Substance in a petri dish (4)

I believe the answer is:

agar

(Other definitions for agar that I've seen before include "Jelly medium" , "Food thickening agent obtained from seaweed" , "Jelly got from seaweeds, used in foods" , "What comes out of seaweed" , "setter" .)

I've seen this clue in The New York Times.

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