The term ACORN SOURCES refers to the various sources of acorns, which are the fruit of OAK trees. OAKS is a four-letter word for a type of tree that produces acorns, while OAK is a three-letter word for the same. SEED is a four-letter word for the reproductive part of a plant, and NUT is a three-letter word for the hard-shelled fruit of some plants. OAKTREE is a seven-letter word for a single OAK tree, and OAKTREES is an eight-letter word for multiple OAK trees.