Cytosolic Ca ²⁺ signaling plays a crucial role in tip growth in moss. (A) A summary of Ca²⁺ flow in the tip of moss caulonemal cells. Ca²⁺ enters the cell via several channels at the tip and is removed from the cytosol by Ca²⁺ pumps (ACAs) further down the cell shaft, generating a concentration gradient. (B) A model of Ca²⁺ contributions to the tip growth. Ca²⁺ gradient directly promotes secretion of new cell material at the tip. In a parallel pathway, Ca²⁺ oscillations modulate the dynamics of the actin spot, which in turn help promote and focus the material secretion.