The Boronia Family - Background

Introduction

The large family of plants which includes the genus Boronia is distributed over many parts of the world. Botanically the family is known as the Rutaceae and it includes a number of commercially important plants such as the citrus group of fruit trees (oranges, lemons, lime, etc) and popular ornamental plants such as Diosma which is native to South Africa. Within Australia there are about 40 genera, many of which are cultivated. The most widely cultivated of these are the genera in the "Boronia group".

Botanically the "Boronia group" is known as the Tribe Boronieae. Within this group are is the well known Boronia, itself, and a number of less well known genera. The table below lists the 19 genera in the Tribe and indicates the approximate number of species in each. All but one of the species in this group are endemic to Australia; the odd one out is Phebalium nudum, which occurs in New Zealand.


Table 1 - Genera in the Tribe Boronieae*

Genus No.of Species** Genus No.of Species**
Acradenia
2
Asterolasia
5
Boronia
95
Chorilaena
1
Correa
11
Crowea
3
Diplolaena
6
Drummondita
4
Eriostemon
2
Geleznowia
1
Leionema
22
Microcybe
3
Muiriantha
1
Nematolepis
7
Phebalium
25
Philotheca
36
Rhadinothamnus
3
Urocarpus
5
Zieria
25
   
*Information from the December 1990 issue of the newsletter of SGAP's Boronia and Allied Genera Study Group, "Australian Plants" June 1999 and from "A Name Change for Most Eriostemons" (see Further Information).
**Approximate number only; some genera contain unnamed species and other genera are in need of botanical revision.

Characteristics

Generally the Boronia group comprises plants of open forests and woodlands. They only rarely are to be found in rainforests or in arid areas. Overall the group is distributed throughout Australia but certain genera within the group may be restricted in their distribution (eg Correa is not found in Western Australia).

The flowers are bisexual and usually have four or five petals (eg. four petals in Boronia and Zieria, five in Crowea, Eriostemon and Philotheca) but it is not unusual for some of the flowers on a particular plant to have an abnormal number of petals. In some cases (eg. most Correa, Nematolepis) the petals are fused into a bell-like tube while in others (Chorilaena, Diplolaena) the petals are small and the stamens are the conspicuous parts of the flowers, similar to the flowers of the well known but unrelated genera Callistemon and Melaleuca. The number of stamens either equals the number of petals or is twice the number of petals (eg. eight stamens in Boronia, four in Zieria). The fruits contain hard, waxy seeds which are expelled over a wide area when ripe.

The following diagram of an Eriostemon flower shows some of the features which can be observed in the Boronia group as a whole.


Diagram of Eriostemon flower
Based on Galbraith, J; "Australian Plants", Vol.1, September 1960

The Boronia group of plants are usually small to medium sized shrubs; none would reach even small tree proportions. A feature of most of the group is the presence of aromatic oils in the foliage and, in some cases, the flowers. When crushed or brushed against, the foliage gives off quite a strong aroma. In most cases this is an attractive feature but a few people find the very strong aroma of some Zieria species (for example) to be unpleasant. A number of the boronias have a very attractive perfume with the "Brown Boronia", B.megastigma, being the most famous. The fragrance of other boronias such as B.serrulata ("Native Rose") and B.florabunda is more subtle and not universally detectable.

Table 2 lists some of the features of each genus.


Table 2 - Some Characteristics in the Tribe Boronieae

Genus Characteristics*
Acradenia 5 petals, 10 stamens; leaves opposite petals
Asterolasia 5 petals, 10 stamens; calyx minute
Boronia 4 petals, 8 stamens; some species highly perfumed
Chorilaena 5 small petals, 10 stamens; flowers in clusters surrounded by large bracts; leaves lobed
Correa 4 petals, 8 stamens; petals fused into a bell-shaped tube
Crowea 5 petals, 10 stamens; hairy appendage on anthers
Diplolaena 5 small petals, 10 stamens; flowers in clusters surrounded by large bracts; leaves not lobed
Drummondita 5 petals, 10 stamens, only five of which are fertile; staminal filaments united into a tube
Eriostemon 5 petals, 10 stamens; distinctive "stellate" hairs on leaves and petals; leaves with 3 or more main veins
Geleznowia 5 petals, 10 stamens; flowers in clusters enclosed by large bracts
Leionema 5 petals, 10 stamens; formerly included in Phebalium
Microcybe 5 petals, 10 stamens; flowers in sessile (unstalked) terminal heads
Muiriantha 5 petals, 10 stamens; petals forming a bell shaped tube but not fused
Nematolepis 5 petals, 10 stamens; petals fused into a bell shaped tube
Phebalium 5 petals, 10 stamens; staminal filaments not hairy
Philotheca 5 petals, 10 fertile stamens; leaves with only one main vein
Rhadinothamnus 5 petals, 10 stamens; leaves closely clustered
Urocarpus 5 petals, 10 stamens; calyx inconspicuous
Zieria 4 petals, 4 stamens; foliage often very aromatic
*Typical characteristics. Some variations often occur, such as the number of petals.

◄◄ Boronia Family Index    Top ▲