518 Introduction to the Study of Science 



combined with carbon, oxygen, hydrogen, and mineral sub- 

 stances, such as sulfur and phosphorus. These nitrogen com- 

 pounds are the proteins. They are less in quantity than the 

 carbohydrates, but not in importance, for they are indispensa- 

 ble to plant life and growth. The substance composing the 

 cells, which is called protoplasm, is very similar in composition to 

 the complex proteins. Proteins are manufactured in the leaves, 

 by a process somewhat like photosynthesis, also in the roots, 

 and possibly in other parts of plants. They are made by plants 

 in the absence of light, but are produced in larger amounts in 

 light. They are present in all the live centers of the plant, in 

 the protoplasm of the cells, some dissolved in cell sap and 

 others contained in seeds, such as cereals, beans, peas, and in 

 roots, tubers, and bulbs. Proteins resemble the albumen or 

 white of an egg, and, like this, coagulate when heated. They 

 become yellow when stained with iodine solution. 



251. Digestion of starch. In the process of photosynthesis 

 a sugar much like cane sugar is first of all produced in the leaves ; 

 but when the amount exceeds that needed for immediate con- 

 sumption by the plant in its growth and work, the excess is 

 converted by the chloroplasts into starch grains. In darkness 

 or at night plants cease to manufacture sugar and starch, but 

 growth continues. Growth requires food and so makes use of 

 the starch stored by day. But starch grains are insoluble in 

 pure water, as you may demonstrate by trial. They are more- 

 over too large to be carried through the very minute sap canals 

 to distant parts of the plant. They must therefore be made 

 soluble for circulation, that is, digested. How then do plants 

 digest starch or other solid food substances? 



Diastase. The cell sap contained in the chloroplasts is not 

 like pure water, but is a very complex solutiont It contains 

 among other things certain substances which render soluble 

 such solid and insoluble foods as starch, and certain proteins. 

 These substances are called digestive ferments or enzymes. 

 Starch, which is the most abundant of the solid foods stored up 



