|Hemerocallis flower, with three flower parts in each whorl|
Monocotyledons are a class of flowering plants (angiosperms), whose embryo (seed) store only one cotyledon. The APG II system recognises a clade called "monocots" but does not assign it to a taxonomic rank. You can tell how to recognize a monocot by its leaves, they have long parallel veins running down the leaf, whereas dicot plant leaves have a complex netted vein pattern on their leaves.
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