Mouse-ear (Pilosella ojficinarum) and king devil (P. praealta), introduced to New Zealand in the 1920's, have rapidly increased in abundance in the modified short-tussock grasslands of the dry montane regions of South Island. Allelopathr and competition effects were investigated to explatn the success of these hawkweed species in ousting other species and to find an agronomically useful species to out-compete them. Evidence of allelopathic effects was found in both laboratory and field experiments. The phytotoxic agents in mouse.-ear and king devil are phenolics of which umbelhfe.rone is the most active. Umbelliferone, present 10 mouse-ear leaves only, is washed into the soil from recently dead leaves but not from living leaves, and is deleterious to root development. Mouse-ear and king devil compete for nitrogen, phosphorus, and water. Pot competition trials with several grass and clover species showed alsike clover to be the best competitor against the two hawkw.eeds .. The nutrient and moisture status of the soll modified the allelopathic effect. In field conditions, aUelopathy is effective as an interference phenomenon only under certain circumstances.

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Polymorphic variation ofleaf shape and size, and the chromosome number were studied in 31 popUlations of Hieracium pilosella L. in New Zealand. Some variation in leaf form persisted after collections of plants were transplanted to a common environment for one growth period. All popUlations were pentaploid (2n = 45). Some ecological and biogeographic implications are discussed.

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