Abstract
SUMMARY: Infection of the flowering culm of Zizania caduciflora by the smut fungus, Ustilago esculenta, results in a swollen structure named‘gau sun’This host‐parasite combination is culivated as a vegetable in eastern Asia. The structure and development of gau sun is described and terms are denned to describe developmental stages. The swollen culm (gau sun) has a volume 517 × that of the unswollen culm and contains 2‐9 × the number of cells which are 14‐9 × the volume of those in the unswollen culm. Mucilaginous cavities, absent from the unswollen culm, make up 16% of the total volume of the swollen culm.