Australian Pea Flowers - Background

Introduction

In its broadest context, the 'pea family' of plants is a very large group comprising over 700 genera and 19,000 species and includes not just the typical 'pea-flowered' plants but others including wattles (Acacia species) as well as Cassia and its relatives. This broadly defined family occurs throughout the world and consists of herbs, shrubs, trees and climbing plants. Some exotics, such as the edible peas and beans, clover, lucerne and sweet peas are well established as commercial or ornamental crops while others, such as English broom (Cytisus scoparius) and gorse (Ulex europaeus), have long since outlived their welcome in some areas and become environmental weeds.

   Root Nodules
   Nodules on the roots of a "pea" plant can often be seen by removing a small plant from its container.
Photo: Brian Walters

The 'pea family' are legumes and are able to take-up ("fix") their nutrient requirements for nitrogen directly from the atmosphere with the aid of soil bacteria (Rhizobium spp.). This occurs in nodules on the roots of the plants.

The botanical classification of the legumes is a little confusing. In Australia, until recently, most authorities classified them as belonging to three distinct families - Fabaceae (typical 'pea-flowered' plants), Mimosaceae (Acacia and relatives) and Caesalpiniaceae (Senna, Cassia and relatives). However, around the world the they have usually been classified in a larger Fabaceae family, with three subfamilies:

  • Sub-family Faboideae (typical 'pea-flowered' plants)
  • Sub-family Mimosoideae (Acacia and relatives)
  • Sub-family Caesalpinioideae (Senna, Cassia and relatives).

This latter classification now seems to have been adopted by Australian herbaria and is also used on these ANPSA pages. The previous classification into three separate families will still be seen in books and in other websites.

Note that the names Leguminoseae and Papilionaceae have also been used as alternative names for Fabaceae and that the sub-family Faboideae is also sometimes referred to as the sub-family Papilionoideae. To minimize confusion, these alternative names will not be used on the ANPSA site.

The following table summarises the 'previous' and 'current' classifications.


Group Previous Classification Current Classification
Typical 'Pea-flowered' Plants Family Fabaceae Family Fabaceae, subfamily Faboideae
Acacia and relatives Family Mimosaceae Family Fabaceae, subfamily Mimosoideae
Senna, Cassia and relatives Family Caesalpiniaceae Family Fabaceae, subfamily Caesalpinioideae

Sub-Family Faboideae - The 'Typical' Peas

This section of the ANPSA site deals with the Sub-family Faboideae - those members of the Fabaceae which have the typical "pea" flowers, as described below and as shown in the diagram.

In Australia, there are about 140 genera and 1,100 species in the sub-family Faboideae. They are found in every state and territory of Australia, from coastal cliffs to alpine areas, from the tropics to the arid inland. Many of Australia's peas are representatives of genera found elsewhere in the world (eg indigo (Indigofera), coral trees (Erythrina) and rattlepods (Crotalaria). However, there are two groups ("tribes") within the family which are almost entirely Australian:

"These are the egg-and-bacon peas, the tribes Bossiaeeae and Mirbelieae. They probably originated from a single common ancestor in Australia millions of years ago. All their ancestors have remained "true blue" except for a single species which has just managed to cross Torres Strait into Papua New Guinea. Together these tribes have 700 species, or more than half of all the peas in this continent. With few exceptions, they can be recognised by their distinctive "egg-and-bacon" flowers, in which the petals are yellow or orange with red markings......"

M D Crisp and J M Taylor, "Australian Plants", June 1993, p102

Characteristics of the Sub-Family Faboideae

   Pea flower diagram

The main identifying characteristic of members of the sub-family Faboideae is the structure of the flower.

The typical "pea" flowers consist of 5 irregularly shaped petals; the "standard", two "wings" and two lower petals joined along their upper edge to form the "keel", as shown in the diagram. The keel encloses 10 stamens, the structure of which helps in distinguishing the various genera within the family. The stamens may be:

  • All individually free (eg. Aotus, Chorizema, Dillwynia, Oxylobium, Phyllota, Pultenaea) as indicated in the diagram
  • All fused into a tube (eg. Bossiaea, Hovea, Platylobium, Templetonia)
  • Nine fused and the tenth one free (eg. Glycine, Hardenbergia, Kennedia)

In some cases the petals may be modified to a great extent such that the "typical" shape is not apparent at first glance (eg. Gastrolobium celsianum, Swainsona formosa). The most common colour among the pea flowers is yellow, often with a red or orange centre but colours of every shade from red through to white can be found in this group.

