Conversation so far:
What was a famous work that came out during the Mystical-abstract period?
Fohn im Marc'schen Garten (Foehn at Marc's Garden) was made after the Turin trip.
What other painters were famous during this period?
in the fashion of Paul Cezanne and Robert Delaunays' cubistic form concepts.
Was World War I important in the Mystical-abstract period?
Under the impression of his military service he created the painting Trauerblumen (Velvetbells) in 1917.
Who were Klee's colleagues during this period?
in the fashion of Paul Cezanne and Robert Delaunays' cubistic form concepts.
What else can you tell me about the Mystical-abstract period?
It took over ten years that Klee worked on experiments and analysis of the color, resulting to an independent artificial work.
What famous work did Cezanne produce during the period?
as in the works In den Hausern von Saint-Germain (In the Houses of Saint-Germain) and Strassencafe (Streetcafe).
What famous works did Robert Delaunays produce during the period?
CANNOTANSWER.

Question: Why was the Mystical-abstract period special?
Self-contained rewrite: Why was the Mystical-abstract period special?