LIVING MATTER 259 



differences, not yet understood, for protoplasm does not 

 always and everywhere behave in the same way, or perform 

 the same kind of work. 



The protoplasm in the tip of the stem of a plant causes 

 growth toward the light; that in the root causes growth 

 away from the light. The protoplasm in a sweat gland 

 under the skin makes perspiration, while that on the 

 outside of a clam or an oyster makes shell. The great 

 number of variations of life found among plants and 

 animals all of them composed of the same living sub- 

 stance, protoplasm, must be due to differences in the way 

 in which protoplasm does its work and not to differences in 

 the protoplasm itself. 



285. The Necessity for Water. Protoplasm contains 

 a considerable amount of water. For this reason all living 

 things require water. The water lily and the cactus show, 

 however, that there is a difference in the quantity of water 

 required by different plants. Water is also used by some 

 plants and animals as a means of transferring dissolved sub- 

 stances from one place in the body to another. Sap is such 

 a liquid in plants; blood, in animals. 



286. The Necessity for Food. We have seen that 

 plants and animals alike require food to furnish materials 

 from which new structures may be made or old ones repaired. 

 Animals must have, for food, organic matter already formed; 

 plants make their own food. This difference distinguishes 

 animals from plants. 



Some plants are green because in the protoplasm of the 

 leaf -cells there are green chlorophyll bodies. Green plants 

 take water from the soil through their roots, and it rises to 

 the extremity of the highest leaf. They take in carbon di- 

 oxide through pores in their leaves. This is not breathing; 

 it is more like collecting material to make food. The ele- 

 ments contained in water and carbon dioxide namely, 

 carbon, hydrogen, and oxygen are made into food for the 



