The life sciences department was a science department aboard some starships, such as the USS Enterprise and the USS Enterprise-D. One section within this department was the botany section. (TOS: "The Man Trap")
In 2265, Doctor Mark Piper was the head of the Enterprise's life sciences department. (TOS: "Where No Man Has Gone Before")
In 2267, Captain James T. Kirk asked both the medical and life sciences departments to investigate the Denevan flying parasite. (TOS: "Operation -- Annihilate!")
This would seem to suggest that the life sciences and medical departments could operate separately from one another.
Life Sciences was also a section aboard the USS Enterprise-D. Data headed up life sciences for a 2366 mission. (TNG: "Tin Man")
Aboard USS Voyager, Life sciences was listed under Sciences in Seven of Nine's 2376 shipwide efficiency analysis. (VOY: "Good Shepherd")