88 NATURE TEACHING 



other at the back of the room where the light is dull. 

 The inside of the upper glass standing in the window 

 soon becomes dull with water settling on it, and after a 

 time actual drops of water will trickle down. The one 

 in the dull light remains bright much longer. The water 

 which settles on the inside of the glass must come from 

 the plant, for the card prevents the water in the lower 

 tumbler being evaporated. We learn, then, as in our 

 earlier experiments, that the leaves give off water, and 

 in addition we have found out that they give off more 

 water when in the light than when in the dark. Repeat 

 this experiment with a shoot from which the leaves have 

 been cut off, and compare results. Make sketches of the 

 apparatus fitted up, and record all the observations 

 made. 



The last experiment can easily be modified to allow 

 us to find out how much water a plant actually gives off 

 in a given time. One method of doing this is as follows. 

 Take a glazed pot without a hole in the bottom for 

 instance, an ordinary jam-pot and plant in it a young 

 sunflower or cabbage, in soil. After two or three days, 

 when the plant has become established, water the plant, 

 and cover up the earth with some thick tin-foil, wrap- 

 ping it round the stem of the plant so that no water 

 can escape except by transpiration through the leaves. 

 Weigh pot and plant together, and record the weight 

 and the time in your note-book. Place the pot in bright 

 sunlight in a window, or out of doors if the weather is 

 fine, until the next day, and then weigh again. The 

 difference in the two weights gives the amount of water 

 transpired in this time. Now lift up one corner 



