A companion's foot slips and there is a loud crack of a snapping branch. The sound of rustling arises from one grove of bamboo-like plants near the stream--and then a spearlike object is flung to land, point first, in the path before you.
This cluster of spear grass is but one of several that line the bank the heroes have walked along for several minutes. The plants track by sound and then communicate with others of their kind using a rustling language. The party has just passed within short range of this cluster and it has flung one spear in an attempt to elicit more noise. Then the spear grass will alert the rest of the clusters to the presence of prey.