Australian Plant Families: Fabaceae and Asteraceae

Another key feature is the pulvinus, a swelling at the base of a petiole or leaflet that allows for movement in response to external stimuli, such as light capture. Additionally, many species possess stipules, which are small appendages at the base of the petiole.

Nitrogen Fixation and Mutualisms

One of the most significant evolutionary advantages of Fabaceae is the ability to perform nitrogen fixation. Through a symbiotic relationship with rhizobia (nitrogen-fixing bacteria), these plants develop root nodules where atmospheric nitrogen is converted into ammonia. This provides a photosynthetic advantage by improving water-use efficiency and allows for rapid growth following germination.

However, high nitrogen levels can attract herbivores. To counter this, legumes have evolved chemical defenses, such as alkaloids (e.g., swainsonine in Swainsona or abrin in Abrus). Some species also engage in complex mutualisms, such as providing extrafloral nectaries or specialized stipules that act as nesting chambers for symbiotic ants.

Flower Biology and Pollination

The way petals are arranged in a bud, known as aestivation, varies by subfamily:

• Faboideae: Exhibit zygomorphic (bilateral) flowers with imbricate aestivation. The petals typically consist of a large "standard" (outermost), two "wings" (lateral), and two "keels" (innermost) that protect the sexual organs.
• Mimosoid Clade: Feature actinomorphic (radially symmetrical) flowers, often grouped into heads or spikes. They frequently produce pollen in polyads (fused grains of 4–32 grains).
• Caesalpinioideae: Also zygomorphic, but the standard petal is the innermost in the bud.

The genus Acacia is a standout in Australia, utilizing protogyny (the female phase occurring before the male phase) and specialized seed features like the hygroscopic hilum and pleurogram to manage water and dispersal.

Exam Tip: When identifying Fabaceae, look for the "pea flower" structure (standard, wings, keel) in Faboideae, or the massive, conspicuous anthers in Mimosoid species. Always check for the presence of a single carpel/legume fruit.

Asteraceae: The Daisy Family

Asteraceae is the world's largest flowering plant family, comprising approximately 34,000 species. Their primary success factor is the "composite" head, where many small flowers are packed together to resemble a single large flower.

The Capitulum (Flower Head)

The inflorescence, or capitulum, is a highly efficient structure. It consists of many small florets attached to a thickened receptacle, often surrounded by a ring of bracts called an involucre. There are three main types of heads:

1. Discoid: Contains only tubular florets.
2. Radiate: Contains both ligulate (ray) and tubular (disc) florets.
3. Ligulifloral: Contains only ligulate florets.

Floral Morphology and Pollination

The individual florets are highly specialized. Most have an inferior ovary and five fused petals. The calyx is often modified into a pappus (scales, bristles, or hairs), which is a critical adaptation for wind or animal dispersal.

Pollination in Asteraceae is often facilitated by secondary pollen presentation. In many bisexual florets, the anthers are fused into a tube around the style. As the style grows, it pushes the pollen out of the tube, acting as a "pollen presenter" for visiting insects. Many species are protandrous, meaning the male phase occurs before the female phase to encourage cross-pollination.

Vegetative and Fruit Characteristics

Asteraceae are mostly herbs, which allows for rapid evolution and the ability to exploit ephemeral environments. Many species use chemical defenses, such as laticifers (vessels containing latex) or resin canals, to deter herbivores.

The fruit is typically a dry, indehiscent cypsela. Some species exhibit fruit polymorphism, producing different types of fruits within a single head (e.g., some with dormancy and some without) to ensure survival in unpredictable climates.

Comparative Summary of Success Factors