When your total earnings reach 25000g Demetrius will come by and tell you he has been doing research. He'll then give you the options if you want bats to leave fruits in your cave or to have mushrooms randomly grow in your cave.
Fruitbats leave various fruits in the Cave on your farm.
Cave
Tree grown on farm
38 15
53 21
68 27
Pelican Town
25 10
35 14
45 18
Certain types of mushrooms will randomly grow, there are a total of six plots for them to grow in.
Hidden Forest
All Year
-50
-70
-90
Exotic Foraging Bundle
125 50
175 70
225 90
Field Research Bundle
75 30
105 42
135 54
20 8
28 11
36 14