Now that I had a bit of time, here is a better implementation.
\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes.multipart, arrows.meta}
\begin{document}
\tikzset{
csw/.style = { % connect from center to south west
edge from parent path={
(\tikzparentnode) -- (\tikzchildnode.south west)
},
},
cse/.style = { % connect from center to south east
edge from parent path={
(\tikzparentnode) -- (\tikzchildnode.south east)
},
},
}
\begin{tikzpicture}[
sibling distance=5em,
level 2/.style = {sibling distance=10em},
-{Stealth[length=5mm, width=2mm]}, grow=up, % use arrowe, grows the graph up.
every node/.style =
{shape=rectangle split, % use two-parts rectangular nodes
rectangle split parts=2, rectangle split horizontal,
draw, thick, align=center, % draw thick lines.
text width=1.5em},
every two node part/.style = { % set the style of the second part
font=\bfseries, % so we can avoid all the \textbf{}
}
]
\node{S1 \nodepart{two} R3}
child [csw] { node {S2 \nodepart{two} R1} }
child [thick] { node {S3 \nodepart{two} R5 }
% we need to reset the thickness here, otherwise is inherited
child [csw, thin] { node {S5 \nodepart{two} R4} }
child [cse, thin] { node {S6 \nodepart{two} R6} }
}
child [cse]{ node {S4 \nodepart{two} R2}
};
\end{tikzpicture}
\end{document}
The result is
Original answer
\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes.multipart}
\begin{document}
\begin{tikzpicture}[sibling distance=10em,
->, >=stealth, grow=up, % use arrowe, grows the graph up.
every node/.style = {shape=rectangle split, % use two-parts rectangular nodes
rectangle split parts=2, rectangle split horizontal,
draw, thick, align=center, % draw thick lines.
top color=white,}]
\node {S1 \nodepart{two}\textbf{R3}}
child { node {S2 \nodepart{two}\textbf{R1}} }
child { node {S3 \nodepart{two}\textbf{R5}}
child { node {S5 \nodepart{two}\textbf{R4}} }
child { node {S6 \nodepart{two}\textbf{R6}} } }
child { node {S4 \nodepart{two}\textbf{R2}} };
\end{tikzpicture}
\end{document}
It should be changed to send the arrows to the option edge from parent path
in the manual, and to make the two parts the same size (look at the Shape Library, shapes with multipart, in the same manual).