CHAPTER XVII 

 DISTRIBUTION OF MATERIAL WITHIN THE BODY 



Questions. 1. Do all animals have blood? 2. Do all animals have 

 hearts ? 3. Is the sap of plants the same as the blood of animals ? 

 4. How does blood help keep us alive ? 5. How can the blood of one 

 person be made to work in the body of another (transfusion)? 6. Can 

 the blood of one animal be transfused into the body of another ? 7. Why 

 does the blood of some people clot more quickly than that of others ? 



143. Transfer of materials in plants. All plants and animals 

 except the very smallest have some way of distributing ma- 

 terials between the surface and the inside cells, or between the 

 different parts of the body. In most of the familiar plants one 

 set of tubes or vessels carries water and dissolved salts from the 

 roots, through the stems, to the leaves, and another set of vessels 

 carries organic food from the leaves to other parts of the plant, 

 where it is either used up in the growth of new organs or stored 

 up, as in seeds or potato tubers. The two currents are inde- 

 pendent of each other, consisting of different materials and be- 

 ing without connections at any point (see section 97). 



144. Blood. In most of the familiar animals the blood moves 

 in a continuous stream. In the clams the blood contains a bluish 

 substance, called hemocyanin, which easily combines with oxy- 

 gen and thus carries oxygen obtained from the surrounding 

 water by diffusion into the blood vessels of the skin and gills. 

 In the earthworm the blood carries in solution a reddish sub- 

 stance, hemoglobin, which behaves in the same way as hemo- 

 cyanin. Among the backboned animals the blood has a more 

 complex structure and flows in an elaborate system of vessels, 

 driven by a pumping organ, the heart. Human blood consists 

 of a colorless fluid, called plasma, and a number of small bodies, 



the corpuscles, floating in it (see Fig. 92). 



176 



