Sunflowers

Everyone knows sunflowers (Helianthus annuus). Giant yellow flowers that look like the sun, painted by Van Gogh, and grown in huge fields, ripe for photographic potential.

Sunflowers are part of the daisy (Asteraceae) family, and they are a composite flower, meaning that each ‘flower’ is actually made up of thousands of tiny flowers. The ones in the centre are called disc florets and are little tubular flowers with insignificant petals. The ones around the outside are ray florets and are asymmetric, with the giant petal on one side. Each of these little florets can be pollinated and produces a sunflower seed.

While they have the shell on, sunflower ‘seeds’ are actually ‘fruit’ (botanically speaking, a fruit is the structure of a flowering plant that encloses the seed). Some sunflower varieties typically produce heaps of seeds/fruit and some (usually those that are bred to be more ornamental) produce less. There are two types of seeds: black and striped. The black seeds or ‘oil seeds’ have a thinner shell and are preferred by birds because they’re easier to crack open and have a higher oil content. The striped ones are generally sold for human consumption - they keep longer because their oil content isn’t as high and they are also a little bigger, making it easier for us to shell. (Alternatively you can buy the small grey ones which have already been shelled for us!)

Sunflowers demonstrate ‘tropism’ which is the movement of a plant in response to an external stimulant. For sunflowers, it’s called ‘heliotropism’ as they move in response to the sun. In actual fact, only the immature sunflower heads that track the sun through the sky. This is caused by the uneven elongation/growth of the stem which grows more on the eastern side during the day (turning the head west) and then evens out over night (resulting in the head returning to anticipate dawn again). Once the flower has matured though, they stay facing East!