Asteraceae is considered as one of the highly evolved families of angiosperms. Give an account of advanced characters in Asteraceae.

Advanced Characters in Asteraceae

Asteraceae exhibits several advanced characters that distinguish it from more primitive angiosperm families. These can be broadly categorized into floral, fruit, and other morphological/anatomical features.

1. Floral Characters

• Inflorescence – Capitulum: The most defining characteristic is the presence of a capitulum, a densely packed inflorescence resembling a single flower. This is a pseudanthium, meaning it appears as a single flower but is actually composed of numerous small florets (ray and disc florets). This structure enhances pollination efficiency by attracting pollinators to a concentrated area.
• Floret Types: Asteraceae exhibits two types of florets: ray florets (ligulate, with strap-shaped petals) and disc florets (tubular, with five lobes). This floral diversity promotes cross-pollination by attracting different pollinators.
• Reduced Perianth: The perianth (calyx and corolla) is often reduced or absent in disc florets, representing a simplification of floral structures.
• Inferior Ovary: The ovary is invariably inferior, meaning it is positioned below the attachment point of the petals and stamens.
• Unique Stamen Structure: Stamens are typically five in number, often with broadened connectives that play a role in pollen dispersal. The anthers are often appendaged or spurred.
• Pollen: Pollen grains are often ornamented and sticky, facilitating adhesion to pollinators.

2. Fruit Characters

• Achene Fruit: The fruit is typically an achene – a small, dry, one-seeded fruit where the seed coat is tightly fused to the pericarp. This fruit type is well-suited for wind and animal dispersal.
• Pappus: A distinctive feature is the presence of a pappus, a modified calyx that forms a parachute-like structure aiding in wind dispersal. The pappus can be of various forms (e.g., bristles, scales, hairs). Taraxacum officinale (dandelion) is a classic example.
• Ribbed or Winged Pericarp: The pericarp (fruit wall) is often ribbed or winged, further enhancing dispersal capabilities.

3. Other Morphological and Anatomical Characters

• Composite Nature: The entire plant body exhibits a composite nature, with the capitulum representing a highly integrated unit.
• Laticiferous Vessels: Many Asteraceae species possess laticiferous vessels containing a milky sap (latex), which may contain various chemical compounds for defense or other functions.
• Presence of Specialized Oils and Alkaloids: Asteraceae is known for producing a wide range of secondary metabolites, including essential oils, alkaloids, and terpenoids, which contribute to their medicinal and aromatic properties.
• Collenchymatous hypodermis: Presence of collenchymatous hypodermis in leaves provides mechanical support.
• Unicellular trichomes: Presence of unicellular, uniseriate trichomes on leaves and stems.

4. Phylogenetic Considerations

Molecular phylogenetic studies have confirmed Asteraceae’s relatively recent evolutionary origin within the Asterales order. The evolution of the capitulum is considered a key innovation that drove the family’s diversification. The family’s success is also linked to its ability to adapt to a wide range of ecological niches, facilitated by its diverse floral and fruit characteristics.

Feature	Asteraceae	Primitive Angiosperms
Inflorescence	Capitulum (head)	Spike, raceme, panicle
Ovary Position	Inferior	Superior or half-inferior
Fruit Type	Achene with pappus	Various (capsule, berry, drupe)
Perianth	Reduced or absent in disc florets	Well-developed calyx and corolla