You'll spot a theme when I tell you that the Dutch variant of Art Nouveau was called Nieuwe Kunst; both titles mean "New Art." In Germany it was called Jugendstil, "youth style," and in Austria, Sezessionstil ("Secession style," that is, created by artists who had seceded from accepted art styles).There's a lot going on in the ideas behind Art Nouveau, and you might enjoy reading more about it on this very informative page. Where I feel it begins is in its drive toward making even everyday useful things beautiful, in its celebration of sinuous lines, and not least in its delight in natural forms, as we see here. This seems to have been a vignette or a small book decoration. There's more examples of Dijsselhof's spot illustrations of creatures here and here.