Mon. Sep 26th, 2022

A parasitic plant with potentially toxic berries might not sound like it’s taking your Christmas decorations to the next level. But, botanically speaking, it is mistletoe.

There are about 1,300 species of this evergreen plant worldwide. Both are parasitic or semi-parasitic, meaning they can only live on host plants. They are not rooted underground, but live on the branches of other trees and shrubs.

Only two types are native to North America. Twelve species of American mistletoe can be found mainly in the southern half of the United States, mainly affecting deciduous trees in the east and some evergreen plants in the west. Sixteen species of leafless dwarf mistletoe infect only trees in the pinaceae family, mainly on the West coast.

American mistletoe is an American Christmas plant of the genus Phoradendron, Greek for “tree thief.” It has green leaves and is capable of photosynthesis, so it can produce most of its own food. But American mistletoe also draws water and other nutrients from its host plant by feeding root-like structures called haustoria into branches and vascular tissues beneath their bark. Even when the mistletoe plant itself is removed, these invasive structures can survive in the tree for many years.

Illustration of cross section of brain showing axonal pathways transitioning from gray matter into white matter.

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Mistletoe is what botanists call dioecious, meaning there are separate male and female versions of the plants. The fruits produced by females, called berries, are usually white, but can be pink or reddish depending on the species. Birds spread seeds widely after eating berries. The seeds of some species can also be shot out of the fruit like cannonballs at speeds of up to 60 MPH (100 km/h) over distances of up to 50 feet (15 meters). The sticky material on the seeds helps them stick to whatever trees they land on until they germinate and start growing.
In general, mistletoe will not kill a tree unless it is heavily infected. Even so, trees don’t usually die from mistletoe. In most cases, death is an indirect effect of disease or insect attacks that take advantage of pressurized trees. Mistletoe’s parasitic manner can cause significant economic losses to forests that are industrially harvested for timber.

For homeowners, however, there is usually no need to control mistletoe — which is good, as getting rid of it can be difficult and requires patience and persistence. You can trim it off, make sure to set aside all those haustoria scattered under the bark of the host, or try chemical control methods such as plant growth regulator ethephon.
You may want to trim a twig for Christmas decorations. Dating back to at least the 1700s, one of the most common traditions associated with mistletoe is that anyone wandering under it welcomes a festive kiss. In my home state of Oklahoma, mistletoe is our state flower symbol, apparently because during the particularly harsh winter of 1889, it was the only green plant that could have been used for a grave. In other parts of the world, mistletoe is believed to give life and fertility, as a peace offering and protection from poison.

About poison: Mistletoe is known as a poisonous plant. While European species of mistletoe are reported to be poisonous, American mistletoe is not deadly. However, it’s best to keep it away from children and pets, and use artificial mistletoe for decorations if you’re worried.
Mistletoe is an important part of the north American ecosystem. Many birds rely on mistletoe berries as a food source, as do elk, deer, squirrels, chipmunks and even porcupines, which also eat leaves when other fresh leaves are scarce. Tangled mistletoe, traditionally known as a witch’s broom, provides nesting sites for birds, including spotted owls and Cooper’s hawks, among other animals. Three species of butterflies in the United States rely entirely on mistletoe. It is also an important nectar and pollen plant for honeybees and other native bees.

Tractography scan showing brain axons.

As a result, the parasitic plant plays an important role in both ecosystems and human traditions. If it grows near you, enjoy it, because you may not be able to get rid of it completely anyway. It might come in handy during the Christmas season.
In recent years, researchers have shown that mechanics, or forces that objects exert on each other, play an important role in the growth and folding of the brain.

Of several hypotheses put forward by scientists to explain how brain folding works, differential tangential growth is the most widely accepted because it is well supported by experimental observations. The theory postulates that the outer layers of the brain grow faster than the inner layers because of how neurons multiply and migrate during development. This mismatch in growth rates leads to more and more compensatory resistance in the outer layers, leading to increasing overall instability in the brain structure. However, folding these layers releases this instability.

To better explain this theory, Jalil made a mechanical model of the brain, assigning the growth rate of the outer layer to the inner layer. As expected, this mismatch in growth rates causes the inner layer to prevent the outer layer from spreading. Because the outer layer cannot expand further due to this blockage, it must fold and snap inside the inner layer to achieve a more stable structure.

Another study using 3D-printed hydrogel brain models also showed that mismatches in growth rates lead to folding.
This buckling happens because folding maximizes the ratio of the brain’s surface area to its volume, or the brain’s surface area relative to its size. A higher surface-to-volume ratio allows the brain to accommodate more neurons in a given space while reducing the relative distance between them.

Jalil’s team also found that other mechanical factors influence the final shape of the developing brain, including the initial thickness of the outer layer of the brain and how hard the two layers are relative to each other.

More recently, our simulation studies have shown that axons are part of the neurons that help transmit electrical signals and play a role in regulating the folding of the brain. Our model suggests that ridges form in areas with a high number of axons and valleys form in areas with low axon density. We confirmed these findings with neuroimaging and tissue samples from real human brains. This reinforces the importance of axon density in brain development and may shed light on the origins of disorders with irregular brain structure and connectivity, such as autism and schizophrenia.

Both of us are now developing more sophisticated brain models based on neuroimaging of real brains, which will provide more detailed simulations of brain development.

Mechanisms of brain disease
Our brain model provides a potential explanation for why the brain may be abnormally formed during development, highlighting the important role that brain structure plays in its normal function.

A brain with an abnormal folding pattern can have devastating consequences. For example, brain models with a thicker than usual outer layer formed fewer ridges and valleys and larger ridges than normal thickness. In extreme cases, this can lead to a condition called ISSencephaly or smooth brain, which is completely free of brain folds. Many children with the disease suffer severe developmental delays and die before the age of 10.

On the other hand, the outer layer of the polycerebellar gyrus is thinner than usual, resulting in overfolding. This situation has also been replicated through mechanical modeling. People with the disorder can have mild to severe neurological problems, including seizures, paralysis and developmental delays.

Scientists have also found abnormal folding patterns in brain diseases such as schizophrenia and epilepsy.

The next step in brain science
Understanding the mechanisms behind brain folding and connectivity will provide researchers with the knowledge base to uncover their role in developmental brain diseases. In the long run, elucidating the connections between brain structure and function could lead to early diagnostic tools for brain diseases.
In the future, ARTIFICIAL intelligence may be able to gain greater insight into the normal growth and folding of the human brain. But even with all these advances in neuroscience, researchers like us still have work to do as they continue to try to decipher the mysteries of the universe’s most complex known structures.

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