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Philodendron is a large genus of flowering plants in the Araceae family. as of September 2015, the World Checklist of Selected Plant Families accepted 489 species; other sources accept different numbers.

Regardless of number of species, the genus is the second-largest member of the arum family.[citation needed] Taxonomically, the genus Philodendron is still poorly known, with many undescribed species. Many are grown as ornamental and indoor plants. The name derives from the Greek words philo- or "love, affection" and dendron or "tree". They are commonly called by their generic name.

Compared to other genera of the family Araceae, philodendrons have an extremely diverse array of growth methods. The habits of growth can be epiphytic, hemiepiphytic, or rarely terrestrially. Others can show a combination of these growth habits depending on the environment. Hemiepiphytic philodendrons can be classified into two types: primary and secondary hemiepiphytes. A primary hemiepiphytic philodendron starts life high up in the canopy where the seed initially sprouts. The plant then grows as an epiphyte. Once it has reached a sufficient size and age, it will begin producing aerial roots that grow toward the forest floor. Once they reach the forest floor, nutrients can be obtained directly from the soil. In this manner, the plant's strategy is to obtain light early in its life at the expense of nutrients. Some primary epiphytic species have a symbiotic relationship with ants. In these species, the ants' nest is grown amongst the plant's roots, which help keep the nest together. Philodendrons have extrafloral nectaries, glands that secrete nectar to attract the ants. The philodendron, in turn, obtains nutrients from the surrounding ant nest, and the aggressive nature of the ants serves to protect the plant from other insects which would eat it.

Secondary hemiepiphytes start life on the ground or on part of a tree trunk very close to the ground, where the seeds sprout. These philodendrons have their roots in the ground early in their lives. They then begin climbing up a tree and eventually may become completely epithytic, doing away with their subterranean roots. Secondary hemiepiphytes do not always start their lives close to a tree. For these philodendrons, the plant will grow with long internodes along the ground until a tree is found. They find a suitable tree by growing towards darker areas, such as the dark shadow of a tree. This trait is called scototropism. After a tree has been found, the scototropic behavior stops and the philodendron switches to a phototropic growth habit and the internodes shorten and thicken. Usually, however, philodendrons germinate on trees.

The leaves are usually large and imposing, often lobed or deeply cut, and may be more or less pinnate. They can also be oval, spear-shaped, or in many other possible shape variations. The leaves are borne alternately on the stem. An interesting quality of philodendrons is that they do not have a single type of leaf on the same plant. Instead, they have juvenile leaves and adult leaves, which can be drastically different from one another. The leaves of seedling philodendrons are usually heart-shaped. Early in the life of the plant, but after it has matured past the seedling stage, the leaves will have acquired the typical juvenile leaf's shape and size. Later in the philodendron's life, it starts producing adult leaves, a process called metamorphosis. Most philodendrons go through metamorphosis gradually; there is no immediately distinct difference between juvenile and adult leaves. Aside from being typically much bigger than the juvenile leaves, the shape of adult leaves can be significantly different. In fact, considerable taxonomic difficulty has occurred in the past due to these differences, causing juvenile and adult plants to mistakenly be classified as different species. The trigger for the transformation to adult leaves can vary considerably. One possible trigger is the height of the plant. Secondary hemiepiphytes start off on the dark forest floor and climb their way up a tree, displaying their juvenile type leaves along the way. Once they reach a sufficient height, they begin developing adult type leaves. The smaller juvenile leaves are used for the darker forest floor where light is in scarce supply, but once they reach a sufficient height in the canopy the light is bright enough that the bigger adult leaves can serve a useful purpose. Another possible trigger occurs in primary hemiepiphytes. These philodendrons typically send their aerial roots downward. Once their roots have reached the ground below, the plant will begin taking up nutrients from the soil, of which it had been previously deprived. As a result, the plant will quickly morph into its adult leaves and gain in size dramatically. Another interesting quality of philodendrons leaves is they are often quite different in shape and size even between two plants of the same species. As a result of all these different possible leaf shapes, it is often difficult to differentiate natural variations from morphogenesis.

Philodendrons also produce cataphylls, which are modified leaves that surround and protect the newly forming leaves. Cataphylls are usually green, leaf-like, and rigid while they are protecting the leaf. In some species, they can even be rather succulent. Once the leaf has been fully formed, the cataphyll usually remains attached where the stem and base of the leaf meet. In philodendrons, cataphylls typically fall into two categories: deciduous and persistent types. A deciduous cataphyll curls away from the leaf once it has formed, eventually turning brown and drying out, and finally falling off the plant, leaving a scar on the stem where it was attached. Deciduous cataphylls are typically found on vining philodendrons, whereas persistent cataphylls are typical of epiphytic philodendrons or appressed climbers. In the latter, the cataphylls are prevented from falling off in a timely manner due to the short internodes of the plant. The cataphylls will remain attached, drying out and becoming nothing more than fibers attached at the nodes. In some philodendrons, the cataphylls build up over time and eventually form a wet mass at the nodes. This may keep emerging roots moist and provide some form of lubrication to new leaves.

Philodendrons have both aerial and subterranean roots. The aerial roots occur in many shapes and sizes and originate from most of the plant's nodes or occasionally from an internode. The size and number of aerial roots per node depends on the presence of a suitable substrate for the roots to attach themselves. Aerial roots serve two primary purposes. They allow the philodendron to attach itself to a tree or other plant, and they allow it to collect water and nutrients. As such, the roots are divided morphologically into these two categories. Aerial roots used for attaching to trees tend to be shorter, more numerous, and sometimes have a layer of root hairs attached; those used for collecting water and nutrients tend to be thicker and longer. These feeder roots tend to attach flush with the substrate to which the philodendron is attached, and make their way directly downwards in search of soil. In general, feeder roots tend to show both positive hydrotropic and negative heliotropic behaviors. Characteristic of roots in philodendrons is the presence of a sclerotic hypodermis, which are cylindrical tubes inside the epidermis that can be one to five cells long. The cells that line the sclerotic hypodermis are elongated and tend to be hardened. Underneath the epidermis is a unique layer of cells in a pattern of long cells followed by short cells.

Some philodendrons have extrafloral nectaries (nectar-producing glands found outside of the flowers). The nectar attracts ants, with which the plant enjoys a protective symbiotic relationship. Nectaries can be found in a variety of locations on the plant, including the stalks, sheaths, lower surfaces of the leaves, and spathes. The nectaries produce a sweet, sticky substance the ants like to eat and which provides an incentive for them to build their nests amongst the roots of the given philodendron. In some cases, the amount of nectar produced can be quite extensive, resulting in the surface becoming entirely covered with it.

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 colour of the base of the spathe contrasts in colour with the upper part, and in others, the inner and outer surfaces of the spathe differ in coloration. The paler colour 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 suggest 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. Interestingly, 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. 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 interesting 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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