"plant which turns its flowers and leaves to the sun," 1620s, from French héliotrope (14c., Old French eliotrope) and directly from Latin heliotropium, from Greek hēliotropion "sundial; heliotropic plant," from hēlios "sun" (from PIE root *sawel- "the sun") + tropos "a turn, change" (from PIE root *trep- "to turn"). In English the word in Latin form was applied c. 1000-1600 to heliotropic plants. Related: Heliotropic.
-ism
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
tropism n.
1899, "tendency of an animal or plant to turn or move in response to a stimulus," 1899, abstracted from geotropism or heliotropism, with the second element taken in an absolute sense; ultimately from Greek tropos "a turning" (from PIE root *trep- "to turn").