Orientated growth of Membranipora membranacea (L.) on the thallus of Saccorhiza polyschides (Lightf.) Batt.

Orientated growth is described in colonies of Membranipora membranacea (L.) growing on the large brown seaweed Saccorhiza polyschides (Lightf.) Batt. On average, 60% of the growth of the colonies was directed proximally along the long axis of the plant, irrespective of the region of the plant on which the colonies were growing or their proximity to its primary meristem. These results seem to rule out the possibility that orientated growth in Membranipora is a response to an age dependent polarity in the host plant, since the tissue ages in opposite directions in the lamina and stipe yet the growth of Membranipora is orientated in the same direction on both. The possibility that directional growth in this species may be a rheopositive response is discussed.