Agar Source is a term used to describe the source of agar, a gelatinous substance derived from SEAWEED and algae. SEAWEED is a seven-letter word for a type of marine plant that grows in shallow coastal waters, while ALGA is a four-letter word for a type of aquatic plant that can be found in both fresh and salt water. Agar is extracted from these plants and used in a variety of applications, including as a thickening agent in food and as a medium for growing bacteria in laboratories.