Water lilies and weeping willows are what Monet made. He painted the light even when he painted the shade.
I occasionally get out in this Florida heat and paint en plein air because it really helps sharpen my eye and my color matching skills. I think many non-painters see "plein air" and wonder what exactly that means. En plein air is a french expression meaning "in the open air." This was most popularized by the impressionists.
Although there were some examples of naturalistic painting in other traditions, in the all important french art world noone was allowed to paint landscapes as they were. Idealized scenes were the only acceptable mode. Impressed by the works of the English painter John Constable, some young artists called the "Barbizon School" were inspired to abandon the formalism of the past and portray nature in a more realistic mode.
The Barbizon painters made studies for their more finished canvases outdoors. They painted farmers in the fields, and many favored the Forest of Fontainebleau because of the wide variety of large trees growing there. The Barbizon painters were active from about 1830-1870.
The Impressionists: Color Theory Meets Plein Air
The next group of young painters came through the forest of Fontainebleau in the 1860s. Back then they had no name, and no theory beyond working in a more naturalistic manner. So they trekked through Fontainebleau and other locales to paint en plein air. Eventually with the application of new theories about light and color to their paintings they developed what came to be known as the Impressionist style.
Fontainebleau Forest by Claude Monet 1865. This work doesn't resemble Monet's later impressionist landscapes at all. This is simply a realist painting done en plein air. Although Impressionists are closely associated with plein air painting, all plein air paintings are not impressionist paintings.
Impression, Sunrise by Claude Monet 1872. The Impressionist got their name from this painting. When it was first exhibited in 1874 a newspaper critic mocked the painting: "Impression—I was certain of it. I was just telling myself that, since I was impressed, there had to be some impression in it ... and what freedom, what ease of workmanship! Wallpaper in its embryonic state is more finished than that seascape." Following this infamous rant the public came to call this group of young painters "impressionists" and they accepted the label.
So what makes a piece Impressionist as opposed to just plein air? An impressionist painting of a landscape must start outdoors en plein air, but there's color theory on top of it. The impressionists were believers in broken color, which means that you would not make perfect blended textures. You put the paint down and leave it. By doing this you see more energetic brush strokes, but the most important part is it leaves "broken color." The idea is that if you put two opposite colors next to eachother then they vibrate and make the painting more vivid. Above are samples of some closeups of Monet's works.
The Houses of Parliament, Sunset, 1903 by Claude Monet. In this painting you can clearly see the unblended brushstrokes, and even though this is a pretty dark scene even the darkest dark doesn't near true black.
Another quality of impressionistic paintings is that they were painting the light. This means that even if something is in the shadow it won't be very dark. These "high key" paintings hang out mostly in the mid-tone range where you can see the most colors. There's no black on your pallet if you're an impressionist. Because of the advent of commercially produced paint in tubes they had more vivid colors that traveled easily and which are readily accessible (all you have to do is squeeze more out of the tube, as opposed to grinding up pigments). The impressionists responded by applying more and brighter paint, and they could do it anywhere they wanted. It was portable.