STEMS 107 



the grass at night, it can be induced to form dew. 

 To do this, set a large glass dish over a vigorous plant 

 of geranium or begonia in such manner that it will 

 be tightly enclosed. If it remains in this position 

 for a few hours, the air inside the vessel becomes 

 saturated with watery vapor, and the leaves are 

 unable to throw off any more. The continued sup- 

 ply from the roots is forced out through the leaf at 

 the edges in the form of large drops. These become 

 so large that they fall off and others collect in the 

 same place, so that moist spots will be formed in the 

 soil where they fall. This demonstration will be 

 most successful if the plant is allowed to remain 

 covered over night. 



144. Lifting power of leaves and branches. It 

 has been shown in a previous experiment ( 135) 

 that a leafy stem will pull colored fluid upward 

 in the stem, and if proper apparatus were at hand 

 it could be proved that it does so with great force. 

 Both the lifting power of the leaves, and the pump- 

 ing power of the roots seem insufficient to send up 

 the necessary amount of water to the leaves, for it 

 will be remembered that a poplar tree uses a bar- 

 rel or more of water every day during the summer, 



