This paper focuses on phenotypic plasticity as a major mode of adaptat
ion in plants. A methodological critique examines difficulties in stud
ying plasticity, including the conceptually critical distinction betwe
en functionally adaptive and inevitable aspects of response. It is arg
ued that plasticity studies depend critically upon the genotypic sampl
e, the choice of environmental factors and factor states, and the defi
nition of phenotypic traits. Examples are drawn from recent studies sh
owing adaptive response by genotypes to physical aspects of the enviro
nment, as well as to biotic factors such as neighbour density and the
presence of bacterial symbionts. Alterations of offspring traits by pa
rental plants of Polygonum persicaria are discussed as a cross-generat
ional aspect of plastic response to environment. Finally, individual p
lasticity and local ecotypes are examined as alternative bases of spec
ies ecological breadth, and methodological problems in distinguishing
these alternatives are discussed.