I'm pushing 30 now, and it baffles the shit out of me the way grown people confuse parsley and cilantro. But I won't ridicule you, unnamed herb-ilitterate friend (even though I should). I will enlighten you. Think of me as your culinary Bill Nye. Let me show you:
|Left: Flat Leaf Parsley / Right: Cilantro|
Parsley was first domesticated in the mediterranean, and owes its grassy smell to a compound called apiol (1-allyl-2,5-dimethoxy-3,4-methylenedioxybenzene). Also known as parsley camphor, apiol is also found in celery, and has been known to cause abortions when ingested in moderate doses since the times of Hipocrates. It was even sold in tablets in the US during the late 19th century for this purpose. Large doses can potentially lead to kidney and liver failure.
Cilantro, on the other hand, has a less grassy and more citrusy overtone, mainly due to the presence of large ammounts of undecadiennal, a volatile compound constituted by at least 12 subfractions. It also shares some aldehydes with the stink bubg nymph, but its detection by humans varies widely. This was the the basis for the first genetic studies in humans, since it is linked to the presence of the TAS2R38 gene. People with this gene detect a much wider range of bitter tastes, and automatically dislike the herb, describing it as "bitter" and "soapy".
Now you know.
Source: Potter, T. L.Essential oil composition of cilantro J. Agric. Food Chem. 1996, 44, 1824– 1826