It is obvious that the compound thus formed still contains a free NH_{2} group and a free COOH group, and is, therefore, capable of linking to another amino-acid molecule in exactly the same way; and so on indefinitely. In actual laboratory experiments, as many as eighteen of these amino-acid units have been caused to unite together in this way, and the resulting compounds thus artificially prepared have been found to possess the characteristic properties of natural proteins. These artificially prepared, protein-like, substances have been called "polypeptides," and the individual amino-acids which unite together to form them are called "peptides." Thus, a compound which contains three such units linked together is called a "tripeptid"; one which contains four, a "tetrapeptid." The use of the term "peptid" was suggested by the fact that these amino-acids are produced from the hydrolysis of proteins by the digestive enzyme _pepsin_. The peptid units of any such complex as those which have been referred to in the preceding paragraphs may be linked together in a great variety of ways. Thus, in a tetrapeptid containing units which may be designated by the letters _a_, _b_, _c_, and _d_, the arrangement may be in the orders _abcd_, _bacd_, _acbd_, _dbca_, etc., etc. Similarly, the same peptid unit may appear in the molecule in two or more different places. Hence, the number of possible combinations of amino-acids into protein molecules is very great. Further, it is possible that the peptid units in natural proteins may be united together through other linkages than the one illustrated above, as they often contain alcoholic OH groups in addition to the basic NH_{2} groups, and these OH groups may form ester-linkages with the acid (COOH) groups of other units. Still other acid and basic groups are present in some of the amino-acids which have been found in natural proteins, so that the possibility of variation in the polypeptid linkages is almost limitless. INDIVIDUAL AMINO-ACIDS FROM PROTEINS About twenty different amino-acids have been isolated from the products of hydrolysis of natural proteins, and this number is being added to from time to time, as the methods of isolation and identification of these compounds are improved. Many of these same amino-acids have been found in free form in plant tissues, particularly in rapidly growing buds, or shoots, or in germinating seeds, where they undoubtedly exist as intermediate products in the transformation of proteins into other types of compounds. These amino-acids, grouped according to the characteristic groups which they contain, are as follows: A. Monoamino-monocarboxylic acids: Glycine, C_{2}H_{5}NO_{2}, CH_{2}NH_{2}·COOH, amino-acetic acid. Alanine, C_{3}H_{7}NO_{2}, CH_{3}·CHNH_{2}·COOH, amino-propionic acid. Serine, C_{3}H_{7}NO_{3}, CH_{2}OH·CHNH_{2}·COOH, oxy-amino-propionic acid.