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Certain plant families generally have a specific number of nominal petals, be it 2, 3, 4, 5, 6, 7, 8, 9, 10 or 12. But just like the 3-leaved clovers can sometimes have 4 leaves, flowers too can have variable number of petals. Thus nominally 6-petalled flowers can sometimes have 5 or 7 or 8, maybe even 9 petals. The Stonecrop Family perhaps exhibits the family with the greatest variation, with some members having 4, 5, 6, 7 or 12 nominal petals, but most Stonecrops have but 5 petals.
NO PETALS
There are some flowers without petals. For example, those belonging to the Pondweed Family, which have no petals but 4 cupped green sepals, which look like petals, but are not. Plants like these are being categorised as if they have petals, however many it may seem. Sea Buckthorn has tiny petal-less green flowers. There are other plants lacking flowers, for example Ferns. Sedges may well have edges but one thing the do lack is petals.
ONE PETAL - COWL
These flowers may have the appearance of having only one petal, but looked at closely enough, it may have an opening which is slightly cut, such as the five triangular teeth at the end of the bell of Cowberry, or it may have a curving 'snout' as in Common Broomrape which has one lip at the opening. It may otherwise have a curving, slightly twisted cowl, like tat of Lords-and-Ladies, which is not a petal at all, but the plants overall appearance is that of having one 'petal'.
It may appear that those flowers with a trumpet-shaped flower (such as Daffodils and Bindweeds) also have just one petal. But in the case of Bindweeds; they have 5 fused petals (there is a crease between the petals and they may also have 5 markings corresponding with the 5 creased petals) and in the case of Daffodils it is 6 fused petals.
TWO PETALS
The aquatic Cape Pondweed seems to be highly unusual in having but two petals. There are extremely few flowers with only two petals arranged openly at 180 degrees.
The aquatic Bladderworts have but two-lipped flowers.
There are also a great many flowers belonging to the mint family which have vertically symmetric two-lipped flowers, flowers such as Red Dead-nettle. Also many saprophytes such as Red Bartsia and orchids have two vertically-symmetric lips.
Many of the zygomorphic plants such as Toadflaxes, Claries, and Mint family plants might also appear at first glance to only have 2 petals, but closer examination will reveal that those petals have lobes summing to 5, and thus 5 petals.
THREE PETALS
It seems that many plants with three petals (but not all) are aquatic. Furthermore no plant that is not aquatic has three petals. Among those aquatic plants that are three-petalled are Flowering-rush, the Water-plantains, the Frogbit Family, the Waterweed Family, and of course Yellow Iris has three very large petals.
A major exception are flowers of the Pondweed Family, which have no petals but 4 cupped green sepals, which look like petals, and New-Zealand Pigmyweed which anomalously has 4 petals and yet belongs to the Stonecrop Family which usually have 5 petals (but some stonecrops have 6 or 7 petals petals).
The aquatic Bladderworts have but two-lipped flowers.
The aquatic Cape Pondweed seems to be highly unusual in having but two petals. There are extremely few flowers with only two petals arranged openly at 180 degrees.
Marsh Cinquefoil, which likes a watery habitat, also has 3 petals (intersected by 3 bracts).
Both 3-petalled and 6-petalled flowers are Monocots
(Monocotyledons
) [EXPAND ON THIS]
It is entirely possible that the three-petalled aquatic flowers represent and are derived from a much earlier epoch of the Earths history when conditions were anoxic (without oxygen). Slowly these plants have evolved to tolerate or even thrive with the upper parts and leaves in an atmosphere containing oxygen, but whose roots still need to exclude oxygen. (Oxygen is removed from muddy organic sediments by other organisms). Ordinary soil does contain some oxygen. Most plants need to have at least some air getting to the roots, and dislike being water-logged. Not so with aquatic plants which positively thrive with roots in oxygen-free conditions.
FOUR PETALS
A large number of plant families have 4 petalled flowers: The Brassicaceae [which includes Wintercresses, Yellow-cresses, Rockets, Rapes, Turnips, Cabbages, Mustards, Watercresses (an aquatic example which does not have 3 petals), Thale Cress, Bittercresses, Rock-cresses, the Scurvygrasses, Pepperworts, Cuckooflowers, Honesty, Stocks and Whitlow-grasses], the Poppy family Papaveraceae, The Plantain family Plantaginaceae (which now includes Speedwells), the Teasel Family Dipsaceae which includes the Scabious genus, the Mezereon family Thymelaeaceae which includes Spurge-laurel, the Bedstraw family Rubiaceae, Tormentil and the Burnets (two 4-petalled anomalies in the normally 5-petalled Rose Family Roseaceae, Traveller's Joy (another 4-petalled anomaly in the normally 5-petalled Buttercup family Ranunculaceae), Figworts and Butterfly-Bushes in the Figwort family Scrophulariaceae and Spindle
FIVE PETALS
Five is a rather strange number. It could be argued that pentagons have less symmetry than do squares, hexagons and octagons. Pentagonal symmetry does not occur readily in the natural world; for many hundreds of years it was thought impossible that any mineral crystal would be found with pentagonal symmetry, simply because unlike triangles, squares and hexagons they do not pack together in an orderly manner. But a few minerals are now known with pentagonal symmetry. They are very rare, and they possess a random structure rather than being in a regular repetitive array. Not so with petals however; possessing five is a very popular number, for they do not need to pack together orderly.
