RESPIRATION AND OTHER FORMS OF METABOLISM. 47 



with diluted sulphuric acid. Close the mouth of the flask with a 

 cork stopper through which extends a short section of glass tubing. 

 To the outer end of this attach a section of rubber tubing 30 cm. 

 in length.- The free end of the rubber tube should be fitted with 

 a small piece of glass tubing drawn to a point and bent at an angle 

 of 45 degrees for introducing the gas into the respiration-tube. 

 (Fig. 42.) 



38. Fermentation. Perennial plants which grow in temper- 

 ate climates store up a supply of reserve food in the roots, 

 rhizomes, or stems, to serve as building material at the begin- 

 ning of the next vegetative period. The seedling cannot 

 obtain nourishment from the soil and air during the first period 

 of its development, because its roots are not sufficiently de- 

 veloped, and because it has not yet enough chlorophyll to build 

 up food, by aid of the sunlight. Before the solid reserve sub- 

 stances can become of use to the plant they must be dissolved, 

 and transported by diffusion where they are needed. The 

 solution of the reserve food is accomplished by means of fer- 

 ments or enzymes, which by their presence induce changes in 

 organic compounds {fermentation) without themselves being 

 thereby in any way affected. On account of this last property 

 a small amount of enzyme may cause fermentation in a large 

 quantity of -the substance acted upon. In the germination of 

 a seed, external moisture and temperature stimulate the proto- 

 plasm to form an enzyme which dissolves the solid starch, 

 protein, or fat in the storage cells. The solution is diffused 

 into the growing cells of the young plant where it is used in 

 building up protoplasm. The starch formed in leaves under- 

 goes solution and transportation in a similar manner. Diastase, 

 the enzyme which changes starch into maltose, is perhaps the 

 most widely distributed ferment. 



