I have draw a tree using tikz package like this
\begin{tikzpicture}
\tikzstyle{level 1}=[sibling distance=40mm]
\tikzstyle{level 2}=[sibling distance=20mm]
\tikzstyle{level 3}=[sibling distance=10mm]
\node{S}
child{node{N} child{node{mary}} }
child{node{VP}
child{node{V} child{node{brought }}}
child{node{NP} child{node{D} child{node{a}}} child{node{N} child{node{cat}}} }
child{node{PP} child{node{IN} child{node{to}}} child{node{N} child{node{school}}} } }
;
\end{tikzpicture}
the result of my picture it is like this
Now i want to draw an ellipse an some text in my picture to describe her like this
How i can do that , any answer will be appreciate