Flower Parts for Plant ID

Large-flowered Trillium, Trillium grandiflorum

Thanks to the four biomes that meet here, Minnesota hosts a diversity of plant life. According to the most recent MNTaxa plant checklist, the state is home to at least 2,250 species and varieties of vascular plants (1). Of those, about 94% are flowering plants, and most guidebooks focus mostly or entirely on that group. To help identify these plants, many wildflower guides rely on flower parts – their presence or absence, their number, their appearance –to arrive at a plant's name.

It’s useful, then, to know how flowers are put together. This post introduces basic flower structure, beginning with the names of flower parts and some common variations and then introducing some terms for flower clusters, called inflorescences. 

Flower parts

In a model flower, parts are organized in four whorls. From outermost to innermost, they are sepals, petals, stamens and one or more pistils. In some flowers, pistils are composed of joined parts, called carpels (seed leaves), to form a compound pistil. In others, the pistils are composed of only one carpel, so they’re called simple pistils. The diagram below shows a simple pistil.

All the sepals together form the calyx, from a Greek word describing a husk or a case for a bud. Many flower buds are enclosed in and protected by a calyx before they open. Similarly, all the petals together form the corolla, from a Latin word meaning wreath or crown.

 

Each of the flowers below has all four parts. Sharp-lobed hepatica has flowers with multiple stamens, multiple simple pistils, five petals, and five sepals. Large-flowered trillium has three sepals, three petals, six stamens, and one compound pistil formed of three joined carpels. Three stigmas emerge from the top of the pistil.

The photo of wild geranium below shows two flowers in different stages of development. In the left flower, stamens are at peak maturity and are releasing pollen. The pistil in this flower is immature and hidden by the stamens. In the right flower, the stamens are past peak and are withering. The pistil, however, is in its prime, with five, curved stigmas ready to accept pollen.

Because the stamens and pistils mature at different times, the flower can’t pollinate itself. This difference in timing, called dichogamy (dy-COG-amee), favors cross-pollination and mixing of genes, creating more diverse -  and perhaps more successful – offspring. 

Tepals

Tepals are petals and sepals that look alike. They are especially common among plants in the lily family and its close relatives. Tulips and day lilies are two garden favorites that have tepals. Native white trout lily and blue-bead lily also have tepals.

Regular (actinomorphic) and irregular (zygomorphic) flowers

When viewed face on, regular flowers look like wheels or stars: Their parts are evenly distributed all the way around. More than one line can be drawn through the center of the flower to create similar halves. Such flowers are also called radially symmetric or actinomorphic, which means star-shaped.

In contrast, irregular flowers have only one plane of symmetry: Only one line can be drawn across their faces to produce similar halves. Irregular flowers are also called bilaterally symmetric or zygomorphic. “Zygo” is a Greek prefix meaning pair.

Complete and incomplete flowers

If flowers have all four parts – sepals, petals, stamens and pistils – they’re complete. If they’re missing one or more of these parts, they’re incomplete. Wild strawberry, for example, has complete flowers. The flowers of Canada anemone, however, have no petals and are incomplete. In that plant, sepals are the large, colorful parts that attract pollinators.

Perfect and imperfect flowers

These terms refer to the reproductive parts of a flower, the pistils and stamens. A perfect flower has both parts, whereas an imperfect flower has only one. A staminate flower has only stamens; a pistillate flower has only pistils.

If a species has imperfect flowers and the staminate and pistillate flowers are on the same plant, the species is monoecious (mon-EE-shus), meaning one house. If the species has staminate and pistillate flowers on different plants, it is dioecious (dy-EE-shus), meaning two houses. Sometimes dioecious plants are said to have separate “female” and “male” individuals.

Most flowers of silver maple, for example, are imperfect. Staminate and pistillate flowers are shown below. They may be on separate trees or on the same tree, so the plants can be dioecious or monoecious, respectively. Occasionally, a tree may also have perfect flowers.

Wind-pollinated flowers

Wind pollinated flowers don’t rely on insects to visit them, so they lack showy petals and sepals. Stamens and stigmas, however, are often numerous and obvious when the flowers mature. Silver maple, shown above, is wind pollinated, as are willows and aspens. Grasses are also wind pollinated. Two  prairie grasses, big bluestem and Indian grass, are shown below.

Inflorescences

Some plants, like tulips or roses, produce flowers singly. Others produce flowers in clusters called inflorescences.

An aster, for example, isn’t one flower but many tiny ones, all clustered on a flat, rounded, or conical receptacle. The flowers in the center are called disk flowers. Those around the edge, often bearing petal-like rays, are called ray flowers. This arrangement, called a head inflorescence, is typical of plants in the aster or sunflower family.

Diagrams and photos of a head inflorescence and other common types are below. In the diagrams, black circles represent flowers. Different sizes of circles indicate that some flowers in an inflorescence mature sooner than others. The larger the circle, the earlier it opens. If all circles are the same size, they mature at the same time.

 

Mixed inflorescences

Adding more challenge, some plants have mixed or combined types of inflorescences. Rough Blazing Star, for example, has heads arranged in a spike, and Showy Goldenrod has heads arranged in a panicle.

