1. A variable speed drive mechanism for a seed planter of the type having a frame, at least one rotatably driven seed dispenser mounted on said frame, at least one ground-engaging wheel rotatably mounted on said frame, and a power transmission means drivingly connecting said wheel to said dispenser for driving the dispenser, said mechanism comprising: support means affixable to said frAme; a first shaft journaled for rotation in said support means, said first shaft being drivably connectable to said wheel by a first portion of said transmission means so that the first shaft may be rotated at a proportionally fixed rotational velocity with respect to the rotational velocity of the wheel; a second shaft journaled in said support means, said second shaft being drivingly connectable to said dispenser by a second portion of said transmission means so that the dispenser may be driven at a proportionally fixed velocity with respect to the rotational velocity of the second shaft; a plurality of selectively engageable drive trains drivingly connecting said first shaft to said second shaft, each of said drive trains consisting of a selectively engageable poweroperated clutch mounted on said first shaft, a first rotary drive element connected to said clutch, and a second rotary drive element affixed to said second shaft, said first element being radially aligned with and drivingly connected to said second element; and means for individually selectively engaging said clutches from a remote location, whereby said second shaft is rotated at different rotational velocities with respect to the rotational velocity of said first shaft upon individual selective engagement of said clutches.