1Botany. The tendency of a plant part, especially a root or shoot, to grow in a straight line or to retain its normal spatial orientation even in the presence of altered tropic stimuli (such as light or gravity).

2Mycology. The tendency of a fungal hypha to grow towards or away from another hypha in the same mycelium, or of the germ-tube of a germinating spore to grow towards or away from a spore of the same species.