Flower Anatomy
From outside to inside — the four whorls
A complete flower contains four whorls of organs attached to the receptacle (the swollen tip of the flower stalk/pedicel). From outside to inside: sepals (calyx), petals (corolla), stamens (androecium — male reproductive organs), and pistil (gynoecium — female reproductive organ). An incomplete flower is missing one or more whorls. A perfect flower has both stamens and pistil; an imperfect flower has only one.
The mnemonic SPSP (Sepals, Petals, Stamens, Pistil) or 'Some Professors Seem Pompous' helps remember the order from outer to inner whorl.
💡 Double Fertilization — Unique to Angiosperms
Double fertilization is the defining feature of angiosperm reproduction — it occurs in no other plant group:
1. Pollen lands on stigma → pollen tube germinates → grows through the style into the ovary → reaches the ovule → enters through the micropyle.
2. The generative cell has divided into TWO sperm cells during pollen tube growth.
3. First fertilization: Sperm 1 + egg cell → zygote (2n) → develops into the embryo.
4. Second fertilization: Sperm 2 + two polar nuclei → endosperm nucleus (3n) → develops into the endosperm (nutritive tissue that feeds the embryo and, in many plants, the seedling after germination).
The endosperm is 3n (triploid) because it results from the fusion of one sperm (n) with two polar nuclei (n + n). This is why corn kernels, rice grains, and wheat endosperm are triploid tissue — the endosperm we eat in most grains.
S
Sepals and Petals — protection and attraction
Sepals (collectively the calyx) are the outermost whorl, usually green and leaf-like. They enclose and protect the flower bud before it opens. In some species, sepals are colorful and petal-like (petaloid sepals) — as in lilies and tulips where the 'petals' you see are actually sepals and petals that look identical (collectively called tepals).
Petals (collectively the corolla) are the usually colorful, conspicuous organs whose primary function is to attract pollinators. Petal characteristics are strongly shaped by the primary pollinator: bee-pollinated flowers are often blue or yellow (bees see UV) with landing platforms; bird-pollinated flowers (hummingbirds) are often red and tubular; moth-pollinated flowers are often white and fragrant (open at night); bat-pollinated flowers are often large, dull-colored, and night-fragrant.
Wind-pollinated flowers (grasses, oaks, corn) have reduced or absent petals — no need to attract animal pollinators.
Memory trick: Sepals = protective outer layer (usually green). Petals = colorful attractors for pollinators. Tepals = sepals + petals look identical (lilies, tulips). Wind-pollinated = no showy petals needed.
St
Stamens — the male reproductive organs
Stamens (collectively the androecium) are the male reproductive organs. Each stamen consists of two parts: the filament (a slender stalk) and the anther (the pollen-producing structure at the tip). The anther contains four microsporangia (pollen sacs) in two pairs. Inside each microsporangium, microspore mother cells undergo meiosis → microspores → each microspore develops into a pollen grain (the male gametophyte).
A mature pollen grain consists of two cells: the generative cell (which will divide to produce two sperm cells) and the tube cell (which produces the pollen tube). The pollen wall (exine) is made of sporopollenin — one of the most resistant biological materials known, surviving millions of years in sediments (basis of palynology — pollen analysis to reconstruct past climates and vegetation).
Memory trick: Stamen = male. Filament + Anther (pollen sacs). Anther makes pollen. Pollen = male gametophyte. Two cells inside pollen: generative cell (becomes 2 sperm) + tube cell (grows the tube).
P
Pistil — the female reproductive organ
The pistil (gynoecium) is the female reproductive organ, located at the center of the flower. It consists of one or more carpels (fused or separate). A single carpel or fused carpels form a pistil with three parts: stigma (the sticky receptive surface at the top, where pollen lands and germinates), style (the stalk connecting stigma to ovary), and ovary (the enlarged base containing one or more ovules).
Inside each ovule, a megaspore mother cell undergoes meiosis → one surviving megaspore → develops into the female gametophyte (embryo sac). The mature embryo sac contains 7 cells with 8 nuclei, including: the egg cell (at the micropyle end), two synergid cells flanking the egg, three antipodal cells (at the opposite end), and the large central cell with two polar nuclei.
After fertilization: the ovule develops into a seed; the ovary wall develops into the fruit (pericarp).
Memory trick: Pistil = female = Stigma + Style + Ovary (SSO). Stigma = landing pad for pollen. Ovary = becomes the fruit. Ovule = becomes the seed. Egg cell + 2 polar nuclei are the key cells inside.
🔬 Applied Scenario — Pollination Biology and Coevolution
Flower structure is the product of coevolution with specific pollinators:
A
Bee pollination syndrome. Bee-pollinated flowers (most flowering plants) show a syndrome of traits: often blue, yellow, or purple (bees see UV and blue, not red); landing platform for bees; nectar guides (UV-absorbing patterns invisible to humans but visible to bees); sweet or spicy scent; open during the day. Bees are the most important pollinators globally — approximately 75% of flowering plant species and 35% of food crop production depends on bee pollination.
