cotyledon [ kot-l-EED-n ] noun: the first leaf or one of the first pair or whorl of leaves developed by the embryo of a seed plant or of some lower plants (such as ferns)
acotyledon [ EY-kot-l-EED-n, ey-kot- ] noun: a plant without cotyledons adjective: acotyledonous
dicotyledon [ dahy-kot-l-EED-n, dahy-kot-l- ] noun: any angiosperms that produce an embryo with two cotyledons; also called dicot
eudicotyledon [ yo͞o-kot-l-EED-n] noun: a member of a monophyletic group of flowering plants included as part of the dicots; also called eudicot
hypocotyl [ hahy-puh–KOT-l ] noun: the part of the axis of a plant embryo or seedling below the cotyledon
monocotyledon [ mon-uh-kot-l-EED-n ] noun: any chiefly herbaceous angiosperms having an embryo with a single cotyledon; also called monocot
If you read about plants, then you have seen the words monocot and dicot. You can surmise that the combining forms “mono” means single and “di” means two, both of Greek origin. But what of “cot”?
It is a shortened version of cotyledon. In angiosperms (flowering plants), it is the first leaf characteristic of monocotyledons or one of the first pair of leaves of the dicotyledons that emerge from a seed. The dicotyledons, or simply dicots, are further divided, with the largest group—comprising two-thirds of all flowering plants—being the monophyletic (derived from a common ancestor) eudicotyledons, with “eu” meaning true. They include the Asterids and Rosids and most leafy trees of the temperate zones with the main exceptions being trees of the Magnoliids and the gymnosperm, Ginkgo.
Left to right: Cotyledons and first true leaves of European fan palm, apple, honey locust, maple, Jerusalem pine.
In gymnosperms (naked seed plants), cycads, Ginkgo, and the gnetophytes have two cotyledons while conifers have as many as 24, although eight is common. A few plants, like orchids, which have no cotyledons, are acotyledonous, with the prefix “a” meaning without.
Photo © B. Domangue CC BY-SA 4.0
Cotyledons are sometimes referred to as the seed leaves or embryo leaves, as they are found in the embryo of the seed. When a seed sprouts, the embryonic root (the radicle) of the plant emerges first, burrowing its way down into the soil. Next, the embryonic stem, connected to the radicle, rises above the surface. This stem, called the hypocotyl, with the prefix “hypo” meaning beneath, supports the cotyledons. When exposed to the light, they turn green with the formation of chloroplasts for photosynthesis. [Watch this time-lapse video comparing dark-grown versus light-grown sunflower cotyledons.] The cotyledons are often oval to lanceolate and usually do not resemble the leaf shape of the plant species. Lastly, the plumule, the shoot of the plant bearing the plant’s true leaves, begins to grow. In some plant species, the cotyledons remain below ground, so it is the plumule that breaks the soil’s surface. [Watch a time-lapse seed germination video—choose among corn, sunflower or Arabidopsis in the left-hand column.]
You do not need to see the cotyledons to determine whether a plant is a dicot/eudicot (such as asterids, brassicas, cucurbits, legumes, maples, mints, oaks, rosids, rues, tomatoes) or a monocot (such as asparagus, bamboo, corn, grains, grasses, irises, lilies, narcissi, palms, tulips, trilliums, yuccas), as each group shares certain traits as described below.
DICOTS/EUDICOTS
Eudicots with floral parts in multiples of four or five (from left to right): Cardamine hirsuta and natives Aronia arbutofolia., Eurybia divaricata, Hibiscus moscheutos.
Left to right: Curcurbita (squash) cotyledons and first true leaves, branching venation of true leaves, stem vascular bundles arranged in a ring, and 5-petalled male and female flowers
MONOCOTS
Monocots with floral parts in multiples of three (from left to right): natives Lilium superbum, Tradescantia virginiana, Trillium grandiflorum, Uvularia grandiflora.
Left to right: Zea (corn) seed showing cotyledon and radicle, parallel venation of true leaves, scattered stem vascular bundles, male and female flowers.
- one cotyledon
- parallel leaf venation
- leaf stomata on upper and lower surfaces
- stem vascular bundles scattered throughout
- fibrous or adventitious roots
- floral parts often in multiples of three
- pollen with one pore
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References
Boeckmann C. Updated March 2024. Vegetable Seedling Identification: Pictures and Descriptions. Almanac.
Chen C, Du X. 2022. LEAFY COTYLEDONs: Connecting different stages of plant development. Frontiers in Plant Science. 13: 916831. doi: 10.3389/fpls.2022.916831. PMID: 36119568; PMCID: PMC9470955.
Cucurbita moschata. FOC Volume 19. Flora of China. (accessed September 24, 2024).
Hangarter RP. Plants-In-Motion. Indiana University.
Helmenstine A. Updated July 2023. Monocot vs Dicot – How to Tell the Difference. Science Notes.
Schader M. Reviewed by: Tremblay S. Updated September 2021. What Is the Function of the Cotyledon in the Seed? Sciencing.