Branched Pathway in TikZ

 I couldn't resist, here are some simple branched pathways using tikz. Note you can change the vertical splay and horizontal distance for the nodes by changing the node distance in the argument to tikzpicture. I also added some colors wshich really don't match, someone with more artistic tallent could do better.

\documentclass[12pt]{article}

\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage{xcolor}

\begin{document}

\begin{tikzpicture}[node distance=0.4cm and 1cm]
\node (Xo) at (0,0) {};
\node[right=of Xo] (S1) {\small $S_1$};
\node[above right=of S1] (S2) {};
\node[below right=of S1] (S3) {};

\draw[-latex, thick, blue] (Xo) -- (S1) node[pos=0.5,above,gray] {$v_1$};

\draw[-latex, thick, orange] (S1) -- (S2) node[above=0.15, left,pos=0.5,gray] {$v_2$};
\draw[-latex, thick, green!45!black] (S1) -- (S3) node[below=0.15, left,pos=0.5,gray] {$v_3$};
\end{tikzpicture}

\vspace{1cm}

\begin{tikzpicture}[node distance=0.4cm and 1cm]
%\draw [help lines,step=.1] (0,-6) grid (6,6);
\node (Xo) at (0,0) {};
\node[right=of Xo,yellow!20!red] (S1) {\small $S_1$};
\node[above right=of S1,blue!80!red] (S2) {\small $S_2$};
\node[below right=of S1,green!80!red] (S3) {\small $S_3$};

\node[above right=of S3] (S4) {};
\node[below right=of S3] (S5) {};

\draw[-latex, thick, blue] (Xo) -- (S1) node[pos=0.5,above,purple] {$v_1$};
\draw[-latex, thick, blue] (S1) -- (S2) node[above=0.15, left,pos=0.5,purple] {$v_2$};
\draw[-latex, thick, blue] (S1) -- (S3) node[below=0.15, left,pos=0.5,purple] {$v_3$};

\draw[-latex, thick, blue] (S3) -- (S4) node[below=-0.12, left,pos=0.5,purple] {$v_4$};
\draw[-latex, thick, blue] (S3) -- (S5) node[below=0.05, left,pos=0.5,purple] {$v_5$};
\end{tikzpicture}

\end{document}