8.2: Flower Morphology

Flowers are sets of highly modified leaves that function to attract a pollinator or, if no animal pollinator is used, to optimize spore dispersal in some way. Over the course of evolutionary history and coevolution, this has lead to an incredible diversity of shape, size, color, smell, and just about any other characteristic you can think of. Because most plants are angiosperms and because flowers are often so diverse, learning the terminology to describe flowers is a major step in learning to identify plants.

Floral Symmetry

Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic. Flowers with only a single line of symmetry (like you) are bilaterally symmetrical, also called zygomorphic.

Figure \(\PageIndex{1}\): On the left is a picture of a radially symmetrical flower. It has five petals that all radiate out from a central point, like a star. We can draw many lines of symmetry through this flower, where if we folded the flower along that line, the two sides would line up with each other. (Note: There may be a line out of place here. Can you find it?) On the right is a bilaterally symmetrical flower. Only one line of symmetry can be drawn through this flower. Much like for humans, this flower is mirrored from side to side, but not top to bottom or any other direction. Diagram by Nikki Harris, CC BY-NC.

Radial Symmetry (Actinomorphic)

Figure \(\PageIndex{2}\): These are two duplicate images of the same flower. This Trillium flower has three lines of symmetry: one that travels from the tip of each petal down between the remaining two petals. The lines are drawn in on the picture on the right so you can compare. Photos by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{3}\): Just as with the set of images above, these are two duplicate images of the same flower and the image on the right has lines of symmetry drawn in. This salmonberry flower has five lines of symmetry, though only three have been drawn in. Can you find the other two? Photos by Maria Morrow, CC BY-NC.

Bilateral Symmetry (Zygomorphic)

Figure \(\PageIndex{4}\): Again we have two duplicate images, but this one includes several florets. This cluster of bleeding hearts has two florets that each have a line of symmetry added in the image on the right. On each heart-shaped floret, the line travels from top to bottom, dividing it into two equal halves. There are no other lines of symmetry we could draw. Photos by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{5}\): Again we have two duplicate images with the image on the right showing a line of symmetry drawn in. This violet has one larger petal on the bottom with two smaller petals winging out to either side. We can draw one line of symmetry through the center of the larger, lower petal and between the 4 smaller petals (as shown on the right). Due to the presence of the single, larger petal, there are no other lines of symmetry we could draw. Photos by Maria Morrow, CC BY-NC.

Whorls

Most flowers are composed of four whorls. If all whorls are present, a flower is said to be both complete and perfect. If any whorl is missing, the flower is incomplete. If one of those missing whorls is either the androecium (pollen-producing) or gynoecium (seed-producing), the flower is also imperfect.

Figure \(\PageIndex{6}\): This wild ginger flower (Asarum caudatum) has no corolla. It is incomplete, but it is still perfect. The hairy sepals are fused together at the base, then split into long, pointed tips that extend out like tendrils. These flowers are normally a deep wine red. Photo by mhays, CC-BY-NC.

Figure \(\PageIndex{7}\): This Sirdavidia solannona flower has no gynoecium, only tightly packed stamens at the center (a staminate flower). This flower is both imperfect and incomplete. Photo by Thomas L.P. Couvreur (Institut de Recherche pour le Développement , Montpellier, France), CC BY 4.0, via Wikimedia Commons.

Ovary Position

We can use the location of the ovary to further distinguish between flowers. If the other whorls of the flower meet below the ovary (the ovary or ovaries look a bit like an egg or eggs in a nest), the ovary is superior (on top of the rest of the flower). This means that the rest of the flower parts are below the gynoecium, so we can also call this flower hypogynous (below the gynoecium). The two terms both describe the same situation, but superior refers only to the ovary while hypogynous refers to the flower, in general.

Superior Ovary (Hypogynous Flowers)

Figure \(\PageIndex{8}\): A drawing of a hypogynous flower. The calyx, corolla, and androecium fuse to the receptacle below the gynoecium. The ovary sits on top of these other whorls, a superior ovary. Image by Gustav Hegi (1876–1932), Gustav Dunzinger, Public domain, via Wikimedia Commons.

Figure \(\PageIndex{9}\): In the image above, we see two flowers (actually two florets of an underground umbel!) of a fetid adderstongue, Scoliopus bigelovii. The one on the left has shed all whorls except the gynoecium. This allows you to see what the gynoecium looks like and where it is positioned in the one on the right. In the complete flower, the sepals, petals, and stamens all meet below the ovary, which sits in the middle like one giant egg in an alien nest. Photo by Maria Morrow, CC BY-NC.

Inferior Ovary (Epigynous Flowers)

In the opposite situation, the other floral whorls join at a point above the ovary. In this case, the ovary is inferior and the flower is epigynous (on top of the gynoecium).

Figure \(\PageIndex{10}\): A drawing of an epiogynous flower. The calyx, corolla, and androecium fuse together above the gynoecium. The ovary sits below these other whorls, an inferior ovary. Image by Gustav Hegi (1876–1932), Gustav Dunzinger, Public domain, via Wikimedia Commons.

Figure \(\PageIndex{11}\): In this flower, the floral whorls are situated above the ovary, which is the elongated, green tube that contains the ovules. Photo by Melissa Ha, CC BY-NC.

Semi-inferior Ovary (Perigynous Flowers)

As always, there are less clear situations. In some flowers, as in the rose family, the floral whorls join together and fuse at a point above the ovary, then travel down, around, and below the ovary as a fused unit. This fused unit is called a hypanthium. The ovary is termed semi-inferior, as it is located below the unfused parts of the floral whorls. Because the floral whorls travel around the ovary as the hypanthium, the flower is perigynous (peri- meaning around).

Figure \(\PageIndex{12}\): A drawing of a perigynous flower. The calyx, corolla, and androecium fuse together to form a layer of tissue (hypanthium) that surrounds and fuses with the ovary. The ovary appears to sit below the other whorls, but is surrounded by the hypanthium (fused or not), a semi-inferior ovary. Image by Gustav Hegi (1876–1932), Gustav Dunzinger, Public domain, via Wikimedia Commons.

Figure \(\PageIndex{13}\): In this flower, the floral whorls join together where the petals and sepals seem to end. They then form a long, tubular hypanthium that looks much like an inferior ovary from the exterior. However, you can see the actually ovary (filled with ovules) is located only at the base of this structure. Photo by Melissa Ha, CC BY-NC.