ENZYMES 21 



ating seed is of value in stimulating' the solution of food 

 material and, in general, in quickening the germination. 



4. Heat. — It has been noted how temperature or amount 

 of heat affects germination. Seeds can germinate over a 

 wide range of temperature; some seeds, wheat, for example, 

 will germinate at nearly 0° C, but most seeds at not 1< 

 than 5° C. The upper limit of germination is about 40° C, 

 although cucumber seeds will germinate at 46° C. Each 

 seed, however, has an optimum temperature, and although 

 this varies for different seeds, it is not far from 30° C. Not 

 all of the necessary heat need come from without; vigorous 

 respiration of the germinating seed will produce some of the 

 necessary heat, as noted in the preceding section. 



5. Food for the Seedling. — The insoluble compounds 

 which supply nourishment to the growing seedling, and which 

 furnish material for oxidation, are starch, oil, and proteins. 

 Starch is a so-called carbohydrate, a carbon, hydrogen, 

 oxygen compound with hydrogen and oxygen in the pro- 

 portion of water. Oil is a glyceryl salt of a fatty acid, a 

 carbon, hydrogen, oxygen compound also, but containing 

 less oxygen than does starch. Proteins are complex com- 

 pounds of carbon, hydrogen, oxygen, and nitrogen, usually 

 sulphur, and sometimes phosphorus. 1 The chemical changes 

 taking place in these compounds are: First, the hydrolytic 

 action which makes them soluble; and second, the oxidation 

 reaction which changes starch and oil ultimately to carbon 

 dioxide and water. It was stated in Sections 2 and 3 that 

 these reactions were brought about by certain catalytic 

 agents called enzymes. 



6. Enzymes. — Enzymes are amorphous substances made 

 by living cells, but which can act independently of the 

 living cell. They are not organized bodies, that is, they 

 have no life in themselves. Thev are soluble in glycerine and 

 in water, insoluble in alcohol, and can be obtained from 

 living tissue by pulverization, extraction with glycerine or 

 water, and purified by alternate precipitation with alcohol 



1 For complete discussion of these and other plant compounds, see 

 Chapter III. 



