Plant Tropism

Skyrail Nature Diary: August 2014

This month we will take a look at the ways plants interact with each other and the environment. We will examine how certain plants do the unique things they do and why. The tropical rainforest of north Queensland is known today as the world’s oldest continually surviving rainforest dating back to the time of the break-up of Gondwana approximately 180 million years ago. Many of the plants we see in the rainforest today are the descendants from ancient times, true survivors having evolved and adapted.

Problems that come with living in a wet tropical rainforest are many. If a plant species cannot handle all that nature throws at it, numbers may dwindle to a point where it becomes extinct.

Let’s first take a look at some of the ways plants protect themselves. Many plants improve their chance of survival and reproduction by reducing the impact of herbivores. Some do this by way of chemical defenses that can act as repellents or toxins to anything disturbing or eating them. Some plants may taste unpleasant or be difficult to digest, while others will have sharp spikes or fine hairs that sting or cause irritation on contact and are therefore left alone. Some may just grow in areas where they are not easily found. It's not so much what the plant ‘wants’, rather it’s what has come about over a very long time.

There are many interesting survival techniques which can be described as plant tropism (derived from the Greek word ‘tropos’ meaning ‘to turn’). For example:

Phototropism  -  growth in response to light waves or colors (to grow towards light).

Skototropism  -  growth or movement away from light.

Traumatropism  -  response to wound lesion.

Hydrotropism  -  growth towards water.

Thigmotropism  -  growth response to touch.

Electrotropism  -  response to electric current.

Chemotropism  -  response to particular substances or chemicals.

Geotropism  -  growth of stems or roots in the response to the force of gravity.

Let’s now have a look at how tropism works. During phototropism, plants produce a light sensing hormone called auxin, upon release from the cells in the apical meristem (the tip of a plant shoot or root where cell division takes place), auxin collects mostly on the darker side of the stem stimulating cell elongation of that side, causing the plant to bend towards the light source. Good examples of this seen at Skyrail are the Wait-a-While, Supple Jack and the native Monsteria.

With chemotropism, roots grow away from harmful acidic type soils displaying negative chemotropism and grow towards useful soil minerals displaying positive chemotropism.

Thigmotropism is another interesting one and is common in climbing vines where the vine will bend towards whatever it touches, coiling the tendrils around an object, this is seen a lot in the rainforest understory.

The Venus Fly Trap and Mimosa when touched display rapid movement known as thigmonasty which is different to tropism. In tropism it is a plant's reaction to a certain stimulus when cell division takes place and the plant grows towards a certain stimulus displaying positive tropism, or away from a certain stimulus in which it would display negative tropism. With nastic movements, a plant’s response to a stimulus is non directional and much quicker. The opening and closing of flowers and in some cases, leaves, are also known as nastic movements caused in response to for example, temperature, humidity or light irradiance.

By Phil Rooney