igo OSMOSIS : ABSORPTION OF WATER 



pressure,' root-pressure being merely a special example of its 

 activity. 



Ex. 108. — Water a well-developed sunflower, tomato, or tobacco plant 

 growing in a pot as in Fig. 84, and place it in a warm shaded situation for 

 two or three hours. Then cut ofif the stem and fasten a glass tube to the 

 stump by means of a piece of rubber-tube (r). Pour in a little water and 

 lap the tube to displace air-bubbles ; mark the height at which the water 

 stands as at a. After a time a considerable amount of sap will be forced from 

 the cut end of the stem and will rise in the glass tube. 



Ex. 109. — Cut off the stem of a young vigorously-growing stinging nettle 

 in spring, and after wiping the cut surface of the stump notice with a lens 

 that the sap which is exuded afterwards comes from the vascular bundles 

 and not from the pith. 



Ex. 110. — Sow a few barley grains in a pot of good garden soil, and when 

 the plants are about 2J or 3 inches high place the pot in a warm shaded or 

 dark place and cover the pot with a bell-glass. Notice after three or four 

 hours that from the tips of the young leaves drops of water are exuded. 

 Remove the bell-glass and leave the plants uncovered until quite dry, then 

 cover again and notice a further execretion of water. 



