DIRECTIONS FOR LABORATORY STUDY. 393 



enough cotton to secure it in the cut cork and set in bottle with 

 roots immersed. 



As the plant grows from day to day watch for the dis- 

 appearance of color in the solution, which will indicate when the 

 alkaline fluid has become acid. Arrange a control experiment in 

 exactly the same way, but without plant. Surround each bottle 

 with opaque shade of heavy paper, to avoid effect of light on the 

 roots and fluid. 



7. To show the corrosion of carbonate of lime by the carbonic acid 

 excreted by the roots* (^j 202.) 



Cover a polished marble slab to a depth of 5 cm. with clean 

 sand, in which plant corn or beans. After the plants are 10-15 

 cm. high, remove sand carefully and rinse off the marble. 

 Examine the surface by reflected light. A little graphite rubbed 

 into lines etched by roots will make them more readily visible. 



8. To show root pressure as a factor in the movement of water in 

 plants. (^ 205, fig. 172.) 



Cut off the stem of an actively growing plant (plants of castor 

 bean and tomato 25-30 cm. high are especially recommended) 

 a short distance above the soil and fasten tightly to the stump, 

 by means of rubber tubing, a piece of glass tubing a meter long, 

 and about the diameter of the stump. Add enough water to rise 

 10 cm. above the rubber connection. Keep roots well watered and 

 mark the height of the water in tube from time to time until it 

 reaches the top or begins to fall. Does the water rise from the first ? 



A more satisfactory record may be reached by attaching to the 

 stump a J-tube as shown in fig. 172. To the horizontal arm 

 attach a mercury manometer. (A manometer may be readily 

 constructed by bending a glass tube, about 5 mm. diameter (3 

 mm. bore) and 80 cm. long, upon itself 30 cm. from one end, so 

 that it forms a U with unequal legs 3-4 cm. apart. Bend 5 cm. 

 of the end of the short leg at right angles, in the plane of the U- 

 Tie the legs to a piece of cork between the legs near top, so that 

 the tube will not be easily broken by the leverage of the legs on 

 the bottom bend.) Fill the space between stump and mercury 

 with water. In the third arm insert a short tube drawn out to a 

 slender point to permit the escape of air and extra water. Seal 

 this with flame after filling. There must be at least 15 cm. of 

 mercury in U portion of manometer. At beginning mark, with a 

 bit of gummed paper, height of mercury in each leg ; measure 

 difference at intervals thereafter until mercury begins to fall. 



