Sentence:  When the smaller facet-planes 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 <WSD>maintain</WSD> the integrity of the picture plane . 
Meaning of the word marked with <WSD> tags:  keep in safety and protect from harm, decay, loss, or destruction