128 BIOLOGY AND HUMAN LIFE 



plant these substances pass from cell to cell, or through special 

 vessels or spaces. Leaves are particularly well fitted to absorb 

 gases from the air and to pass gases into the air (see Fig. 6i and 

 Fig. 64). Roots are generally well fitted to absorb water and 

 dissolved salts from the soil, the surface being greatly enlarged 

 by the outgrowth of root hairs (see Fig. 66). 



104. Animal income. The sugars are the only nutrients that 

 can dissolve in water. Starches and proteins, while capable of 

 absorbing water, cannot dissolve. Such substances are called 

 colloids (which means "like glue"), to distinguish them from 

 sugars and salts, called crystalloids, which can dissolve in water 

 and diffuse through cellulose and other membranes. Moreover, 

 most of our food consists not of pure sugars, starches, and pro- 

 teins, but of plant and animal tissues the nutrients of which are 

 locked up in cells. Finally, seeds, stems, and other portions of 

 many plants contain colloid starch, which we have considered as 

 food for the growth of the plant. How can plant and animal 

 cells make use of such colloids ? 



105. Digestion. Experiments show that colloids can be 

 changed into crystalloids ; and then the material can pass 

 through cell walls by osmosis. The process of transformation is 

 called digestion and can easily be demonstrated. 



In the grains and in seeds containing starch the absorption of water 

 leads to the development of a substance called diastase. This can con- 

 vert starch into sugar. Diastase has been extracted from malted barley 

 (that is, barley kept moist until the grains have sprouted), from rice, 

 and from many other seeds. It can now be bought in the stores. A sub- 

 stance that behaves in many ways like diastase is found in human saliva 

 (spit) and in the digestive juices of many other animals. 



Substances like diastase and the active part of the saliva are 

 called ferments, or enzyms, and many different kinds are known. 

 They are peculiar in that they seem to induce chemical changes 

 in other substances, without, however, undergoing any changes 

 themselves. As a result of this peculiarity a comparatively 

 large amount of material may be made to undergo chemical 

 change through the activity of a very small amount of enzym. 



