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How is a 1-foliolate leaf (e.g., Hardenbergia) different from a simple leaf?

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Short answer:

Look for a pulvinus at the leaf-stalk/leaf-lamina junction.

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Long Answer:

So from a strictly anatomical identification standpoint, this can be a bit confusing.

According to "Plant Identification Terminology: An Illustrated Glossary" (Harris & Harris, 2001):

Simple: Undivided, as a leaf blade which is not separated into leaflets.

Where:

Leaflet: A division of a compound leaf

and

Compound Leaf: A leaf separated into two or more distinct leaflets.

So the definition of a compound leaf is to have "two or more distinct" leaflets.

But wait...

From plantnet:

one-foliolate (1-foliolate, unifoliolate): a compound leaf reduced to a single leaflet, usually recognized by the articulated or jointed ‘petiole’, which is in fact a petiole plus a petiolule.

So what gives?

Perhaps this is due to a deficiency in terminology, as discussed (and corrected) by Jiménez-Saa 2011.

But, again Harris & Harris (2001) hint toward how this might arise [emphasis mine]:

Unifoliolate: a leaf theoretically compound, though only expressing as a single leaflet and appearing simple.

So there is still a fundamental difference between the two.

Structurally, we can see the difference at the basal joint of the foliar structure:

Simple or unifoliolate: The leaf is undivided...with (unifoliolate) or without (simple) an articulation between the leaf lamina and leaf stalk. [source]

Put differently:

Unifoliolate leaf is usually recognized from a simple leaf by the presence of a distinct joint or a pulvinus at the lamina-petiole junction, which is in fact a petiole plus a petiolule.

  • Note: Pulvinus = a distinct enlargement/swelling at the base of (sometimes at the apex of) a petiole or of a petiolule.

Note: a discussion about the developmental evolution of unifoliolate characteristics in Cercis canadensis is discussed here.

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