  We study the first derivative of the staggered magnetization squared
$dm^\dag(\theta)^2/d\theta$ and the second derivative
$d^2e_0(\theta)/d\theta^2$ of the ground state energy per site. The parameter
$\theta$ controls the anisotropy between horizontal and vertical couplings in a
two-dimensional (2D) spin-1/2 antiferromagnetic Heisenberg model. It is shown,
that both derivatives diverge at $\theta=1$, where the anisotropic 2D model
reduces to the 1D model.
