Common Glowcap

The Common Glowcap is a luminescent fungus and is used as scenery in some areas and caves. They are a source of light when "awake", which can be used in the leaders' favor.

In the first two games, they are purely aesthetic, and as scenery, the Glowcaps do not change gameplay at all, unlike their supposed "relative", the Glowstem, which may cause bomb-rocks to fall upon contact with it.

In Pikmin 3 these plants are initially found buried in the ground with only the tops showing; they do not give light in this state. Contact with them will make them rise out and light up a small area around them. The light from these plants can damage and kill Phosbats, as well as provide shelter from the Vehemoth Phosbat. Two variations exist: a cyan one and a pink one. They can also have different sizes, sometimes. The color influences the light that comes out (of the same color), but nothing else.

In Pikmin Adventure, they are referred to as Glowing Mushrooms, and provide light to help the player navigate. Moreover, if the player touches them, they will glow and emit light themselves for a small period of time.

Pikmin 2 Nintendo Player's Guide
"Providing illumination in dark areas, the Common Glowcap is a wonder to observe. Olimar notes that the mushroom's glow comes from a compound called Hocotatium 111."

Pikmin 3 Prima Guide
"These luminescent mushrooms can be very useful when you're exploring a dark cave. When a Glowcap is dormant, it looks like nothing more than a spot of light on the ground. When you pass over a dormant Glowcap, however, it springs out of the ground and illuminates the immediate area."

Trivia

 * If the player looks at a cloaked Vehemoth Phosbat through the light of a Common Glowcap, they'll be able to see its silhouette.