126 



LEAVES AND THEIR WORK 



the water's surface. If some of this plant or other green water 

 weed is placed in a large battery jar or fruit jar in a sunny 

 window, bubbles of gas will be seen to arise from it, the amount 

 increasing as the water is warmed by the sun's rays. 



If a glass funnel is placed upside down so as to cover the plants, 

 and then--a test tube full of water inverted over the mouth of the 

 funnel, the gas may be collected by displacement. After two or 

 three days of hot sun, enough of the gas can be obtained to make 

 the oxygen test. 



That oxygen is given off as a by-product by green plants is a fact 

 of far-reaching importance. Parks are in a city true " breathing 

 spaces." The green covering of the earth is giving to animals an 

 element that they must have, while the animals in their turn are 



supplying to the plants carbon 

 dioxide, a compound used in 

 food-making. Thus a relation 

 of mutual helpfulness exists 

 between plants and animals. 



Evaporation of Excess Water. 

 — In the manufacture of starch 

 and proteid, an enormous 

 amoimt of water is taken up 

 by the roots and' passed to 

 the leaves to supply the 

 needed amount of mineral 

 matter. The excess of water 

 is evaporated through the sto- 

 mata. That water is passed 

 through the blade of the leaf 

 in the form of moisture is 

 shown by the photograph 

 above, drops of water having 

 gathered on the inside of the 

 bell jar. A small grass plant 

 on a summer's day evaporates 

 more than its own weight in 

 water. This would make 

 nearly half a ton of water- 



Experiment to show transpiration. Notice 

 that roots covered with root hairs have 

 grown out of the main stem of the plant 

 in response to the moist condition exist- 

 ing outside of the rubber-covered flower- 

 pot and within the bell jar. 



