The family from the skies

“Aster” is the greek name for “Star”, which is also the name of the type genus of one of the most diverse families of flowering plants: Asteraceae (also known in Botany under the old name of Compositae).

When I think of the name Asteraceae I cannot avoid thinking about the supreme star that lights and heats the planet we live in. In fact, I think the name Aster is well attributed due to morphological similarities that we might find between an Asteraceae flower and a star, or even the Sun itself. Stars usually have a core and irradiate light through rays of light. In Asteraceae inflorescences do also resemble stars by having the core flowers tightly grouped in the centre and the ray flowers on the edges.

The fact of having this extremely cosmopolitan and diverse family under the name of “Star”, makes me think that they can be found anywhere in the world as easily as finding stars in the dark sky above. So let’s explore a bit the secret recipe for the high success of this family which became the largest among core eudicots. They which are so widespread that can be found naturalized in all continents (except Antartica) and in most environments. Such success must be related with their floral morphology! And if it’s not directly related with the floral morphology it doesn’t matter because I will introduce it anyway.

The inflorescence type is an exclusive of the family, it is very odd and it comes in a pseudanthium-like inflorescence. I bet some of you are now wondering what the pseudanthium is, or scrolled down the window to check on the glossary. Botanists call pseudanthium to some types of inflorescences where flowers are organized in such a way forming a structure that looks like a single flower, even though it’s a group of flowers! We find these pseudanthia in all Asteraceae, and in this family the pseudanthia are special as they are organized in a disc. The botanical name Compositae was also based on the morphology of such inflorescences, which can also be called compound flowers, meaning a flower made out of many flowers, or opposite to single flower.

When it comes to Asteraceae, even some structures have special names as they don’t occur in other plant groups, starting with the inflorescence, the capitulum. Also the calyx usually differentiates into a pappus (a series of hairs or bristles) instead of the typical 5 sepals. Petals are always fused, forming a stamen-petal tube (disc flowers) or forming the monosymmetric corolla we find in peripherical flowers. Finally, the tubular flowers are bisexual but the peripherical ligulate flowers are usually pistillate (lacking male organs) or sterile.

Florets of an Asteraceae cappitulum

One of the most interesting characters of these flowers is known by the secondary pollination mechanism. This mechanism is simple and effective at the same time. The tube formed around the stigma on tubular flowers is the basis of this mechanism, as stamens are organized all very tight around the gynoecium, and when it pops outside the flower, it is covered with pollen on the outer (abaxial) side of the stigma. This mechanism is interestingly effective as it avoids self-pollination (as the pollen is attached to the abaxial side of the stigma and the fertile region is the inner or adaxial side, which only opens when it sticks totally out from the floral tube).

The pollen gets attached to the abaxial side of the stigma, the receptive part of the stigma is the adaxial side. Mature anthers in the left side and mature stigmas in the right side. The ovaries are always bicarpellate and inferior.