Sunday, March 13, 2022

Leaf Morphology for Plant ID

A panel of three leaves: quaking aspen, wild rose, and false Solomon's seal.
 









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.)

An illustration of leaf parts, pointing out the blade, apex, base, veins and petiole.



Leaf apices

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

A panel of four leaves showing acute, acuminate, mucronate and obtuse apices.


Leaf bases

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

A panel of three leaves showing acute, obtuse and oblique bases.












A panel of three leaves showing cordate, truncate and hastate bases.













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.  

A panel of three leaves showing entire, serrate and dentate margins.


A panel of three leaves showing crenate, undulate and lobed margins.


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.

A panel of three leaves showing glabrous, glaucous and pubescent surfaces.
 














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. 

A panel of three leaves showing petiolate, sessile and perfoliate attachments.










A panel of two leaves showing clasping and sheathing attachments.











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.

 The leaf-like stipules of smooth wild rose and the thread-like stipules of sweet clover.


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. 

Cordate leaf of lilac, deltoid leaf of cottonwood, and elliptic leaf of black cherry.

Lanceolate leaf of coneflower, oblong leaflets of prairie clover, and ovate leaf of snakeroot.












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.

Oblanceolate leaf of black chokeberry, obcordate leaves of yellow oxalis, and obovate leaf of chokecherry.


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.  

A branch of Japanese lilac.

 









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. 

A stem and attached leaves of swamp milkweed.

 












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.

Leaves of wild ginger, riverbank grape and tall bellflower.

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. 

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