Phenolic compounds are an important component of brown algal cell walls. Physodes, packed full of phenolic compounds, accumulate in the peripheral cells and around the male and female conceptacles of Hormosira banksii. After periods of sun exposure, parts of the thallus appear dark brown (burnt). The brown layer consists of oxidised phenolic compounds that have been released from physodes in the peripheral cells. This brown layer only penetrates a few cells deep, thereby forming a protective layer for the photosynthetic tissue beneath. Physodes in the peripheral cells of H. banksii therefore appear to protect the plant from excess UV radiation. The physodes autofluoresce blue/blue-green under UV excitation, and provide a simple but accurate system of assessing phenolic location within the plant.
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