Examples:

> with(DEtools):
dfieldplot([diff(x(t),t)=x(t)*(1-y(t)),diff(y(t),t)=.3*y(t)*(x(t)-1)], \
[x(t),y(t)],t=-2..2,x=-1..2,y=-1..2,arrows=LARGE,title=`Lotka-Volterra model`,\
color=[.3*y(t)*(x(t)-1),x(t)*(1-y(t)),.1]);
dfieldplot(diff(y(x),x)=1/2*(-x-(x^2+4*y(x))^(1/2)),y(x),x=-3..3,y=-3..2,\
title=`Restricted domain`,color=1/2*(-x-(x^2+4*y)));

[Maple Plot]

[Maple Plot]