Platanus

The genus *Platanus* consists of a small number of tree species. These trees are the only living members of the family Platanaceae. Most mature members of this genus are very tall. They are often called plane trees or planes in English. In North America, some species are known as sycamores.

This name is most common for *Platanus occidentalis*. The genus name itself comes from an Ancient Greek word. That word originally referred to the *Platanus orientalis*, or the Oriental plane. These trees are important to many different ecosystems around the world.

Plane trees have a very distinct way of growing and reproducing. They are monoecious, which means male and female flowers grow on the same plant. The flowers are small and grow in round shapes called globose heads. These trees are wind-pollinated. The male flower-heads fall off after they release their pollen. After pollination, the female flowers develop into small units called achenes. These achenes form an aggregate ball that serves as the fruit.

The fruit ball is usually 2.5 to 4 centimeters in diameter. It contains several hundred achenes, each with a single seed. Each achene has a tuft of stiff, yellow-green bristles. These bristles act like tiny parachutes to help the wind carry the seeds away.

There are two main subgenera within the *Platanus* genus. The first is subgenus *Castaneophyllum*, which contains only one species: *P. kerrii*. The second is subgenus *Platanus*, which includes almost all other species. The leaves of the trees in subgenus *Platanus* are simple and alternate. They have a palmate outline, which means they are shaped like a hand.

A unique feature is the leaf stalk, or petiole. The base of the petiole is enlarged and wraps around the young stem bud. This protects the bud until the leaf falls off in the autumn. Most species are deciduous, meaning they lose their leaves every year. The only exception is *P. kerrii*.

Scientists have discovered that these trees have a complex evolutionary history. Fossil records of *Platanus* leaves and fruit date back to the Paleocene. Recent studies using gene sequences have identified two major lineages. One is called the ANA clade, or the Atlantic North American lineage. The other is the PNA-E clade, the Pacific North American-European lineage.

These lineages show that the trees have experienced reticulate evolution. This means they have undergone ancient or recent hybridization. Hybridization occurs when two different species breed to create a new type of tree. This process makes their family tree look more like a network than a simple branch.

One of the most famous examples of this is the London plane. This tree is a hybrid known as *Platanus × hispanica*. It was created by crossing the North American *P. occidentalis* with the Mediterranean *P. orientalis*. Because of this mix, it is very tolerant of urban conditions. People plant it widely in cities like London and throughout the temperate world. It is a very hardy tree for park environments. It can even survive in areas with high pollution. The hybrid often backcrosses with its parent species in the wild.

Plane trees can also face challenges from various diseases. A fungal disease called anthracnose can cause trees to lose their leaves. This usually happens during cold and wet spring weather. *P. occidentalis* is quite susceptible to this, but *P. orientalis* is more resistant.

Another serious issue is a wilt disease called *Ceratocystis platani*. This disease is a major problem in much of Europe. While North American species are mostly resistant, Old World species are very sensitive to it. Other fungi, like powdery mildew or Massaria disease, can also affect the trees.

Finally, these trees can sometimes affect the people living near them. All parts of a *Platanus* tree have tiny, star-shaped hairs called trichomes. During long dry periods, strong winds can break these hairs off. This creates a fine dust that can irritate the eyes, nose, and throat. This irritation is sometimes called "platanus cough." It is not a true allergy, but a physical irritation from the dust. To stay safe, people can wear masks or protective glasses during windy, dry weather. Sweeping up fallen leaves in cities can also help reduce the dust.