  Let $V$ and $V'$ be $2n$-dimensional vector spaces over fields $F$ and $F'$.
Let also $\Omega: V\times V\to F$ and $\Omega': V'\times V'\to F'$ be
non-degenerate symplectic forms. Denote by $\Pi$ and $\Pi'$ the associated
$(2n-1)$-dimensional projective spaces. The sets of $k$-dimensional totally
isotropic subspaces of $\Pi$ and $\Pi'$ will denoted by ${\mathcal G}_{k}$ and
${\mathcal G}'_{k}$, respectively. Apartments of the associated buildings
intersect ${\mathcal G}_{k}$ and ${\mathcal G}'_{k}$ by so-called base subsets.
We show that every mapping of ${\mathcal G}_{k}$ to ${\mathcal G}'_{k}$ sending
base subsets to base subsets is induced by a symplectic embedding of $\Pi$ to
$\Pi'$.

Combinatorics;Group Theory