In a large number of cases the conveyance of the seeds to a distant
point is accomplished by the adoption of the screw-propeller principle.
An excellent example of this is to be seen in the fruits of the
sycamore. Here the actual seed is large and heavy, but it is attached to
a wing-like expansion. When the fruit falls from the tree the wing
revolves with great rapidity, very much on the lines of a propeller
blade. This has the effect of controlling the rate of fall, so that the
whole contrivance is carried to some distance before the seed is
actually brought to earth.