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is a large genus of flowering plants in the Araceae
family, consisting of about 900 species according to TROPICOS (a service of the Missouri Botanical Garden). Other sources quote different numbers of species: S.J. Mayo reports about 350-400 formally recognised species, whereas Croat lists about 700. Regardless of number of species, the genus is the second-largest member of the arum family. 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" and dendron or "tree". They are commonly called by their generic name.
Philodendron species can be found in many diverse habitats in the tropical Americas and the West Indies. Most occur in humid tropical forests, but can also be found in swamps and on river banks, roadsides and rock outcrops. They are also found throughout the diverse range of elevations from sea level to over 2000 m above sea level. Species of this genus are often found clambering over other plants, or climbing the trunks of trees with the aid of aerial roots. Philodendrons usually distinguish themselves in their environment by their large numbers compared to other plants, making them a highly noticeable component of the ecosystems in which they are found. They are found in great numbers in road clearings.
Philodendrons can also be found in Australia, some Pacific islands, Africa and Asia, although they are not indigenous and were introduced or accidentally escaped.
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.
A leaf of Philodendron maximum
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.
Philodendrons can contain as much as 0.7% of oxalates in the form of calcium oxalate crystals as raphides. The risk of death, if even possible, is extremely low if ingested by an average adult, although its consumption is generally considered unhealthy. In general, the calcium oxalate crystals have a very mild effect on humans, and large quantities have to be consumed for symptoms to even appear. Possible symptoms include increased salivation, a sensation of burning of the mouth, swelling of the tongue, stomatitis, dysphagia, an inability to speak, and edema. Cases of mild dermatitis due to contact with the leaves have also been reported, with symptoms including vesiculation and erythema. The chemical derivatives of alkenyl resorcinol are believed to be responsible for the dermatitis in some people. Contact with philodendron oils or fluids with the eyes have also been known to result in conjunctivitis. Fatal poisonings are extremely rare; one case of an infant eating small quantities of a philodendron resulting in hospitalization and death has been reported. This one case study, however, was found to be inconsistent with the findings from a second study. In this study, 127 cases of children ingesting philodendrons were studied, and they found only one child showed symptoms; a 10-month-old had minor upper lip swelling when he chewed on a philodendron leaf. The study also found the symptoms could subside without treatment and that previously reported cases of severe complications were exaggerated.
As to the toxicity of philodendrons in cats, the evidence is conflicting. In one study, 72 cases of cat poisonings were examined, of which 37 resulted in the death of the cat. The symptoms of the poisoned cats included excitability, spasms, seizures, renal failure, and encephalitis. However, in another study, three cats were tube fed Philodendron cordatum and showed no signs of acute poisoning. In this study, two adult cats and one kitten were fed a puréed leaf and water mixture, observed afterward, then euthanized, and finally a necropsy was performed. Dosages of 2.8, 5.6, and 9.1 g/kg were used, with the highest dose administered being considerably more than any house cat could consume. The cats showed none of the symptoms found in past epidemiological studies and appeared normal. Necropsies showed nothing that would suggest toxicity. The past epidemiological studies have been suggested to be wrong, since sick cats may be inclined to eat plants to alleviate their illnesses. If this were the case, then such studies would be incorrectly attributing the sickness of the cats to the philodendrons. The forced feeding study may have failed to show signs of philodendron toxicity because the tube feeding bypassed the mouth and hence minimized the typical signs of irritation.
Some philodendrons are, however, known to be toxic to mice and rats. In one study, 100 mg of Philodendron cordatum leaves suspended in distilled water were fed to six mice. Three of the mice died. The same experiment was done with 100 mg of P. cordatum stems on three mice and none of them died. Leaves and flowers of Philodendron sagittifolium were also orally administered in 100-mg doses to the mice. Three mice were used for each of the leaves and flowers; none of the mice died. A similar experiment was done on rats with the leaves and stems of P. cordatum, but instead of oral administration of the dose, it was injected intraperitoneally using 3 g of plant extract from either the leaves or stems. Six rats were injected with the leaf extract and five of them died. Eight rats were injected with the stem extract and two of them died.