Final answer:
Paul Cézanne continued to paint 'en plein air' after 1890, focusing on still life, landscapes, and portraits, and developing a unique style that influenced future art movements like Cubism and Fauvism.
Step-by-step explanation:
Paul Cézanne, after 1890, continued to paint en plein air, aligning with the Impressionist practice of painting outdoors to capture natural light and scenes. However, as Cézanne progressed in his career, he focused on developing his unique style, placing emphasis on the beauty of the work itself rather than the technique of its rendering. He concentrated on themes such as still life, landscapes, and portraits of people he was close with. His innovative approach involved using short parallel lines that slanted obliquely to create texture and form. This technique would later influence movements such as Cubism and Fauvism. Cézanne's unique method of building form with color and his analytical approach to nature were key forces in the transition from 19th century art to the new art world of the 20th century.