On the one hand

§ 28. As shown by A. I. Smirnitsky ¹, words derived from different roots may be recognized as suppletive only under the following conditions:

1. 
   When they are identical as to their lexical meaning.
   

1. 
   When they mutually complement one another, having no parallel opposemes. For example, better has no other oppo­site of the positive degree but good and good has no opposite of the comparative degree but better.
   
2. 
   When other lexemes of the same class build up a given opposeme without suppletivity, i. e. from one root. Thus, we recognize the words go — went as suppletive because they express exactly the same grammatical meanings as the oppo­semes come — came, work — worked, finish — finished, etc.
   

Of these conditions only the first two seem indispensable. The words am and is, for example, are suppletive in Modern English in spite of the fact that other verb lexemes do not build up the given opposeme (of person) without suppletivity.
§ 29. The above-mentioned criteria serve to prove the identity of lexical morphemes in spite of their difference in form. The same criteria can be used to prove the identity of any morphemes.
¹ Op. cit. pp. 56—57.
w
H. Gleason writes: "Two elements can be considered as the same morpheme if (1) they have some common range of meaning, and (2) they are in complementary distribution..." \
By means of these criteria it is possible to prove, for instance, the identity of the 'plural' morphemes -s (in cows) and -en (in oxen):

1. 
   They are identical as to their grammatical meaning.
   
2. 
   They complement each other or, in other words, their distribution is complementary: they are not used with the same lexical morpheme. The word ox has no other 'plural' opposite but oxen (not oxes, for instance) and the word cow has no 'plural' opposite but cows (not cowen) ².
   

§ 30. We have already spoken (§§ 14, 15, 18) about lexico-grammatical morphemes and their functions as.stem-building elements. Now we are to see their role in building up classes of words.
A lexico-grammatical morpheme like -er or -ize resembles a lexical morpheme in being common to all the words of a lexeme. Comp. teacher, teacher's, teachers, teachers'; realize, realizes, realized, will realize, has realized, is realized, etc.
But it resembles a grammatical morpheme in being common to many different lexemes. Comp. teacher, worker, leader, writer, reader, realize, nationalize, individualize, naturalize, industrialize, etc.
Hence we may draw the following conclusions:
1) The words of a lexeme are united not only by a lexical morpheme functioning as its root, but also by its lexico-gram­matical morphemes functioning as its stem-building elements. In short, it is the stem that unites words into- a lexeme. To lay stress on the content we may say that a lexeme is a group of words united by the same lexical and lexico-grammatical
¹ H. A. Gleason. An Introduction to Descriptive Linguistics. N. Y.,
1958, p. 80.
² Though there exists the obsolete and dialectal plural kine. Charles
F. Hocket-t (Language. Vol, 23, No. 4, Oct. Dec. 1947) suggests to replace
"complementary distribution' by 'non-contrastive distribution', i. e.
two elements may be in partial complementation, and in those environ­
ments in which both occur, they are in free alternation. "Thus the unaspi-
rated [t] of stick and the aspirated [V] of tick are both found as utterance-
final and in other positions: He's in the skit may end with [t] and [t'j.
hoof and hoove /s/ and /z/ are in non-contrastive distribution: hoofs and
hooves do not differ in meaning"... "The plural of brother 'fellow lodge-
or church-member of the male sex' is either brothers or brethren in free
alternation."
27
meanings. Though the words person, personal, personality, personify, personification have the same lexical morpheme, they belong to different lexemes owing to their lexico-grammat­ical morphemes.
2) Lexico-grammatical morphemes unite lexemes into groups possessing common lexico-grammatical properties.