A text is cohesive when the elements are tied together and considered meaningful to the reader

Ellipsis

Ellipsis as a type of cohesive relation is very similar to substitution. While substitution referred to the replacement of one textual element by another, ellipsis is simply characterized by “the omission of an item” (HALLIDAY & HASAN 1994:88). The process can, therefore, be “interpreted as that form of substitution in which [an] item is replaced by nothing” (HALLIDAY & HASAN 1994: 88) or as “substitution by zero” (HALLIDAY & HASAN 1994: 142). Example (8) illustrates such a cohesive tie of ellipsis. In the given example the predicator “ate” is left out in the second half of the sentence and is presupposed because it already occurred before. It would, of course, also be possible to repeat the predicator again at the position where it has been left out.
(8) Mary ate some chocolate chip cookies, and Robert [blank] some gummi bears.
It is possible to say that “[w]here there is ellipsis, there is presupposition, in the structure, that something is to be supplied, or ‘understood’” (HALLIDAY & HASAN 1994: 144). In other words, “ellipsis occurs when something that is structurally necessary is left unsaid” (HALLIDAY & HASAN 1994: 144). As with substitution, there are also three different types of ellipsis, i.e. nominal ellipsis, verbal ellipsis, and clausal ellipsis. Ellipsis s also “a relation within the text, and in the great majority of instances the presupposed item is present in the preceding text” (HALLIDAY & HASAN 1994: 144). This is also true for ubstitution and renders the two kinds of cohesive tie to relations which are normally anaphoric. Table 1 summarizes the main features of reference, substitution and ellipsis once again.

'	Reference	Substitution / Ellipsis
Level of abstraction	Semantic	lexicogrammatical
Primary source of presupposition	Situation	Text
What is presupposed?	Meanings	items ( i.e. words, groups, clauses)
Is class preserved?	not necessarily	Yes
Is replacement possible?	not necessarily	Yes
Use as cohesive device	yes; anaphoric and cataphoric	yes; anaphoric (occasionally cataphoric)

Table 1: Reference vs. Substitution/Ellipsis (HALLIDAY & HASAN 1994:145)