References

(1) MNTaxa: The State of Minnesota Vascular Plant Checklist. Minnesota Department of Natural Resources. Accessed March 30, 2022, online at https://www.dnr.state.mn.us/eco/mcbs/plant_lists.html.

(2) Minnesota Wildflowers: A field guide to the flora of Minnesota. Online at minnesotawildflowers.info.

Leaf Morphology for Plant ID

 

The last two posts introduced leaf arrangement and leaf divisions, two features used in many guides to help identify plants. This post goes deeper into the weeds to introduce leaf morphology – the shape and structure of leaf blades, stalks and edges.

Leaves are tremendously variable in shape and structure, so there are many terms to describe them. It’s impractical – and overwhelming – to cover all of them here, so this post introduces only those that are commonly used in technical keys. To learn more, see the resources at the end of the post.

Leaf parts

Before diving into morphology, it’s helpful to know the names of leaf parts. They are marked below on the simple leaf of Common Lilac, Syringa vulgaris. Most of the terms also apply to the leaflets of a compound leaf. (See the previous post for a tutorial on simple and compound leaves.)

Leaf apices

Dozens of terms describe leaf apices, but four are especially common: acute, acuminate, mucronate and obtuse. 

Leaf bases

Leaf bases are also diverse. Common terms for their shapes are acute, obtuse, oblique, cordate, truncate and sagittate. 

Leaf margins

Leaf margins that are continuous – not toothed, notched or lobed– are called entire. Margins that aren’t entire are variously shaped and have several terms to describe them, including dentate, serrate, crenate, undulate and lobed.  

Leaf surfaces

Leaves are surprisingly diverse in surface texture. If the surfaces are smooth, they’re called glabrous. If they have a white or bluish, waxy coating that can be rubbed off, they’re glaucous. Hairy leaves have many terms to describe them, but a common overall term is pubescent.

 

Leaf attachments

Petiolate leaves are attached to a stem with a petiole, or leaf stalk. Sessile leaves lack petioles; they are unstalked and attached directly to the stems. Perfoliate leaves wrap around and are pierced by the stem. Clasping leaves, as the name implies, clasp the stem with the base of the leaf. Sheathing leaves wrap around the stem and extend down its length to form a sheath. 

Stipules

Stipules are pairs of leaf-like or thread-like appendages at the base of the petioles of some leaves. Not all species have them, but if stipules are present, their size and shape are useful for identification.

 

Leaf shapes

This is where terminology really takes off. Because leaves come in a wide variety of shapes, there are many words to describe them. Common terms are cordate, deltoid, elliptic, lanceolate, oblong and ovate. 

Adding “ob” to the beginning of cordate, lanceolate or ovate means the shape is reversed. An obcordate leaf, for example, looks upside down compared to a cordate leaf.

Expect inconsistency, intermediates and combinations

Leaves are variable even on the same plant. In the folowing photo of Japanese Lilac, Syringa reticulata, the bases of older leaves look truncate or obtuse, whereas the bases of younger leaves look acute. It’s best to look at several leaves to get a sense of what’s typical.  

 

In some cases, leaves look intermediate between two morphologies. It’s common to find combination terms for their shapes, like ovate-elliptic or lanceolate-ovate. For example, the leaves of Swamp Milkweed, Asclepias incarnata, below, are described as oblong-lanceolate or lanceolate, with an acute to acuminate apex. 

 

Test your knowledge

Here’s a description that could be found in a technical guide: Leaves petiolate, lobed, margins coarsely serrate to dentate, blades glabrous, base broadly cordate, apex acute to acuminate. Which of these leaves best matches this description? Scroll down for the answer.

The answer is Riverbank Grape. Wild Ginger has entire leaves that are not lobed, although the leaf base makes them appear so. Tall Bellflower has an acute leaf base and a margin that is not coarsely toothed.

 

More resources

Plant Identification Terminology: An Illustrated Glossary, by James G. Harris and Melinda Woolf Harris. Second Edition. Spring Lake Publishing, Spring Lake, Utah, 2001. 206 pp.

Botany Primer: Understanding Botany for Nature’s Notebook. This public-domain primer from the USA National Phenology Network covers many aspects of botany, including leaf morphology. The full citation for this reference is:

Guertin, P., Barnett, L., Denny, E.G., Schaffer, S.N. 2015. USA National Phenology Network Botany Primer. USA-NPN Education and Engagement Series 2015-001. www.usanpn.org.

Biology and botany textbooks also cover plant morphology. School and public libraries may have some on their shelves. Another choice is LibreTexts™, a non-profit collaboration that offers free online access to postsecondary textbooks. At the website, open the Explore the Libraries menu and choose Biology. Then choose Bookshelves and look for Botany.

References

(1)    Minnesota Wildflowers: A Field Guide to the Flora of Minnesota. Maintained by Katy Chayka. Accessed March 12, 2022. 

(2)    Minnesota Biodiversity Atlas. University of Minnesota, Bell Museum. Accessed March 12, 2022.