Defence Mechanisms in Plants

Skyrail Nature Diary: December 2013


This month we will be looking at the various methods plants use to defend themselves against the environment, there are certainly more than enough things a plant has to be wary of. There are animals that eat their flowers, leaves, sap and bark as well as some that make their homes in trees by gouging out part of the trunk. Then you have animals that climb the trees all the while ripping deep gouges into the bark with their claws, goannas being a prime example. Yet others lay their eggs in their leaves and trunks. Some plants even attack other plants, like mistletoes and dodder laurel. In addition to this, there are numerous diseases of the leaves, bark and roots caused by various micro-organisms like fungi and bacteria. Farmers worldwide lose millions of dollars every year due to fungi in particular. Examples include rust, smut and root rot. The environment itself also throws a few challenges their way. Consider the occurrence of cyclones, fires, earthquakes and floods just for starters. All of this has to be taken into account by a plant as far as what strategies they will employ to protect themselves. The easiest solution is often simply producing so many offspring that at least some of them will survive the trials and tribulations of everyday life.  The downside to this strategy is a rather brief life span and very little in the way of size and specialisations. Below are some of the ways plants protect themselves against attacks from the surrounding environment.

 

Physical protection against ravenous plant eating animals, sapsuckers and parasites are numerous. The two most common are the use of spines and fiberglass hairs. Spines are very common in Barron Gorge as all of the rangers can attest to. The two most commonly encountered plants that make use of this strategy are the Rattans and the Mother-in-law vines. The Rattans are spiny palms that grow as vines over much of Barron Gorge. The spines are found on the trunks, undersides of leaves and on specialised climbing tendrils. Once these palms reach the canopy they shed all the lower spines and fronds as a skin and become what is known as lawyer cane. The Mother-in-law vines have mimosa like leaves and the whole plant is covered with tiny spines that cling tenaciously to anyone unlucky enough to rub against them. The use of fiberglass hairs is best documented in the Stinger Tree, a herbaceous shrub that grows up to 6 metres tall. The whole plant and especially the large heart shaped leaves are covered in hollow fiberglass hairs attached to a poison gland. The hairs break off easily and create an instant sensation of broken glass while the two reported poisons sensitise the skin to pain as well as inflaming the lymph nodes.

 

Animals occasionally protect plants in return for certain favours. Ants and mites constitute the best examples of this phenomenon. Ants have entered into an interesting relationship with the unusual ant plants of which we have three species in the Wet Tropics. These rather small plants are epiphytes most commonly found on Paperbarks in swampy areas. The leaves are leathery while the main body of the plant is a bulbous warty mass full of tunnels. These tunnels usually contain ants that  use the bulb as their nest, vigorously defending the plant against any intruders as well as fertilising the plant with their droppings. Mites are most commonly found on Quandongs. These trees have leaves with special structures known as domatia. Domatia are always found along the midrib of the Quandong. There are two types - hairy pockets and foveoles. The latter are fleshy outgrowths of the leaf itself. Mites live in the domatia and defend the leaves against being smothered by copious amounts of algae and fungi. These often grow on the surface of the leaf in communities known as epiphylls. The mites eat these communities and allow the leaf to continue its photosynthesis.

 

Poisons are the most common method employed against plant eating animals. Leaves often contain various kinds of alkaloids that either kill the animal outright, or make them very sick. Over time the animals develop the antidote against these chemicals and the plants have to develop new effective ones. This chemical warfare is typical of most ecosystems but is most noticeable in tropical rainforests. Typically the quality of the soil the plant is growing on dictates the amount of poison produced. In Barron Gorge the soils are very poor and all of the trees have large quantities of poison. This is why there are no possums or tree kangaroos in Barron Gorge. On the Tablelands, the trees are less stressed and produce less poison. A special kind of poison is white sticky sap that deters sapsuckers. This type of poison is associated with the oleander, mulberry/fig and star apple families.

 

Natural disasters are often hard to defend against but there are some surprising defences against cyclones here in Australia. The most common defence is the tendency to shed branches when the cyclone hits and have a flexible trunk, presenting as little resistance to the wind as possible. Examples in the Barron Gorge include Blue Quandong and Milky Pine. A more unusual strategy is shedding bark. The Kauri sheds copious quantities of bark at regular intervals to get rid of vines and epiphytes. In addition to this all of the lower branches consistently shed throughout the tree’s life and the wounds smoothed out. This creates a smooth round trunk that the cyclone will have difficulty impacting.

 

By Tore Lien Linde