Sentence:  Large planes juxtaposed with other large planes tend to assert themselves as independent shapes , and to the extent that they are flat , they also assert themselves as silhouettes ; and independent silhouettes are <WSD>apt</WSD> 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:  at risk of or subject to experiencing something usually unpleasant