documentclass[border=10pt]{standalone}
usepackage{verbatim}
usepackage{tikz}
begin{document}
begin{tikzpicture}[sibling distance=10em,
every node/.style = {shape=rectangle, rounded corners,
draw, align=center,
top color=white, bottom color=blue!20}]]
node {Calculus}
    child { node {Limit}
child { node {Differential}
child { node {Tangent line\ to a curve} } }}
child { node {Limit}
   child { node {Integral}
child { node {Area under\ a curve} } }  } ;
end{tikzpicture}
end{document}

documentclass[tikz, border=10pt]{standalone}
usetikzlibrary{arrows.meta,
                matrix}

begin{document}
begin{tikzpicture}[
          > = Stealth,
base/.style = {shape=rectangle, draw,
               text width=11ex, align=center, inner xsep=0.5ex,
               top color=white, bottom color=blue!20},
  RB/.style = {base, rounded corners, font=sffamily},
  SB/.style = {base, font=itshapebfseries},
                    ]
matrix (m) [matrix of nodes,
             nodes in empty cells,
             column sep=-6mm, row sep=3mm
             ]
{
    &   & |[RB]| Calculus
            &   &                               \
    & |[RB]| {Limit}
        &   & |[RB]| Limit
                &                               \
|[RB]| Differential
    &   & |[SB]| FFT
            &   & |[RB]| Integral               \
|[SB,text width=15ex]| Tangent line to a curve
    &   &   &   & |[SB,text width=15ex]| Area under a curve   \
includegraphics[width=15ex]{example-image-a}
    &   &   &   & includegraphics[width=15ex]{example-image-b}   \
};
draw[<->]  (m-1-3) -- (m-2-2);
draw[<->]  (m-1-3) -- (m-2-4);
%
draw[ ->]  (m-2-2) -- (m-3-1);
draw[ ->]  (m-2-4) -- (m-3-5);
%
draw[ ->]  (m-3-1) -- (m-4-1);
draw[ ->]  (m-3-5) -- (m-4-5);
%
draw[ ->]  (m-3-3) -- (m-3-1);
draw[ ->]  (m-3-3) -- (m-3-5);
end{tikzpicture}
end{document}