THE ROLE OF GREEN PLANTS 



269 



The Rise of Water in Plants 



We have spoken of the passage of water from the root up the stem 

 into the leaf. Osmotic pressure has been shown to be sufficient to 

 start this column of water on its way up the stem, but it is not enough 

 to account for the rise of water sometimes hundreds of feet into the 

 air in the stems of trees. Several theories have been advanced to 

 account for this phenomenon. The most satisfactory of these is the 

 theory that such a column of water is held together by the force of 

 cohesion. Experimental evidence shows that the cohesive quality of 

 water in capillary tubes is very great. The core of water acts as a 

 fine, extremely ductile wire. When we realize that a core of water in 

 a tube 2^ of an inch in diameter will withstand a pressure of over 

 4600 pounds to the square inch, it will be seen 

 that such resistance is a factor in the rise of 

 water through the very tiny tubes found in 

 the vascular bundles of a tree. Another 

 factor in the rise of water in a plant or tree is 

 the evaporation that takes place through the 

 leaves, causing a pull on the cores of water in 

 the tubes of the vascular bundles. During 

 the daytime this is undoubtedly the chief 

 factor in causing the rise of fluids in the 

 stem. 



Production of Oxygen by Plants 



A good many years ago the botanist Sachs 

 proved that a green plant placed in the sun- 

 light will give off oxygen, an experiment easily 

 shown in the laboratory. If an aquatic plant 

 such as Elodca is placed under an inverted 

 funnel in a bell jar of water, and an inverted 

 test tube of water is placed over the mouth 

 of the funnel, bubbles of a gas are seen to 

 leave the plant and gradually displace the 

 water in the test tube. If a sufficient amount of this gas is collected, 

 it can be tested with a glowing splint of wood and proved to be 

 oxygen. The amount of the gas can be shown to depend approxi- 

 mately on the amount of sunlight and consequently the rate of 

 photosynthesis. Going back to the formula which shows the making 



How would you pro\e 

 that the gas on the test 

 tube w as oxygen ? 



