22 GERMINATION OF THE SEED 



and solution in water. Chemically they are protein-like 

 in character, although they have never been obtained pure 

 enough to have their constitution accurately determined. 

 The mode of action is catalytic, that is, they start a reaction 

 or change its rate. In most cases it is thought that a given 

 reaction in the seed will go on by itself, but so slowly as 

 to be almost imperceptible. A catalytic agent hastens the 

 reaction so that the changes take place quickly . The catalytic 

 agent suffers no permanent change by the reaction, and a 

 small amount of catalyst under optimum conditions can cause 

 an almost indefinite amount of chemical change. Catalytic 

 agents are not confined to enzymes. An example of an 

 inorganic catalyst is manganese dioxide in the generation of 

 oxygen from potassium chlorate. Manganese dioxide comes 

 out of the reaction unchanged; potassium chlorate is reduced 

 to potassium chloride. The manganese dioxide serves to 

 hasten the reaction and to make it take place at a lower 

 temperature than if the potassium chlorate were decomposed 

 alone. Another example is platinized asbestos in the contact 

 process for sulphuric acid, causing sulphur dioxide and oxygen 

 to unite easily, remaining unchanged itself. 



Enzymes are specific in their activity. There are numerous 

 classes, each one causing or hastening a particular reaction, 

 and no other. The principal classes that are effective in 

 changing insoluble stored food to soluble, movable foods 

 in the seed are amylases, lipases, and proteases. Enzymes 

 which cause oxidation are called oxidases. 



7. Amylases. — These enzymes, commonly known as 

 starch splitters, are the best known of the seed enzymes. 

 Diastase is a general name for certain members of this 

 group which occur in plants. In the presence of water and 

 sufficient heat, and from the energy derived by oxidation, 

 the embryo cells, for the most part, produce quantities 

 of these diastatic enzymes which pass through the cell 

 walls into the starch packed cells of the endosperm or 

 cotyledons. There they cause the starch to unite with water 

 and, after going through a series of changes, form maltose as 

 an end product. In some cases, although not so often in 

 seeds, dextrose is formed from maltose by another enzyme. 



