Sunday, November 25, 2007

What makes a plant a PARASITIC plant?

- Parasitic plants are defined as vascular plants which have developed specialized organs that penetrate the tissues of other vascular plants (hosts).They establish connections to the vascular tissue of the host in order for the parasite to absorb the host's nutrients.

- Despite their wide diversity, parasitic plants, like normal plants, have a basic body structure. Like other plants, they have a shoot system and a root system, but what sets parasitic plants apart is that the root meristem can develop a whole new organ: the haustorium.
* Upon contact with a host, a part of the parasite's root will actually develop its attachment organ, the haustorium.
* The haustorium consists of a host attachment structure, a heavily vascularized infection peg, and thin filaments that infiltrate the host vascular bundles.
* The filaments that invade the host can enter the xylem, the phloem, or even both.

- In order to successfully invade a host, a parasitic plant must be able to read and respond to the host's cells. By doing so, the host plant is unaware of its attacker and allows the parasitic plant into its "developmental social circle."

- Along with the nutrients gained by the host plant, some parasitic plants still gather their own nutrients and use photosynthesis for energy.

As in most things, there is a wide variety amongst parasitic plants, and not all of them act in the way just described. Here is a list of the 6 basic ways to characterize a parasitic plant:
  1. Obligate parasite - a parasite that cannot complete its life cycle without a host.
  2. Facultative parasite - a parasite that can complete its life cycle independent of a host.
  3. Stem parasite - a parasite that attaches to the host stem.
  4. Root parasite - a parasite that attaches to the host root.
  5. Holoparasite - a plant that is completely parasitic on other plants and has virtually no chlorophyll.
  6. Hemiparasite - a plant that is parasitic under natural conditions and is also photosynthetic to some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant. Many obtain at least part of their organic nutrients from the host as well.
(Nuytsia floribunda, seen above, is an obligate root hemiparasite)

Types of Parasitic Plants

There are two main types of parasitic plants Holoparasites and Hemiparasites.

Holoparasites: These parasites are also known as "obligate" because of their obligation to find a host in order to survive.

Nonphotosynthetic Vines- They are found in temperate and tropical regions of the world. These species are a part of the Cuscutacea family and are related in to the well-known vine, Morning Glory. Nonphotosynthetic vines are also known as Stem Parasites because their connection to hosts is through the stem.

Root Parasites- These are fleshy-stemmed parasites that fall into the Orobanchacease family. Root parasites have their parasitic connection the roots of their hosts.
(Ivy Broomrape, seen above, is a holoparasitic root parasite. Like other Broomrape species, it is named after its specific hosts, which in this case, are different types of ivy.)

Hemiparasites: These are also known as facultative parasites, meaning optional, because they are not completely parasitic. They are capable of both photosynthesis and parasitism.

These have green photosynthetic leaves, yet the plant uses carbon from the host plant's roots. Common types are hemiparasites are Indian paint brush, owl's clover (seen on the right) and lousewort. Because they are not completely parasitic, they are capable of living on their own, but studies show the plant is usually smaller and less vigorous if it lives without invading a host.

Details on Common Species

There a thousands of parasitic plant species, but here are a few common ones you may have heard of.

Mistletoe: Mistletoes are parasitic plants that grow on the branches of shrubs and trees. Nearly all mistletoes are hemiparasites with evergreen leaves that perform photosynthesis and berries in a variety of colors.

(Above we see clusters of mistletoe invading a large group of trees.)

The name "mistletoe" incorporates a multitude of species, including Desert Mistletoe (Phoradendron californicum), Dwarf Mistletoe (Arceuthobium campylopodum), and the species we are familiar with, Eastern Mistletoe (Phoradendron serotinum).

Mistletoes grow on a wide variety of hosts and can eventually prove fatal, although they usually only stunt growth. While most plants have a root that grows downward (gravitropism), mistletoe roots, along with other parasitic plants, have roots that grow in the direction of their host.

Mistletoe seeds are generally spread by birds which eat the berries and later excrete them onto trees and shrubs. The seeds are coated with a sticky substance that hardens and attaches the seed to its future host. While most plant seeds germinate in the soil, mistletoe seeds germinate directly on the branches they are attached to.

Dodder: Dodder is a thin and wispy plant, usually orange or yellow (occasionally tinged with purple or red.) The stems are threadlike and short. When dodder flowers they are usually white, pink, or yellowish. A few species include California Dodder (Cuscuta californica) and Salt Marsh Dodder (Cuscuta salina).

Dodder is parasitic to various kinds of wild and cultivated plants, especially alfalfa, closer, and potatoes. Dodder is difficult to "weed out" because its seeds are often mixed in with those of the farmed crops.

