Australian Daisies - Background
Introduction
Daisies are members of the Asteraceae, a diverse family of plants comprising about 20,000 species worldwide. In Australia there are almost 1,000 indigenous species comprising shrubs, sub-shrubs, perennial herbs, annuals and a few biennials. There are too many Australian daisies to generalize about the growing conditions they enjoy. Some prefer full sun, others like shade, some like open conditions, others prefer overhead protection. They all appreciate group planting whereby they benefit from the increased possibilities for cross-pollination.
There are daisies for all soils from clay to sand, wet to dry, but usually improved growth results if the soil is well drained, enriched with fertilizers and slightly acid.
The apparent 'flower' of a daisy is a condensed inflorescence composed of a large number of small stalkless flowers arranged on an expanded stem called a 'receptacle'. The inflorescence is known as a 'capitulum' or 'head', and each individual flower is called a 'floret'. Florets are of two main types:
- Tubular (or disc) florets have a regular corolla of 5 (sometimes 4) united petals with 5 (or 4) lobes. The calyx (if present) consists of a ring of hairs or scales known as the 'pappus'. There are 5 stamens united by their anthers to form a tube around the style. The style is divided at the top into two 'arms'. The ovary is 'inferior', (ie it occurs below the calyx) - Figure 1
- Ligulate (or ray) florets have an irregular corolla of 5 (or 3) petals which are united at the base and then extend into a flat strap-shaped structure called a 'ligule' or ray which has 5 (or 3) small lobes. A pappus of bristles, scales or awns may be present. The stamens are united by their anthers to form a tube, the style is divided, and the ovary is inferior - Figure 2
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The ligules are the apparent 'petals' in the typical daisy flowers such as Brachyscome, Olearia and Celmesia.
The structure of a daisy head depends on the arrangement of the two main types of florets.
- In ligulate heads all the florets are ligulate and 5-lobed and usually they are all bisexual. An example is the genus Microseris or 'Yam Daisies'.
- Discoid heads have all tubular florets which may be all bisexual, or there may be female or sterile florets around the periphery of the head. An example is the genera Rhodanthe.
- Heads with both ligulate and tubular florets are called radiate heads. The tubular florets are usually bisexual and are found in the centre of the receptacle with the ligulate florets arranged around the outside in one or two rows. The ligulate florets are usually either female or sterile. An example is the genus Brachyscome - Figure 3.
Each head is generally surrounded by an 'involucre' of bracts which occur in several rows, and may be soft and herbaceous as in Brachyscome or stiff and papery as in Rhodanthe.
The fruit is dry, one-seeded and indehiscent (that is, it does not open to disperse its contents) and is known as a cypsela.
The criteria for identification are many but one of the most important is the appearance of the mature cypsela. Under a microscope the presence or absence of a pappus, whether it is feathery or barbed, the size, shape and colour of the body of the cypsela, whether it is smooth, hairy, warted or winged, all help with identification. Other criteria include the appearance of the receptacle, the bracts, anthers and style-arms as well as the characteristics of habit, leaf, stem and arrangement of heads - Figures 4 and 5.
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Cypselas (seeds) of daisies |