Biology · anatomy
Anatomy of the sunflower head
From involucre to achene: the name of each part, the difference between ray and disc florets, and the precise place where the seed forms.
In short: From outside in, the head is built from the involucre (phyllaries), the receptacle, a ring of sterile ray florets and hundreds of fertile disc florets. The seed — botanically an achene — forms at the base of each disc floret, from the ovary after pollination.
The parts, from outside in
The sunflower head is an inflorescence, not a single flower, and so it has a layered build. At the base sits the receptacle: a widened, fleshy disc into which all the florets are set. Around it, green bracts called phyllaries together form the involucre, which protects the bud and stays as a green collar around the ripening head. The Naturalis Biodiversity Center describes this build as characteristic of the whole composite family.
On the disc stand two floret types. At the rim the ray florets with their large yellow strap-shaped corolla; inward, the densely packed disc florets. Between the disc florets sit small, stiff scales (palea) that support each flower and hold the ripening seeds in place. For the wider context — why this is a false flower or pseudanthium — see the page on the biology of the sunflower.
Ray floret and disc floret: what is the difference?
The yellow "petals" are not petals but complete flowers. Each ray floret has a corolla fused into a flat strap and in the sunflower is almost always sterile: it sets no seed and serves as a signal to pollinators. The disc florets in the centre are small, five-parted and bisexual — that is where the whole reproductive machinery sits. Royal Botanic Gardens, Kew describes this division of labour as typical of Helianthus annuus: the rim attracts, the centre produces.
Within a disc floret sit five stamens whose anthers are fused into a small tube (the stamen tube). The style grows up through it and pushes the pollen out, after which the stigma splits into two lobes to catch foreign pollen. Because the male part matures before the female, self-pollination within a single floret is limited. The spiral arrangement of all those disc florets is covered on the page about Fibonacci.
Where does the seed come from?
What we loosely call a "sunflower seed" is botanically not a seed but a fruit: an achene. At the base of each disc floret sits one ovary with a single ovule. When pollen lands on the stigma, a pollen tube grows down to the ovule and fertilisation occurs. The ovule becomes the seed; the ovary wall hardens into the striped shell around it. One disc floret thus yields exactly one kernel.
Because a large head holds hundreds to more than two thousand disc florets, it yields that many kernels. Ripening runs from the outside in, in the same wave in which the disc florets opened. Which heads give the most and largest seed depends strongly on cultivar and cultivation; that is covered on the pages about species and cultivars and growing sunflowers. How the genome behind seed traits such as oil content works is on the genetics page.
Not a petal, but a flower
A handy rule of thumb: count the yellow "petals" of a sunflower and you count flowers, not petals. Each strap is one sterile ray floret. The real work happens in the brown centre.
Sources
- Naturalis Biodiversity Center — taxonomic and morphological description of Asteraceae and Helianthus annuus (accessed 2026).
- Royal Botanic Gardens, Kew — Plants of the World Online, Helianthus annuus (accessed 2026).