Sentence:  Large planes juxtaposed with other large planes tend to assert themselves as independent shapes , and to the extent that they <WSD>are</WSD> flat , they also assert themselves as silhouettes ; and independent silhouettes are apt to coincide with the recognizable contours of the subject from which a picture starts ( if it does start from a subject ) . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)