Did you know that a sunflower is not a flower?

Today, March 30, 2022, marks 35 years since Vincent van Gogh’s legendary painting Sunflowers was auctioned off for the impressive price of £22 million.

Sunflowers, daisies, or dandelions are often mistakenly assumed to be flowers . But the matter is a little more complicated .

How is a flower?

The anatomy of a flower is generally made up of four concentric layers called whorls : the calyx on the periphery, made up of sepals , which normally have the appearance of small green leaves in the form of scales; the corolla , which is made up of the petals ; the androecium , which has the stamens , the male reproductive apparatus that produces the pollen grains; and the gynoecium in the center, formed by one or several carpels , the female reproductive apparatus that contains the ovaries, and that once pollinated, will form the fruit.

This general anatomy has some exceptions. There are flowers, such as the lily, whose sepals and petals are the same and form an intermediate structure called a tepal . Some plants have many whorls on one side, such as the garden carnation with multiple concentric corollas , or the plum blossom, with a large number of stamens arranged in many layers.

In certain species the reproductive function is separated into two distinct flowers , a male without carpels, and a female without stamens. There are also flowers, such as rosemary, in which the petals are fused to form a tube that divides into lobes at the end. Or species that have some of the layers so modified that it is hardly recognizable, such as grasses , whose flowers are so strange that their structures are given different botanical names.

However, if we were to look closely at the sunflower, daisy, or dandelion—or any other plant in the Composite Plant family —we would see that what is commonly thought of as a flower does not actually have the structure of one. When one of those pieces that look like petals is torn off, a female reproductive system can be seen inside. And if we look carefully at the center, we do not find stamens surrounding the gynoecium, but there are hundreds of tubular pieces, and inside them, a complete androecium surrounding the gynoecium. In reality, each of those structures is a flower .

The multiple flowers of a sunflower

In a sunflower we find two types of flowers that receive their own names. On the one hand, the flowers that form the periphery of the disc, with the shape of a large petal, which are called ligules . These have only the female reproductive system —they lack stamens— and their five petals are fused to form a very short tube that then opens, expanding like a sheet.

On the other hand, we have the flowers that cover the rest of the disk, which are called florets . These have both reproductive apparatus, and the petals form an enclosing tube, ending in five small beak-shaped lobes, one for each fused petal.

The same structure is also found in the daisy, and in most of the plants of the Compositae family. However, there are some species that only contain ligule-type flowers —but with both sexual organs—, such as the dandelion, and others that only have florets , such as the species Matricaria discoidea , a type of chamomile.

In some species, such as the protagonist of van Gogh’s painting, the calyx of the flower has disappeared. But, others like the dandelion have sepals at the base of each flower, modified in the form of hairs, which we call pappus . When it matures, this pappus remains part of the fruit and helps it to fly away with the wind , with or without the help of someone who, making a wish, blows on it.

It’s not a flower, it’s an inflorescence

An inflorescence is a plant unit that contains flowers . There are many types of inflorescences in nature. Examples are the racemes , like that of the vine, which has the flowers at the end of peduncles on a long central axis; the spikes , with the flowers on a central axis but without a peduncle, which can be simple like those of gladioli, or composed of subunits called spikelets like wheat; or the umbel , which arranges the flowers in a structure reminiscent of an umbrella, like the flowers of anise or carrot.

In the case of the sunflower and the rest of the Compositae family, the flowers also form an inflorescence, called a capitulum . In it, the stem is thickened and is called a receptacle , and is covered on the outside by a series of green leaflets called involutional bracts . On the receptacle the flowers are arranged in concentric spirals. In addition, at the base of each flower there is usually a small bract called the palea , whose variable shape can be decisive for the identification of some species.

When the flowers mature, dry fruits are formed that receive the botanical name of cypsela , which in the sunflower, would be the pipes that we eat or that we use to produce the oil.

REFERENCES:

 

Berendí González, C. et al. 2014. CLIX. Compositae. In Iberian Flora: Vol. XVI . Royal Botanical Garden, CSIC.

Pérez Morales, C. 1999. Morphology of spermatophytes . Ed. Celarayn.