  We study the S=1/2 Heisenberg antiferromagnet on the floret pentagonal
lattice by numerical diagonalization method. This system shows various
behaviours that are different from that of the Cairo-pentagonal-lattice
antiferromagnet. The ground-state energy without magnetic field and the
magnetization process of this system are reported. Magnetization plateaux
appear at one-ninth height of the saturation magnetization, at one-third
height, and at seven-ninth height. The magnetization plateaux at one-third and
seven-ninth heights come from interactions linking the sixfold-coordinated spin
sites. A magnetization jump appears from the plateau at one-ninth height to the
plateau at one-third height. Another magnetization jump is observed between the
heights corresponding to the one-third and seven-ninth plateaux; however the
jump is away from the two plateaux, namely, the jump is not accompanied with
any magnetization plateaux. The jump is a peculiar phenomenon that has not been
reported.
