documentclass[a4paper,10pt]{article}

usepackage{tikz}
usepackage{verbatim}
usepackage[active,tightpage]{preview}
PreviewEnvironment{tikzpicture}
setlengthPreviewBorder{5pt}%
usetikzlibrary{arrows,chains,matrix,positioning,scopes}
makeatletter
tikzset{join/.code=tikzset{after node path={%
ifxtikzchainpreviouspgfutil@emptyelse(tikzchainprevious)%
edge[every join]#1(tikzchaincurrent)fi}}}
makeatother
tikzset{>=stealth',every on chain/.append style={join},
         every join/.style={->}}
tikzstyle{labeled}=[execute at begin node=$scriptstyle,
   execute at end node=$]
%
begin{document}
begin{tikzpicture}
  matrix (m) [matrix of math nodes, row sep=3em, column sep=3em]
    { group , 1 & N & phantom{N} & M &  S,M  & phantom{S,M} \
      group , 2 & N & S,M &  & phantom{S,M} & phantom{S,M} \ };
  { [start chain] chainin (m-1-2);
    chainin (m-1-4); %[join={node[above,labeled] {psi}}];
    chainin (m-1-5);
    chainin (m-1-6);}

   { [start chain] chainin (m-2-2);
    chainin (m-2-3);
        { [start branch=A] chainin (m-1-3)
        [join={node[right,labeled] {text}}];}
    chainin (m-2-6);}
end{tikzpicture}
end{document}