Contributed by Peter Schroth / Some subjects are immune to age or aesthetic trends. Like the sprout that powers through the crack in the sidewalk, plant life in general and flowers in particular are irrepressible inspirations for art and have breached the territories of artists primarily known for other, more rigorous forms, from Piet Mondrian to Amy Sillman. Ruby Palmer’s show “Garden Theory” at Morgan Lehman Gallery demonstrates yet again that flowers are forever.