  We present observations of the power spectral anisotropy in wave-vector space
of solar wind turbulence, and study how it evolves in interplanetary space with
increasing heliocentric distance. For this purpose we use magnetic field
measurements made by the Helios-2 spacecraft at three positions between 0.29
and 0.9 AU. To derive the power spectral density (PSD) in $(k_\parallel,
k_\bot)$-space based on single-satellite measurements is a challenging task not
yet accomplished previously. Here we derive the spectrum
$\rm{PSD}_{\rm{2D}}$($\rm{k}_\parallel$, $\rm{k}_\bot$) from the spatial
correlation function $\rm{CF}_{\rm{2D}}(r_\parallel, r_\bot)$ by a
transformation according to the projection-slice theorem. We find the so
constructed PSDs to be distributed in k-space mainly along a ridge that is more
inclined toward the $\rm{k}_\bot$ than $\rm{k}_\parallel$ axis, a new result
which probably indicates preferential cascading of turbulent energy along the
$\rm{k}_\bot$ direction. Furthermore, this ridge of the distribution is found
to gradually get closer to the $\rm{k}_\bot$ axis, as the outer scale length of
the turbulence becomes larger while the solar wind flows further away from the
Sun. In the vicinity of the $\rm{k}_\parallel$ axis, there appears a minor
spectral component that probably corresponds to quasi-parallel Alfv\'enic
fluctuations. Their relative contribution to the total spectral density tends
to decrease with radial distance. These findings suggest that solar wind
turbulence undergoes an anisotropic cascade transporting most of its magnetic
energy towards larger $\rm{k}_\bot$, and that the anisotropy in the inertial
range is radially developing further at scales that are relatively far from the
ever increasing outer scale.