But some with five petals do have one peculiar property. Many flowers (but not all) with five petals have asymmetrically shaped petals, rather like propeller blades where the leading edge is of a different shape to the trailing edge. The Periwinkle Family are such flowers, another is some members of the Mallow Family. Soapwort and Bouncing Bett also exhibit this property which seems to be peculiarly restricted to those having 5 petals at 72 degree intervals around a circle. However, not all those which have five petals separated at 72 degrees have this asymmetric characteristic. Some are totally symmetric.
Of those that are totally and rotationally symmetric at 72 degrees (1/5 of a circle), members of the Rose, Borage, Bellflower, Flax, Wood-Sorrel, Crane's-bill, Buttercup, Pink & Carnation, Saxifrage, Primrose, Rock-Rose, Nightshade, Thrift, Wintergreen, and St John's Wort Families are prominent. Plus some, but not all, members of the Stonecrop, Figwort and Gentian Families. Members of the Bindweed Family, although having only a single trumpet-shaped flower, have 5-fold symmetry markings on the flowers. Minor Families with five petals are the Bogbean, Jacob's-Ladder and Verbena Families, with the Mulleins.
But there are some with five petals, although symmetrical about a centre line, are not rotationally symmetric. The Violet, Bladderwort and Balsam Families. Some members of the Dead-Nettle Family appears to be 5-petalled but some are joined and reminiscent of orchids. Toadflaxes, Valerians and Monkey Flowers also appear to have 5 petals, but symmetric about a centre line (that is, they are zygomorphic).
SIX PETALS
Both 6-petalled (and 3-petalled) flowers are Monocots
(Monocotyledons
). Many of the flowers with what look like 6 petals actually have an inner and outer row of 'petals', an inner circle of 3 petals and an outer circle of 3 (but not of petals, rather of petal lookee-likees called tepals) which can take the place of sepals. The petals and tepals are often the same colour as each other.
Flowers with 3 petals and 3 tepals include : Cape Lily
, Snowdrops (Galanthus species), Snowflake
s (Leucojum species), Starflower
s (tristagma species), species, Iris
's, Montbretias and other Crocosmia species, Solomon's Seals (Polygonatum species), Star-of-Betlehem
s (Ornithogalum species) and Rushes of the Juncus and Lazulus genera.
Flower families possessing 6 actual petals are also quite numerous: Winter Aconite and Anemones such as Poppy Anemone), Purple Loosestrife (5 to 7 petals),
6 Tepalled flowers (it is quite possible that many of these actually have 3 sepals and 3 tepals, particularly any in the Alliaceae family): Bog Asphodel, Bluebell and Hyacinthoides species, Blue-eyed-Grass and Sysyrinchium species, Black Bryony, African Lily and other Agapanthus species, Ramsons and other Allium (Garlic
s and Leeks etc) species, Daffodil
s (Narcissus species), Crocus
species, Day-lilies (Hemerocaulis species), Red-hot-poker
s (Knipholia species), New-Zealand Flaxes (Phormium species) and Wild Mignonette.
SEVEN PETALS
For plants with the higher numbers of petals more variation in number of petals results. For instance, those which usually have 7 petals, there could be some specimens with 6 petals or 8 petals. Both Yellow-wort and Broad-Leaved Ragwort usually have 7 flowers, but some specimens may have 6 or 8 petals.
EIGHT PETALS
Flowers with normally 8 petals include Mountain Avens, Lesser Calendine
where the median is 8 flowers but it can have between 7 to 12 inclusive. Canadian Goldenrod has about 8 petals but it can have 7 to 11. Sneezewort can have 5 to 12 petals but the dedian is 8.
NINE PETALS
Groundsel more likely has 9 petals but it can range from 7 to 11.
TEN PETALS
Field Fleawort 10±2, Hop actually does have 10 petals as does Passionflower
(in two overlapping circles of 5 + 5).
TWELVE PETALS
Houseleek is the only member of the Stonecrop Family to have 12 petals; most Stonecrops have but 5.
MANY
This is dominated by the Asteraceae family (Dandelion & Daisy) but even here some Asteraceae species have as few as 5 petals, such as Leopardplant and Gallant Soldier
CLEFT and DEEPLY CLEFT
If the petals are deeply cleft, to over half-way, then the flower can appear to have twice as many petals than what it does. An example is Bog Stitchwort, which appears to have 10 petals, but it really only has 5 deeply-cleft petals. In Ragged-Robin each petal is cleft twice, giving the appearance of three times as many petals.
