Concept:
Phytochemistry deals with chemical constituents present in plants.
Step 1: Medicinal plants contain alkaloids, glycosides, flavonoids, tannins, and other constituents.
Step 2: Phytochemical investigation studies these constituents.
Step 3: The aim is to identify, isolate, and characterize active compounds.
Step 4: It is not mainly done to increase yield or cosmetic value.
\[
\boxed{\text{Identify and characterize chemical constituents}}
\]