The study has revealed double nature of semantic relations between neighbouring sentences. On the one hand, sentences contain syntactical features determining their semantic dependance on other sentences, on the other hand - there is a structure of meaning in sentences which makes some of them logically indispensable, and some - not. When the dispensable sentences are extracted from the text, the content of the latter may become less rich, while the syntactical and logical structure of the text do not suffer. It is important to learn therefore how such reduction of texts effects understanding of them by readers and in what way it may be used for making synopses and abstracts.