Among the artist's favorite subjects to paint, these haystacks attracted an entire year of Monet's almost full attention, and in that time he produced over twenty paintings…just of haystacks. He painted them in different seasons, in different lights, and from different angles, and even though the subject matter never changed (haystacks), the artist was able to produce a broad variety of paintings based on the differences of sunlight, time of day, season of the year, etc., in which he painted them. This was a series, vaguely like Thomas Cole's Course of the Empire, but here nature is not pictured as constant or eternally immutable. Nature changes—and by no interfering influence from people. It changes organically. Nature is naturally fluid, fluctuating, changeable, and unpredictable (remember that new views of the natural world were developing out of the published works of Charles Darwin). Monet's haystacks series, in a sense, showed the evolution of nature's constantly shifting atmosphere; that sometimes the haystacks are hit with sun, shadowed by clouds, rained on, or covered by snow. In this particular image from the series, the haystacks are vibrantly colorful because they are subject to the sun's bright afternoon light, but they lose their color at dusk and in fog. They never simply look the same, is I guess the point to be taken from this exhaustive series.
This provides Monet with a chance to perfect his craft. Like working on a still life, the artist used these haystacks as subjects for stylistic experimentation. Art for him no longer became about depicting a subject but the technique through which one depicts it (otherwise I think he'd have gotten bored with haystacks). The Impressionists in that sense made a science of art, formulating the most accurate method in which to paint, making it all about the craft, the act of creation and experimentation, not necessarily about the product. Put it another way, painting no longer needs to look pretty or impressive or even finished; it needs to accomplish the artist's end. For the Impressionists, this was capturing an impression of real-world environments through the effects of atmosphere and sunlight on subject matter. The image isn't enough. Plein air painting brought artists into the experience of nature, as I said; and Impressionism became focused on conveying that experience, not just the visual appearance of surroundings and items within an area. When we look at Monet's haystacks, the subject doesn't change, and we are never looking at anything different; the experience is what changes, and our feelings and reactions to the paintings change as we see day turned to night, summer turned to winter, and so on. This is why so many otherwise commonplace subjects appear in the artwork of Impressionists at this time. The subject matter almost didn't matter so much anymore as the communication of immaterial elements.