74 WATER AND MINERAL NUTRIENTS 



maple and grape particularly exhibit strong sap-pressure. To 

 illustrate root-pressure, grow squash or cucumber plants, and 

 when they are about a foot or more in height cut off the 

 plant close to the ground. To the plant stem attach a small 

 piece of rubber tubing. Fill it with water and then connect 

 it to a glass tube. (Fig. 124.) At intervals note the rise of 

 water due to root-pressure. The root-pressure in a large 

 cucumber plant may force sap to a height of five feet or more 

 in a tube of five millimeters diameter. 



Review. — What is meant by plant-food? Plant nutrient? De- 

 scribe the root structure. What are root-hairs? Their function? How- 

 do water plants secure water? Do they have roots and root-hairs? 

 Explain osmosis. Exosmosis. How does water enter the root? Why? 

 How can you illustrate the path of water-ascent? What is root- or sap- 

 pressure? Why do plants "bleed?" Have you ever actually seen root- 

 hairs? Explain where and when. Make a drawing as they appeared 

 to you. 