The flowers are followed by pods (legumes) which contain few to many hard seeds and which vary greatly in shape.....flat, short, elongated or cylindrical.

Like the acacias, many pea-flowered species occur in areas where bushfires are common, such as dry forests and woodlands. In these habitats they are often "pioneer" species, quickly recolonizing burnt-out areas and then being gradually replaced by other species in the plant community. They are often helped in this role by ants which store the seeds underground. The seeds themselves often have a very long viability.

Some of the most common Australian genera are listed in the following table:


Genus Brief Details
All Stamens Free
Aotus Small shrubs, mainly from Western Australia; about 15 species
Castanospermum A single species (C.australe, the black bean); large tree native to Queensland and NSW
Chorizema Small shrubs/twiners. all but one native to Western Australia; about 18 species
Daviesia Small to medium shrubs from all states but mainly western Australian; about 120 species
Dillwynia Small to medium shrubs from all states with most in NSW; about 24 species
Gastrolobium Small to medium shrubs mainly from Western Australia; some toxic; over 100 species, including all species previously included in Brachysema, Nemcia and most Oxylobium
Jacksonia Shrubs or small trees widespread except in southern Australia; about 40 species
Gompholobium Small shrubs from all states; about 30 Australian species
Oxylobium Small to medium shrubs from easten Australia. Western species now included in Gastrolobium
Podolobium Small to medium shrubs from most states; 6 species formerly included in Oxylobium
Phyllota Small shrubs from most states; about 10 species
Pultenaea Small to large shrubs from all states; about 120 species
Swainsona Annual and perennial herbs from semi arid and arid areas; about 84 species
All Stamens United
Bossiaea Small to medium shrubs from all states; about 50 species
Crotalaria Herbs or shrubs; about 26 Australian species. Some toxic and some exotic species have naturalised
Goodia Small shrubs from all states; 2 species
Hovea Small shrubs usually with blue or purple flowers; widespread; about 20 species
Platylobium Small shrubs; all states except Western Australia; 4 species
Templetonia Small shrubs from all mainland states; about 11 species
Nine Stamens United; One Free
Glycine Light twiners from all states; about 15 Australian species
Hardenbergia Normally climbers; 3 species from eastern and western Australia
Indigofera Small to medium shrubs; pinnate leaves; about 30 Australian species
Kennedia Climbers or prostrate plants from all states; about 16 species

Toxins and Tucker

Although no Australian member of this group has been seriously exploited as a food crop, some have been important as food for both aborigines and for the early European colonists. However, as some of the latter found to their cost, many of the native "peas" are also toxic unless the seed is prepared correctly. This usually involves soaking to leach out the toxin or heating to destroy the toxic chemicals.

Some of the species used for food include:

  • Castanospermum australe, the black bean. The seeds of this species are toxic when raw and are prepared by roasting or soaking in water.
  • Coastal Jack, Canavalia rosea (toxin removed by heat). This plant was eaten by members of James Cook's expedition in 1770.
  • Vigna lanceolata, maloga bean, from northern Australia
  • Mucuna gigantea, burny bean.

The toxic properties of some species have also had an impact on agriculture by other means; ie. by being implicated in livestock poisoning. Nervous system malfunction is seen in poisoning of livestock by some Darling peas, (Swainsona species), and by Gastrolobium species.

The rattlepods, Crotalaria species, are also implicated in livestock poisoning. They contain pyrrolizidine alkaloids which accumulate in the liver and produce long-term damage which is often fatal.

"Horses and cattle are more susceptible to poisoning than sheep, but sheep have been poisoned by bluebush pea (Crotalaria eremaea ssp. eremaea) in western Queensland. Poisoned horses develop a condition called "walk-about disease" or "Kimberley horse disease" in which they become unaware of their surroundings and wander blindly. A major cause of this poisoning is Crotalaria crispata, a small plant common in the Kimberley region of Western Australia. Other Crotalaria species cause the disease in the remainder of tropical Australia.

In central and northern Queensland an unusual disease of horses is caused by two other species of Crotalaria, C. aridicola (Chillagoe horse poison) and C. medicaginea (trefoil rattlepod). Horses may develop a taste for these plants which damage the oesophagus (gullet) producing ulceration severe enough to stop the horse swallowing food."

Dr Ross McKenzie, 1993 Bill Tulloch Memorial lecture to the Queensland Region of the Society for Growing Australian Plants

◄◄ Australian Pea Flowers Index    Top ▲