Dodder absorb water, minerals, and carbohydrates from the host through its haustoria. In dodder, the haustoria are modified adventitious roots. When dodder produce seeds, they germinate the next growing season if a host is present. However, if there is not, the seeds can remain dormant for up to five years, waiting for their opportune moment. Once the seeds germinate, they must find a host within a few days or else they die.

Wood rose: Wood rose (New Zealand) is a fully parasitic plant (holoparasite). It's a root parasite in the family Balanophoraceae. It's New Zealand's only fully parasitic flowering plant. It has no green leaves or shoots of its own.

The small flowers which make up the large buds (inflorescences) are adapted to attract bats which then pollinate the plant. Wood rose is also often pollinated by mice and rats. These inflorescences are extremely skewed in gender ratio as there are five male "flowers" for each female.

Wood rose's latin name is Dactylanthus taylorii and it is also known as "the flower of Hades." When wood rose invades its host, usually a shrub or tree, the host's roots enlarge in repsonse. As the roots enlarge, the wood rose creates a solid attachment, and from then on gains its nutrients though this connection.

Exotic and Rare Species!

Rafflesia: Rafflesia (Rafflesia arnoldii) is a flowering parasitic plant that is found in the rainforests of Indonesia. The plant does not produce leaves, stems or roots, therefore it is holoparasite. Rafflesia is a nonphotosynthetic vine which grows as individual threads. These thread-like strands grow within their host, usually the Tetrastigma vine.

Rafflesia can only be seen when it is ready to reproduce because then, and only then do the parasitic growths on the vine develop and burst through their host's bark as an strange-looking cabbage.

After nine months the bud opens and becomes the well-known
rafflesia flower which smells of rotting flesh. The flower is the largest in the world and can grow up to three feet in diameter. Its putrid smell is used to attract insects which then pollinate the flower.

Inside the flower are numerous spikes whose functions are unknown. These mysterious spikes hold gallons of nectar.

Thurber's Stemsucker: Thurber's Stemsucker (Pilostyles thurberi) is a rare parasitic plant in the family of Rafflesiaceae. It is a wildflower stem parasite that lives completely within the stems of a small desert shrub called dyeweed. The parasite is only visible when the buds appear.

Thurber's Stemsucker is a
holoparasite with no true stem, roots, or leaves and is completely dependent on its host dyweed plant. From this shrub, the parasite gains its vital nutrients and water.

Once a year small buds protrude from the hosts stem and resemble small, red pimples. After a certain period of time, these buds bloom into small flowers with no true petals and are so small they are only 2-3mm in diameter.

These buds produce a saffron yellow dye. This dye is used in local Indian artwork in California, Arizona, and Colorado.

Cytinus Ruber: (Cytinus hypocistis) These root parasites get thier nutrients entirely from host plant therefore they are holoparasites. They are part of the genus Cytinus and are related to Rafflesia. Cytinus Ruber lives inside its host and can only be seen from March to May when it flowers. It is found in Algeria, Corsica, Crete, Croatia, Cyprus, East Aegean Islands, France, Greece, Italy and Libya.

It has bright red and white fleshy flowers that grow in clusters at ground level. The flowers are usually around three inches tall and are hidden beneath the base of the host plant so they are difficult to see.

The plant is edable and is also used medicinally for astringent, which constricts body tissue.

Side-By-Side Comparison to Non-parasitic Plants

Finally, here's an easy to read list of many SIMILARITIES and DIFFERENCES between parasitic plants and non-parasitic plants.
  • Both are in the Kingdom Plantae.

  • Both parasitic and non-parasitic plants need water and nutrients in order to survive.

  • Non-parasitic plants gain their nutrients and water by absorbing them through their root systems, and while some parasites are also capable of doing this, most parasites depend on infiltrating a hosts vascular tissue in order to obtain their water and nutrients, something a regular plant does not do.

  • All regular plants perform photosynthesis while parasites do not, except for those in the category "hemiparasites."

  • Parasitic and non-parasitic plants have a shoot and root system.

  • Parasitic roots grow towards the host, while regular plants have roots that grow downward due to gravitropism.

  • Only parasitic plants develop a haustorium.

  • Both parasitic plants and non-parasitic plants have vascular tissue known as xylem and phloem.

  • Parasitic plant seeds can germinate on top/attached to their host while non-parasitic plants have seeds that germinate in soil, water, or some other medium. Keep in mind, however, some parasitic plants do germinate in the soil.

  • In plain terms, parasitic plants are the "predator" while the non-parasitic plants on their "prey."