AN INTRODUCTION TO PLANT BIOLOGY 



89 



straight tube and put in a small quantity of water instead of 

 mercury, which is used with the S-tube. Geranium, dahlia, tomato, 

 sunflower, corn, many young shrubs and trees, and grape-vines have 

 stems which make it easy to attach the tube. Make marks on the 

 tube and note the rise of water in the tube for several days. 



Instead of a straight tube, botanists often use an S-shaped 

 tube, and fill the lower bend of the tube with mercury. 

 Since mercury is 13.6 times heavier than 

 water, it is easy to compute the height to 

 which a column of water might be forced. 

 It has been found that in the grape the 

 root-pressure is sufficient to force a column 

 of water up 10 meters (how many feet?). 



87. Work of the Root-Hairs. How 

 does the water concerned in root-pressure 

 get into the root from the soil? This is 

 the question which naturally arises in our 

 minds when we observe the preceding ex- 

 periment. Scientists have studied the mi- 

 croscopic structure of the roots of many 

 plants and have found no openings or pores FlG 

 through which water can get into the root, 

 and so they have concluded that the water 

 must soak through or be absorbed through 

 the walls of the surface cells of the root and 

 especially through the walls of root-hairs. 



In order to make this method of absorption clearer we must 

 try some experiments. 



88. Osmosis. (D) Select a cork that will fit firmly in the mouth 

 of a "diffusion shell " (a membrane bag which may be purchased from 

 dealers in scientific apparatus), bore a small hole in the cork, and into 

 the hole fit a glass tube of small caliber and 18 to 40 inches long. 

 Fill the shell with dark-colored molasses, insert the cork in the shell, 

 wrap and tie around the cork a strong coarse thread, and support with 

 a retort-stand, wooden tripod, or otherwise, so that the diffusion- 

 shell will hang in water (Fig. 36). Note the movement of the 



tube 



(g,h) with mercury, 

 attached to a stem 

 (s) by rubber (r), 

 to measure root- 

 pressure. (Det- 

 mer.) 



