flowers are separated on the spadix by a sterile zone or staminodal region composed of sterile male flowers. This barrier of sterile male flowers ensures fertile male flowers do not fertilize the female flowers. The arrangement tends to be vertical, with fertile male flowers at the top of the spadix followed by sterile male flowers, and fertile female flowers very close to the bottom in the region known as the spathe tube or chamber. In some philodendrons, an additional region of sterile male flowers is found at the very top of the spadix. The fertile female flowers are often not receptive to fertilization when the fertile males are producing pollen, which again prevents self-pollination. The pollen itself is thread-like and appears to project out from the region where the fertile male flowers are located.

Sexual reproduction is achieved by means of beetles, with many philodendron species requiring the presence of a specific beetle species to achieve pollination. The reverse is not always the case, as many beetle species will pollinate more than one philodendron species. These same beetles could also pollinate other genera outside of philodendron, as well as outside of the family Araceae. The pollinating beetles are males and members of the subfamily Rutelinae and Dynastinae, and to date the only beetles seen to pollinate the inflorescence are in the genera Cyclocephala or Erioscelis. Other smaller types of beetles in the genus Neelia visit the inflorescences, as well, but they are not believed to be involved in pollinating philodendrons. To attract the beetles, the sterile male flowers give off pheromones to attract the male beetles, usually at dusk. This process, female anthesis, is followed by male anthesis, in which the pollen is produced. Female anthesis typically lasts up to two days and includes the gradual opening of the spathe to allow the beetles to enter. Some evidence suggests the timing of opening of the spathe is dependent on light levels, where cloudy, darker days result in the spathe opening up earlier than on clear days. During female anthesis, the spadix will project forward at roughly 45° relative to the spathe.

The spathe provides a safe breeding area for the beetles. As such, the male beetles are often followed by female beetles with the intent of mating with the males within the spathe. The philodendrons benefit from this symbiotic relationship because the males will eventually leave the spathe covered in pollen and repeat the process at another philodendron, pollinating it in the process and thus providing philodendrons a means of sexual reproduction. In addition to gaining a safe location to mate, the male beetles may benefit from having a central location, because it allows them to broadcast to females that they are willing and able to mate. Females which see a male beetle headed for a philodendron flower know he does so with intention of mating, and females which are sexually receptive and need to mate know that they can find males if they follow the pheromones produced by the philodendron flowers. As a result, the male beetles benefit from this relationship with the philodendrons because they do not have to produce pheromones to attract females, since the philodendrons do it for them. Additionally, male beetles benefit because they are ensured of mating with only sexually receptive females, which is not necessarily certain otherwise. In doing so, the philodendron provides male beetles a more efficient way to find females than what they could achieve on their own. Pheromones produced by the philodendrons may be similar to those produced by female beetles when they wish to attract males to mate. Also, the pheromones have a sweet, fruity smell in many species and no noticeable smell for others. Not all beetle behavior directly benefits the philodendrons in this symbiosis: in addition to the reproductive benefits to beetles, the philodendrons provide food in two forms. Pollen from the fertile male flowers is eaten by the beetles throughout the night. Secondly, the sterile male flowers consumed by the beetles are rich in lipids.

The male beetles will stay overnight in the spathe, eating and mating throughout the night due to the benefits provided by the spathe and spadix. Typically, five to 12 beetles will be within the spathe throughout the night. Rarely, cases of 200 beetles at a time have been observed and almost always the beetles are of the same species. Another feature of this symbiotic relationship, less well understood, is the series of events in which the spadix begins to heat up prior to the spathe opening up for the beetles. This process is known as thermogenesis. By the time the spathe is open and the beetles have arrived, the spadix is usually quite hot; up to around 46 °C in some species, but usually around 35 °C. The thermogenesis coincides with the arrival of the beetles and appears to increase their presence. The maximum temperature reached by the spadix remains about 20 °C higher than the outside ambient temperature. The time dependence of the temperature can vary from species to species. In some species, the temperature of the spadix will peak on the arrival of the beetles, then decrease, and finally increase reaching a maximum once again when the philodendron is ready for the beetles to leave. Other species, though, only show a maximum temperature on the arrival of the beetles, which remains roughly constant for about a day, and then steadily decreases. A few species will show three peaks in temperature during the flowering. The increased temperature increases the metabolism of the beetles, causing them to move about more within the spathe and increasing the likelihood they will be sufficiently coated with pollen. A sticky resin is also produced in drops attached to the spadix which help to keep the pollen attached to the beetles. This resin producing quality is unique to Philodendron and Monstera, as other genera of Araceae do not produce it on their spadices. The resin is also found on the stems, leaves, and roots of philodendrons. Its color can be red, orange, yellow, or colorless when it is first produced. Yet, over time, it will turn brown as it is exposed to air. Also, some evidence suggests the thermogenesis triggers the beetles to mate. It also appears to distribute the pheromones into the air. The reason for the spadix being held at 45° relative to the spathe may be to maximize the heat's ability to waft the pheromones into the air. Oxidizing stored carbohydrates and lipids has been found to be the energy source for thermogenesis. The part of the spadix that heats up is the sterile zone. As it heats up, carbohydrates are used, but once the spadix has reached its maximum temperature, lipids are oxidized. The lipids are not first converted to carbohydrates, but rather are directly oxidized. The thermogenic reaction is triggered when concentrations of acetosalicytic acid form in the sterile zone. The acid sets off the mitochondria in the cells that make up the sterile zone to switch to an electron transport chain called the cyanide-resistant pathway, which results in the production of heat. Philodendrons consume oxygen during thermogenesis. The rate at which oxygen is used is remarkably high, close to that of hummingbirds and sphinx moths. The spadix has been shown to generate infrared radiation. As the beetles home in on the inflorescence, they first move in a zig-zag pattern until they get reasonably close, when they switch to a straight-line path. The beetles may be using scent to find the inflorescence when they are far away, but once within range, they find it by means of the infrared radiation. This would account for the two different types of paths the beetles follow.

Once female anthesis is nearing its end and the female flowers have been pollinated, the spathe will be fully open and male anthesis begins. In the beginning of male anthesis, the fertile male flowers complete the process of producing the pollen and the female flowers become unreceptive to further pollination. Additionally, the spadix moves from its 45° position and presses up flush to the spathe. Towards the end of male anthesis, the spathe begins to close from the bottom, working its way up and forcing the beetles to move up and across the upper region of the spathe, where the fertile male flowers are located. In doing so, the philodendron controls when the beetles come and when they leave and forces them to rub against the top of the spadix where the pollen is located as they exit, thus ensuring they are well-coated with pollen. One would expect the beetles to stay indefinitely if they could due to the very favorable conditions the inflorescence provides. After male anthesis, the males will go off and find another philodendron undergoing female anthesis, so will pollinate the female flowers with the pollen it had collected from its previous night of mating.
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