74 ELEMENTARY BIOLOGY 



106. Breathing and leaves. It is sometimes said that 

 "plants breathe in what animals breathe out, while animals 

 breathe in what plants breathe out." This statement is heard 

 so often that many people accept it as true without taking the 

 trouble to consider just what it means. The statement is true 

 only if we are careless enough to jumble up the meaning of 

 the word breathe. The fact is that plants and animals both 

 breathe in oxygen and breathe out carbon dioxid. This oxygen 

 is used in the same way in both plants and animals, and the 

 carbon dioxid originates in the same way. In addition to the 

 breathing, green plants carry on another process in which 

 gases are involved. In the process of starch-making, plants 

 use up carbon dioxid, which they take in from outside, and 

 they give off oxygen, which is separated out in the course of 

 the starch-making. Now breathing has to do with the gas 

 exchange concerned in oxidation and the release of energy. 

 The gas exchange concerned in photosynthesis is not breath- 

 ing. The statement that the breathing of plants differs from 

 the breathing of animals is therefore misleading and not true. 



107. Uses of leaves. Aside from the fact that it is in the 

 leaves of plants that our food supplies are originally worked 

 up, the leaves of many plants are of use to us directly. Some 

 are eaten, as, for example, cabbage, lettuce, spinach, water- 

 cress, dandelion. The leafstalks of some plants, as the rhubarb 

 and celery, are also used as food, although they do not contain 

 very much protein, fat, or carbohydrates. 



The tea and tobacco industries are founded upon peculiar 

 substances found in certain leaves. It is not the food value, 

 but the presence of an alkaloid^ that makes the leaves of these 

 plants interesting to human beings. 



The fact that in the course of its activities a plant throws 

 into the air large quantities of oxygen makes the plants valu- 

 able neighbors, especially in the cities, where oxygen is used 



1 An alkaloid (that is, " like an alkali ") is an organic compound containing 

 nitrogen, capable of combining with acids. 



