tropism

1. (uncountable, biology) The turning of an organism (chiefly a plant) or part of an organism either towards or away from a stimulus; (countable) an instance of this.Examples: "Orientation and tropism are synonymous expressions; that gatherings [of Paramecium] take place is a consequence of the orientation, and therefore throws no light whatever upon the tropisms. (Gatherings, however, are not always indicative of a tropism, i.e., of an orientation, but may be due to kinesis […]"; "Phototropic orientation of an embryo plant in zero gravity […] On earth, a germinated seedling can be placed in any position and the radicle will invariably turn the embryo plant "downward" and the hypocotle "upward". This is because positive root tropism toward gravity and negative stem tropism away from gravity. In the absence of gravity, will the plumules and the stem of the embryo plant reacting positively to phototropism guide the entire plant including the root system for its directional orientation?"; "Tropism means that the roots or branches move by turning towards or turning away from environmental stimuli. We have implemented this behavior in the roots of the plantoid [i.e., a robot or synthetic organism designed to act like a plant], integrating sensors and using an adaptive material for growing."uncountable

2. (countable, by extension, chiefly virology) A capability or tendency for a pathogen (chiefly a virus) to infect a type of cell, tissue, organ, or host organism.Examples: "host tropism    tissue tropism"; "Studies similar to those describe above for cells cultivated in vitro were performed using homogenates of tissues and organs from humans and rhesus monkeys to investigate tissue tropisms in susceptible species […] McLaren (personal communication) found an excellent correlation between poliovirus tissue tropisms and presence or absence of receptor activity in human and rhesus tissues and organs."; "Virus tropism can be defined at two levels: host tropism being the range of host species that a virus can productively infect, and tissue tropism reflecting the particular tissues or organs within the host that are the site(s) of virus replication. […] The primary determinant of virus tropism is the presence on the cell surface of a suitable receptor which the virus can utilize to gain entry to the cell."broadlycountable

3. (countable, figurative) Of a person: an instinctive predilection or tendency; also (generally), a liking, a preference.Examples: "It is reasonable to suppose that tropism will be less conspicuous in animals which are endowed with intelligence than in others; but well-marked tropisms are still to be found among the higher animals. Gregarious animals have a positive tropism toward the members of their own flock; the dog has a negative tropism toward his own kind and a positive tropism toward mankind."countablefiguratively