APERTURE PRIORITY: You adjust the aperture and the camera adjusts the shutter speed.   Aperture refers to the amount of light that hits your camera’s sensor.  Counter-intuitively, an f-2 aperture stop lets in more light than an f-16 stop aperture.  In other words, smaller numbers represent a large wide-open aperture.  Higher numbers represent a small aperture. So if you are in dark woods you might want to set your aperture at the lower end, like f2. However, aperture has an effect on the depth of field of your photos, the region of your photo that is in focus (i.e. from the front to the back of your mushroom). At wide-open apertures (f.2) DOF is narrower. At small apertures (f.16 and up) all of the mushroom and much of the background will be in focus.

At a wide open f2 setting, only the front of your mushroom will be in focus.  The rest, including the stem, will be out of focus  (a dreamy look). A better solution is to set the aperture between f5-f11 to balance the need for light while creating an adequate DOF.  I recommend the use of Aperture priority almost exclusively, because this mode gives you control of your DOF.