A revision of South American Myriophyllum (Haloragaceae) and its repercussions on some Australian and North American species.

The three species of Myriophyllum occurring naturally in South America (M. aquaticum, M. quitense and M. mattogrossensis) are keyed and described. M. quitense is an earlier legitimate name for the plant generally known as M. elatinoides, a species until now thought to extend from South America into North America, New Zealand and Australia. Plants of 'M. elatinoides' in North America and Australia have been critically re-examined, and the status of closely related taxa has been reconsidered. It has been found that, in North America, M. quitense exists as two isolated populations in Arizona and Oregon. The status of M. spicatum, M. exalbescens and M. magdalenense, the three previously described species of the 'elatinoides' complex is discussed, and it is suggested that two varieties should be recognized, M. spicatum var. spicatum and M. spicatum var. exalbescens. M. magdalenense is considered to be a minor variant of M. spicatum var. spicatum. Aus- tralian plants known until now as M. elatinoides are shown to be distinct from M. quitense, and to constitute three new species, M. salsugineum, M. caput-medusae and M. porcatum, which are here described.