132 



BOTANY 



Oxygen given off by Green Plants. — It is possible to prove that 



oxygen is given off by green plants in sunlight. The common 



green frog scum seen in shallow ponds is often so full of bubbles 



that it is buoyed up by this means at 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. Carefully remove 

 the tube, holding the thumb over the end of 

 the tube to prevent the escape of the gas. 

 A glowing match end thrust into the tube 

 will burst into flame. 



Leaf Structure and Functions. — To 



understand the process fully we must 

 refer to a small portion of the leaf. 

 Here we find that the cells of the green 

 layer of the leaf, under the upper epi- 

 dermis, perform most of the w^ork. 

 The carbon dioxide is taken in through 

 the stomata and reaches the green cells 

 by way of the intercellular spaces and 

 by diffusion from cell to cell. Water 

 reaches the green cells through the 

 tracheal tubes of the veins. It then 

 passes into the cells by osmosis and there becomes part of the cell 

 sap. The light of the sun easily penetrates to the cells of the 

 palisade layer, giving the energy needed to make the food. This 

 whole process is a very delicate one, and will take place only 

 when external conditions are favorable. For example, too much 

 heat or too little heat stops starch making ; the presence of stored 

 food in the leaf, or of 10 per cent of carbon dioxide in the atmos- 

 phere, may stop its work. 



Experiment to show that oxygen 

 is given off by green plants in 

 the sunhght. 



