Mushrooms: click the picture of the mushrooms you want.
If the button is dark it is selected, if it is light that mushroom will not show on screen.
All: click the button to select or deselect the row of mushrooms.
Number of mushrooms: Set the slider to select the number of mushrooms. The more to the right the more mushrooms. Mushrooms don't cost you a lot of CPU-time.
No mushrooms: check the box to make no mushrooms appear.