  Kagome magnets provide a fascinating platform for the realization of
correlated topological quantum phases under various magnetic ground states.
However, the intricate effect of the magnetic spin configurations on the
characteristic electronic structure directly from the kagome lattice layer
remains still elusive. Here, utilizing angle-resolved photoemission
spectroscopy and density functional theory calculations, we report the
spectroscopic evidence for the spin-reorientation effect of a kagome
ferromagnet Fe$_3$Ge, which is composed only of the kagome planes. There are
two kinds of kagome-derived Dirac fermions due to the structural
three-dimensionality -- one is less dispersive ($k_z$ $\sim$ 0) and the other
disperses linearly ($k_z$ $\sim$ $\pi$). As the Fe moments cant from the $c$
axis into the $ab$ plane upon cooling, the Dirac fermion in $k_z$ $\sim$ 0
plane with a mixture of the Fe-$3d_{xy}$ and Fe-$3d_{x^2-y^2}$ components
evolves from gapped into nearly gapless, while the Dirac cone in $k_z$ $\sim$
$\pi$ plane mainly of the Fe-$3d_{x^2-y^2}$ orbital character remains intact,
suggesting that the effect of spin reorientation on the Dirac fermions has an
orbital selectivity. Our unambiguous observations provide a feasible route to
design and manipulate the mass of Dirac fermions for realizing the novel
quantum phases. We also perform comparative studies between the
non-charge-ordered Fe$_3$Ge and its sibling compound FeGe, a newly established
charge-density-wave kagome magnet, the results suggest that the
orbital-selective van Hove singularities near the Fermi level play an
indispensable part in driving the charge order on a magnetic kagome lattice.
