I] INTRODITCTION 7 



phloroglucin, might at some stage be formed from a hexose by conversion 

 of the aliphatic chain into a closed ring : 



OH H OH OH 

 OHO— 0- C -C— 0" CH.,OH — 3H.,0 = CO— CH,— CO— CH„— CO— OH., 



H OH H H j " , 



( (Ivicose 



Ha H 



OC CO HOC COH 



' I = II I 



H.,C CH., HC CH 



\c/ \c^ 



O OH 



Phloroglucin 



There is evidence that aromatic compounds, such as phloroglucin, 

 tannins, flavones and anthocyanins are synthesized in the leaves, and 

 that sugar-feeding, by floating leaves in sugar solutions, leads to the 

 increase of aromatics in the tissues. When the ring structure has been 

 once synthesized, further changes can take place either by the addition 

 of side-chains to the ring or by the condensation of two or more rings. 

 In this way the great multitude of aromatic products present in the 

 higher plants may arise. 



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

 endowed with the properties of n/atter 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 protoplasm 

 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 

 processes 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 



