R. S. Ginzburg, S. S. Khidekel, G. Y. Knyazeva, A. A. Sankin a course in modern english

§ 1. Segmentation of Words into Morphemes

Close observation and comparison of words clearly shows that a great many words have a composite nature and are made up of smaller units, each possessing sound-form and meaning. These are generally referred to as morphemes defined as the smallest indivisible two-facet language units. For instance, words like boiler, driller fall into the morphemes boil-, drill- and -er by virtue of the recurrence of the morpheme -er in these and other similar words and of the morphemes boil- and drill- in to boil, a boil, boiling and to drill, a drill, drilling, a drill-press, etc. Likewise, words like flower-pot and shoe-lace are segmented into the morphemes flower-, pot-, shoe- and lace- (cf. flower-show, flowerful, etc., shoe-brush, shoeless, etc., on the one hand; and pot-lid, pottery, etc., lace-boots, lacing, etc., on the other).
Like a word a morpheme is a two-facet language unit, an association of a certain meaning with a certain sound-pattern. Unlike a word a morpheme is not an autonomous unit and can occur in speech only as a constituent part of the word.
Morphemes cannot be segmented into smaller units without losing their constitutive essence, i.e. two-facetedness, association of a certain meaning with a given sound-pattern, cf. the morpheme lace- denoting 'a string or cord put through small holes in shoes', etc.; 'to draw edges together' and the constituent phonemes [l], [ei], [s] entirely without meaning.
Identification of morphemes in various texts shows that morphemes may have different phonemic shapes.
In the word-cluster please, pleasing, pleasure, pleasant the root-morpheme is represented by phonemic shapes: [pli:z] in please, pleasing, [plez] in pleasure and [plez] in pleasant. In such cases we say that the phonemic shapes of the word stand in complementary distribution or in alternation with each other. All the representations of the given morpheme that manifest alteration are called allomorphs of that morpheme or morpheme variants. Thus [pli:z, plez] and [рlез] are allomorphs of оде and the same morpheme. The root-morphemes in the word-cluster duke, ducal, duchess, duchy or poor, poverty may also serve as examples of the allomorphs of one morpheme.