What Do Lungs and Gills Do? 



Breathing in Man^ The lungs are soft bags consisting of air-tubes and 

 air-sacs, which are lined by a layer of thin-walled cells and surrounded by 

 very fine blood vessels. They are suspended in the thorax, or chest cavity, 

 and air comes into the air-sacs of the lungs, and also passes out, by way of 

 the windpipe, or trachea (see illustration opposite). The trachea divides and 

 branches again and again into the bronchial tubes. While the air-sacs are 

 filled with air, oxygen diffuses from these spaces into the lymph and blood 

 of the surrounding vessels, and carbon dioxide diffuses in the opposite 

 direction (see illustration, p. 208). 



The lungs are filled with fresh air and emptied again by the action of 

 (1) muscles attached to the ribs and (2) a large muscular organ called the 



Rutherford Piatt 



BREATHING ARMS OF SWAMP PLANTS 



In cypress trees, which are typical swamp plants, the roots breathe through the 

 so-called "knees", which rise above the level of the water. The roots of many trees 

 spread out, as in the tamarack, soft maple, pin oak, spruce, hemlock, and cedar, in 

 drier soil they form deeper roots; in swamps they spread roots near the surface. 

 Trees that form tap-roots, such as hickory and ash, are never found in swamps 



iSee Nos. 4, 5, and 6, pp. 212-213. 



204 



