Mysteries of Cotyledon Leaves

Over the last year or two, I’ve found a new challenge to amuse myself—learning to identify plants by their cotyledon leaves. Dicots get their name from the two cotyledon leaves that emerge from a newly germinated seed. This first pair of leaves often, if not usually, bears no resemblance to the regular leaves found later on the plant. Most likely, the form of these leaves is defined more by the shape or some other characteristic of the seed. Typically, they are more or less oval. Sometimes they have distinct petioles, sometimes they are sessile. One would be hard pressed to identify these unless they were growing in quantity under the mother plant. Some plants, however, have very unusual cotyledon leaves. These really pique my curiosity.

Lotus micranthus

The compound true leaves of two tiny Lotus micranthus solve the puzzle of the red-striped cotyledons

Oemleria cerasiformis?

Might these waterlily-like cotyledons belong to Oemleria cerasiformis? Note also the tiny Nemophila parviflora with its cotyledon leaves still attached.

A couple of years ago, while out on Heckletooth Mountain just east of Oakridge in the fall, some little cotyledon leaves caught my eye. While the shape of the leaf was fairly generic, each one had a distinct red stripe starting at the base and going halfway up the leaf. What could they be? There were plenty of them, and I surmised they were one of many common annuals growing on this low elevation mountain. I brought one home and put it in a pot, hoping to find my answer in the spring. Naturally, I forgot about it over the winter. The following spring, however, there it was again on Tire Mountain, this time with the first true leaf appearing. It was Lotus micranthus, a little annual member of the clover family that is abundant on many of the lower elevation mountains in the area. What a surprise! I checked my pot when I got home, and sure enough, there was a tiny Lotus micranthus. Since then I’ve seen it several times with both the red-striped cotyledon leaves and the first tiny compound true leaves. The stems are red, so perhaps this has something to do with the unusual red stripe on the cotyledons.

Phlox diffusa seedlings

The cotyledon leaves of Phlox diffusa seedlings are nothing like the needle-like true leaves.

Geranium robertianum

A large clump of seedlings of the pernicious weed Geranium robertianum show the notched cotyledons typical of geraniums

This spring, I’ve been noticing another unusual cotyledon leaf. It is quite round, with prominent basal lobes. The true leaves are appearing now, and my best guess from the shape and quality of these new leaves is that they are osoberry (Oemleria cerasiformis). Interestly, they are scattered about my property, not near any Oemleria or anything else in particular. If it is osoberry, this would make sense, as the seeds are probably dropped at random by birds, rather than falling below a mature plant. I’ll have to keep an eye on a few of them as they grow to see if my ID is correct.

Some other plants with distinctive cotyledon leaves include maples, with narrowly oblong leaves, and geraniums which have an obvious notch in the first leaves. A good way to learn cotyledons is to grow things from seed. The very first needle-like leaves have begun to grow from the run-of-the-mill cotyledons of newly germinated Phlox diffusa seed I’m growing for my rock garden. If anyone has found some other interesting cotyledon leaves, I’d love to see them.

4 Responses to “Mysteries of Cotyledon Leaves”

  • Stewart Wechsler:

    I too have been noticing some of these same distinctive cotyledons. I see that you also have noted the seemingly distinctive cotyledons of Geranium robertianum. While I freely pull these at the cotyledon stage, I have become hesitant to instruct all other people to do the same. I don’t know if some rarer native Geraniums, whose cotyledons I haven’t familiarized myself with would have look-a-like cotyledons. Do you know if and how similar or different any of our less common native Geranium cotyledons would look?

  • Hi Stewart,

    I can’t say that I can tell the difference between the cotyledon leaves of various species of Geranium. Most of ours in western Oregon are non-native weeds, some of them, including G. robertianum and G. lucidum are really aggressive and are listed as class B noxious weeds by the ODA. I am familiar with the garden Geraniums I have planted in my garden. Their cotyledons are similar. They have turned out to be more trouble than they are worth because of their amazing ability to send seeds all over the garden.

    As for telling people to pull these nasty seedlings, I wouldn’t worry too much if they are in an area with an obvious weed problem. It is highly unlikely that our native Geranium oreganum is hiding in a bed of G. robertianum. And if they wait until they see the first true leaves, like in my photo, it’ll be pretty clear they aren’t the natives. It’s a good question, and I’ll have to see if I can find any seedlings in the few places I run into Geranium oreganum.

  • Tanya-

    You might be interested in our series of cotyledon photos from our nursery:

  • What an excellent resource, Ben! Thanks for sharing your site and for documenting your seedlings like that. I hope you’ll continue to add other plants to your photo collection. And I’m so glad your photos have confirmed my mystery cotyledons are indeed Oemleria cerasiformis. Now I’d love to know why they are so different. The rest of your Rose family cotyledons look pretty standard, so it isn’t a family trait. Always more questions to ponder.

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