A.T. Robertson Commentary Mark 4:29

"But when the fruit is ripe, straightway he putteth forth the sickle, because the harvest is come." — Mark 4:29 (ASV)

Is ripe (παραδο, second aorist subjunctive with οταν). Whenever the fruit yields itself or permits.

Putteth forth (αποστελλε). Sends forth the sickle. The word for apostle comes from this verb. See Joh 4:38 "I sent you forth to reap" (εγο απεστειλα υμας θεριζειν). Sickle (δρεπανον) here by metonymy stands for the reapers who use it when the harvest stands ready for it (παρεστηκεν, stands by the side, present perfect indicative).