Whenever there is a plant near you, it will be displayed on the mini-map. Different kinds of plants are marked with different colors. After you harvest them, you can use them to prepare a variety of syringes.
There are four categories of such plants: Healing, Hunting, Fight, Exploration, Special. All in all, they are very useful because they enable you e.g. to stay underwater for an extended period of time, heal wounds, or grant temporary invulnerability. New recipes can be obtained along your progress in the game, by finding hidden items, (e.g.. cult objects) and by performing side missions.