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.
“The light emitted from this rare mushroom is neither a reflection, nor a release of stored light. It grows not only on decomposing trees, but also on soil and rocks. This mushroom's fungal filaments are capable of stabilizing and concentrating pure Hocotatium 111. It is none other than the Hocotatium 111 approaching the point of critical mass that causes the glowcap's blue luminescence.”
“Rapturous fresh or sauteed, this illuminating fungus will be hot in the galaxy's trendiest restaurants.”
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."
Names in other languages