chemotaxis n.

before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.

1620s, "science of arranging military forces for combat," from Modern Latin tactica (17c.), from Greek taktike techne "art of arrangement," noun use of fem. of taktikos "of or pertaining to arrangement," especially "tactics in war," adjective to taxis "arrangement, an arranging, the order or disposition of an army, battle array; order, regularity," verbal noun of tassein "arrange," from PIE root *tag- "to touch, handle."