proc glm data=plants;
   class i;
   model y=i;
   estimate "h-l" i 0 -1 1 -1 1/divisor=2;
run;
data plants;
   set plants;
   dark=(i=1);
   if (i>1) and (i<4) then type="A"; else type="B";
   if (i=2) or (i=4) then intensity="L"; else intensity="H";
run;
proc glm;
   class dark type intensity;
   model y=dark type(dark) intensity(dark) type*intensity(dark);
run;