Samstag, 15. September 2012

Flowers and inflorescences of the Asteraceae - an introduction

1) Introduction

In my previous four postings, I've shown you four different species from the Asteraceae. With 1500 different genera and 24000 species, the Asteraceae are one of the largest plant-families in the world. The most of them are herbaceous plants (Bellis perennis, Taraxacum officinale, Tanacetum vulgare) but there are also shrubs and even trees.

 Astericus serecius - a shrub, native to the Canary Islands

Members of the Asteraceae can be found all over the world (except Antarctica). In Europe, they are one of the most variable families. Many plants from this family are used as vegetable, medicinal plant or spice (like Stevia rebaudiana; the “natural” sugar).

The most distinctive characters of the Asteraceae are their inflorescences and their fruits.

2) The inflorescences of the Asteraceae and their flowers

The inflorescences of the Asteraceae, often called “head”, are unique. They consist of many small flowers, which form a large “fake” flower in order to seem more attractive for pollinators. This is similar to the Apiaceae and their umbels, but the floral axis of the Asteraceae is reduced so extremely, that the flowers seem to sit directly together. Their bracts are forming a circle of fake sepals: the Inculucrum.

The flowers of these heads have the basic formula K 5 C (5) A 5 G (2): five free sepals, five fused petals, 5 stamens and 2 hypogenous carpels. However, the flowers haven't a uniform appearance. Some of them are cygomorphic, others are radial. In Botany, the cygomorphic flowers are called “ray flowers” (“Zungenblüten” in German) and the radial flowers “disc flowers” (“Röhrenblüten” in German).

  Depending on the composition and morphology of these two types of flowers, the Asteraceae are subdivided into twelve sub-families today. The most important are

  • Asteroideae: the inflorescences of the Asteroideae consist of disc and ray flowers. In the most cases, the ray flowers are sterile and arranged in the periphery of the inflorescence in order to simulate petals. The fertile disc flowers are located in the centre. Typical species from this sub-family are Helianthus annuus (sunflower), Senecio inaequidens (South African ragwort) or Bellis perennis (Daisy)
Senecio inaequidens from the Asteroideae

Helianthus annus - here, you can see the three "teeth"
of the yellow ray flowers
  • Cichorioideae: the heads of Cichorioideae consists only of fertile ray flowers. All five petals of this flowers show in the same direction; so the ray flowers of this family seem to have five teeth instead of three, what is an important difference towards the Asterioideae sub-family. Another distinctive feature of the Cichorioideae is the milky liquid in their stalks. Typical species are Cichorium intybus (Chicory) or Taraxacum officinale (Dandelion)
Cichorium inytbus - ray flowers with five teeth

Taraxacum officinale

  • Carduoideae: The inflorescences from species of this subfamily have only disc flowers. The most popular Genera of the Carduoideae are Cirsium (Thistles), Centaurea (centaury), Arctium (Burdock) and Cynara, the genus of the artichoke (Cynara cardunculus)

    Arctium lappa from the Carduoideae

    Cirsium vulgaris - a species, which only has disc flowers
However, their different morphology isn't the only special character of the flowers of Asteraceae. Another point of interest is their biology of pollination. As you can see in the formula, the stamens are not grown together but they are arranged close together and form a tubular structure. The stylus grows through this tube and slips off the pollen. The stigma is closed in order to avoid self-pollination. After the pollinators like bees have taken the pollen away, the stigma opens for pollination.

3) The fruits of the Asteraceae

The fruits of the Asteraceae are normal, one-seeded nuts. What makes them so special is their kind of distribution. The pericarp is linked with the testa of the nut and the sepals. The sepals are modified and form a kind of “paraglider”: the Pappus, whose morphology can be an important distinctive feature.
In the most cases the pappus is used as paraglider, which gives the fruit the ability to fly with the wind. However in some cases, the Pappus is used as a kind of “hook”, which hang on clothes or fur for distribution.

Together, Pappus, pericarp and nut forms the unique propagule of the Asteraceae: the Achene.

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