8 INTRODUCTION [ch. 



Thus the cell can be pictured as a colloidal solution of proteins 

 endoAved with the properties of matter in mass and surrounded by a 

 permeable cell-wall of cellulose. The colloidal solution contains liquid 

 and solid particles of very varied chemical composition. In the proto- 

 plasm are spaces, vacuoles, filled with cell-sap also containing many 

 and various substances in solution. Throughout the protoplasm, which 

 probably has an organized structure, many kinds of chemical reactions 

 are continually in progress, some being the converse of others, as for 

 instance those of oxidation and reduction which can take place side by 

 side in the same cell. 



Next will be considered the chemical reactions by which the various 

 metabolic changes in the plant are brought about. How are these pro- 

 cesses controlled and how do they take place ? 



There is a large group of organic substances, termed enzymes, many 

 of which are present in every plant. They have a certain characteristic 

 in common, i.e. they bring about chemical reactions in the plant without 

 undergoing any permanent change : in other words they are organic 

 catalysts. Many of these reactions, which take place in the cell at 

 ordinary temperatures with considerable rapidity, need prolonged heating 

 at high temperatures when brought about by artificial means. Enzymes 

 can generally be extracted from the plant by water, especially if the 

 tissues are thoroughly disintegrated. Their chemical constitution is at 

 present unknown, and they are usually destroyed by temperatures greater 

 than 60° C. Moreover, many of the processes which they control in the 

 plant can be brought about by them in vitro under suitable conditions, 

 and it is by means of such experiments that information as to their role 

 in plant metabolism has been ascertained. The majority of known 

 enzymes control both hydrolysis and its converse, synthesis by conden- 

 sation with elimination of water, but under artificial conditions hydrolysis 

 most frequently occurs. The enzyme, diastase, for instance, found in all 

 starch -containing plants hydrolyzes in vitro starch to dextrin and maltose. 

 Similarly the enzyme, maltase, hydrolyzes maltose into glucose. Other 

 enzymes hydrolyze proteins into amino-acids, and others, again, hydro- 

 lyze fats into fatty acids and glycerol. 



Until fairly recently the fact escaped notice that such reactions are 

 reversible, and that these enzymes in situ in the plant may, according 

 to the conditions, control not only the hydrolytic but also the corre- 

 sponding synthetic process. The latter may also be brought about, though 

 not readily, in vitro. This, and other evidence, leads us to believe that 

 enzymes in the plant control the reactions in both directions. 



