essary for all life. We know that all animals need this oxygen 

 in order to breathe, or respire. In fact they have become accus- 

 tomed to it in just the proportions found in the air ; and this is 

 now best for them. 



When animals breathe the air once, they make it foul, because 

 they use some of the oxygen and give off more carbon dioxid. 

 Likewise, all parts of the plant must have a constant supply of 

 oxygen. Roots need it, and this has already been emphasized in 

 Reading- Lesson 4. 



The ox3^gen passes into the air spaces and into the living proto- 

 plasm, performing a function of purification, as in animals. It is 

 interesting to note that the airspaces in the leaf are equal in bulk 

 to the tissues themselves. As a result of the use of this oxygen 

 alone at night, plants give off carbon dioxid just as animals do. 

 Plants respire ; but since they are stationary, and more or less 

 inactive, the}' do not need so much oxygen as animals ; and they 

 do not give off so much carbon dioxid. 



During the day plants use so much more of the foul-gas car- 

 bon dioxid than of oxygen that plants are said to purify the air. 

 The carbon dioxid which plants give off at night is very slight 

 in comparison with that given off by animals ; so that a few 

 plants in a sleeping room need not disturb one more than a family 

 of mice, perhaps. Plants usually grow most rapidly in darkness. 



14. The plant has a?i important connection zvith the water vapor 

 of the air. — In addition to obtaining much of its food supply from 

 the air, the plant has an important relation to the humidity of the 

 atmosphere. Cut off a succulent shoot of any plant, stick the 

 end of it through a hole in a cork and stand it in a small 

 bottle of water. Invert over this bottle a large-mouthed bottle 

 (as a fruit-jar), and notice that a mist soon accumulates on the 

 inside of the glass. . In time drops of water form. The plant 

 gives off water from its leaves and from other succulent parts. 



It has been mentioned that the plant takes its food from the 

 soil in very dilute solutions. Then much more water is absorbed 

 by the roots than is used in growth ; and it is this surplus water 

 which is given off from the leaves into the atmosphere by an 

 evapoiation process known as transpiration. 



Transpiration takes place more abundantly from the under sur- 



