Sentence:  When the smaller <WSD>facet-planes</WSD> of Analytical Cubism were placed upon or juxtaposed with the large , dense shapes formed by the affixed materials of the collage , they had to coalesce - become `` synthesized '' - into larger planar shapes themselves simply in order to maintain the integrity of the picture plane . 
Meaning of the word marked with <WSD> tags:  the plane of a facet of an object (as of a cube)