368 THE BIOLOGY OF THE PLANT 



water lost by a leaf freely exposed to the air with 

 that lost by another leaf kept under a bell-jar or 

 in a closed box. 



c. Take a small plant like a, primrose or hegonia, 

 growing in a flower-pot, closely wrap the entire 

 flower-pot and the lower portion of the stem of 

 the plant in rubber cloth such as dentists use, thus 

 preventing any evaporation of water from the sur- 

 face of the soil or through the flower-pot. Insert 

 through the rubber into the soil a small thistle 

 tube, so that water may be supplied to the roots 

 as needed. Weigh the plant as thus arranged 

 and make a note of the weight. At the end of a 

 day weigh again and compare with the previous 

 weight. How much water is transpired by the 

 leaves in twenty-four hours in an ordinary living- 

 room ? Does a collection of plants in a room help 

 to keep the air moist? If the weather be favora- 

 ble, set the plant out of doors overnight and find 

 how much moisture is transpired. Compare this 

 with the amount lost during the same number of 

 hours of exposure to the sun. Judging from your 

 experiments, what relations exist between the 

 transpiration of water by plants and the humid- 

 ity of the atmosphere ? 



£. — Assimilation. 



With a piece of tin-foil wrap both surfaces of 

 the lower end of a leaf of a Geranium, Begonia, 

 or corn plant, so as to exclude the light from 

 that part of the leaf. Let the plant stand for 

 several days exposed to the sunlight. At the end 

 of that time cut off the leaf, place it for a few 



