  We study the low-energy excitations of the spin-1/2 antiferromagnetic
Heisenberg chain and $N$-leg ($N$=2, 3, 4) ladders in a staggered magnetic
field $h_s$. We show that $h_s$ induces gap and midgap states in all the cases
and examine their field scaling behavior. A modified boundary scheme is devised
to extract accurate bulk excitation behavior. The gap values converge rapidly
as $N$ increases, leading to a field scaling exponent $\gamma=1/2$ for both the
longitudinal and transverse gaps of the square lattice ($N \to \infty$). The
midgap states induced by the boundary edge effects share the bulk gap scaling
exponents but their overall scaling behavior in the large-N limit needs further
investigation.