B
Wind pollination — no petals needed. Wind-pollinated plants (grasses, oaks, birches, corn) produce vast quantities of small, smooth, dry pollen (designed to remain airborne). They have reduced or absent petals (no need to attract animals), large feathery stigmas (to capture airborne pollen), and often flower before leaves emerge (leaves would block pollen dispersal). Wind pollination is energetically costly in pollen production but cheap per pollination event — no energy invested in nectar or showy petals.
C
Darwin's orchid and long-tongued moth. Angraecum sesquipedale orchid from Madagascar has a nectar spur 30 cm deep. Darwin predicted in 1862 that a pollinator with a 30-cm proboscis must exist. The hawk moth Xanthopan morganii praedicta (with 'praedicta' acknowledging Darwin's prediction) was discovered in 1903. This is the textbook example of coevolution between flower structure and pollinator morphology.
D
Fruit development after fertilization. After double fertilization, the flower transforms: petals, stamens, and style wither; the ovary wall develops into the pericarp (fruit wall); the ovule develops into the seed. The fruit type reflects the ovary structure: simple fruits from a single ovary (apple, tomato, peach); aggregate fruits from multiple carpels of one flower (strawberry, raspberry); multiple fruits from multiple flowers (pineapple, mulberry). Botanically, a tomato is a fruit (derived from the ovary) and a strawberry is an accessory fruit (the fleshy part is the receptacle, not the ovary).
📌 Exam Application
1. Four whorls (outside to inside): Sepals → Petals → Stamens → Pistil. Complete flower has all four. Perfect flower has both stamens and pistil.
2. Stamen: Filament + anther. Anther produces pollen (male gametophyte). Two cells in pollen: generative (→ 2 sperm) + tube cell.
3. Pistil: Stigma + Style + Ovary. Ovary → fruit. Ovule → seed. Embryo sac (female gametophyte): egg + 2 synergids + 3 antipodals + central cell with 2 polar nuclei.
4. Double fertilization: Sperm 1 + egg → zygote (2n) → embryo. Sperm 2 + 2 polar nuclei → endosperm (3n). Unique to angiosperms.
5. Pollination syndromes: Bee = blue/yellow/UV. Bird = red/tubular. Wind = no petals, dry smooth pollen, feathery stigma.
⚠️ Most Common Flower Anatomy Mistakes
Ovary → fruit; ovule → seed. Students confuse which structure becomes what. The ovary wall (pericarp) becomes the fruit. The ovule inside the ovary becomes the seed. A tomato's fleshy part = pericarp (from ovary wall). The seeds inside = from ovules. 'Ovary Outside becomes fruit; Ovule becomes seed.'
Endosperm is 3n (triploid) — not 2n. The endosperm forms from the fusion of one sperm (n) with two polar nuclei (n + n) = 3n total. The embryo is 2n (from egg + sperm). The endosperm is 3n. This is why the second fertilization in double fertilization produces a triploid tissue, not a diploid one.
Tepals = when sepals and petals look identical. In lilies and tulips, the three 'petals' and three sepals look alike — they are all called tepals. Students sometimes think all flowers have clearly distinct green sepals and colored petals. Many monocots have identical sepals and petals (tepals). This distinction matters in plant identification.
✓ Quick Self-Test
1. What are the four whorls of a complete flower from outside to inside?
2. What are the parts of the stamen and what does each do?
3. What are the three parts of the pistil and what does each become after fertilization?
4. What is double fertilization and why is it unique to angiosperms?
5. How does flower structure reflect pollinator type?
Answers:
1. From outside to inside: Sepals (calyx — protective, usually green), Petals (corolla — colorful, attract pollinators), Stamens (androecium — male reproductive organs, produce pollen), Pistil (gynoecium — female reproductive organ, contains ovules).
2. Each stamen has a filament (slender stalk) and an anther (pollen-producing structure at the tip). The anther contains four microsporangia (pollen sacs) where meiosis occurs to produce microspores, each of which develops into a pollen grain (male gametophyte) containing the generative cell (divides to form two sperm) and the tube cell (forms the pollen tube).
3. The pistil has three parts: stigma (sticky surface at the top where pollen lands and germinates → becomes nothing), style (stalk connecting stigma to ovary → withers after fertilization), and ovary (enlarged base containing ovules → develops into the fruit). Inside the ovary, each ovule develops into a seed after fertilization.
4. Double fertilization is the fusion of two sperm cells with two different targets in the same ovule. Sperm 1 fuses with the egg cell → zygote (2n) → develops into the embryo. Sperm 2 fuses with the two polar nuclei in the central cell → endosperm nucleus (3n) → develops into the nutritive endosperm. This process is unique to angiosperms (flowering plants) — no other plant group has double fertilization.
5. Flower structure coevolves with its pollinator: bee-pollinated flowers are often blue/yellow, have landing platforms, and produce UV nectar guides; hummingbird-pollinated flowers are red and tubular (hummingbirds see red, have long beaks); moth-pollinated flowers are white and fragrant at night; bat-pollinated flowers are large and night-fragrant; wind-pollinated flowers have reduced petals, produce dry smooth pollen in large quantities, and have large feathery stigmas.