ABSORPTION OF WATER AND MINERALS. 67 



Note that in one case we have innumerable tiny root-hairs, 

 and in the other, as it were, a single gigantic one. 

 But what other differences exist between the two? 



This root-absorption (not, of course, root-pressure) apparatus can 

 easily be made autographic by placing a frictionless float (see page 124) 

 on the liquid and connecting it with a magnifying wheel carrying a pen 

 against the recording cylinder (see page 103). It is more convenient for 

 this purpose to use a burette only half the usual length, that the threads 

 may not need to be too long. To determine whether the internal pro- 

 cesses show any periodicity independently of outside influences, the ex- 

 periment should be carried on in the dark-chamber with as constant 

 temperature as possible. Since watering the plant affects the process 

 very much, the plant should either be watered well at the beginning of 

 the experiment, and not again (evaporation being hindered by a wrapping 

 of rubber or equivalent method see page 78, or else a self-regulating 

 apparatus should be used see page 109). An eight-day clock should 

 be used to turn the cylinder, so the record need not be disturbed during 

 the experiment. 



25. Do living 1 roots exert active pressure in water absorption 

 as the artificial cells already studied do? 



Answer by Experiment 10. 



EXPERIMENT 10. This may be tested by leading the water given 

 off by the roots (as shown in the preceding experiment) into a gauge 

 where its pressure, if any, may be measured. Select a vigorously 

 growing potted plant, such as a Ricinus or Sunflower. Take a piece 

 of glass tubing whose inner bore is about the diameter of the plant ; 

 some 15 cm. from the end draw it to a capillary open point as abrupt 

 as possible (by use of a very small flame) and bent over at right 

 angles (effected by using little heat and bending with an iron instru- 

 ment). Attach this gauge by a water- and pressure-tight joint (see 

 page 40) to the stump of the plant cut off an inch from the ground 

 (see Fig. 12). Attach by wire to the tube a mm. scale (unless it is 

 already graduated). Wait until the liquid inside the tube reaches 

 the zero mark, when the capillary point is to be quickly sealed in the 

 flame of a spirit-lamp, care being taken (by interposing a piece of 

 cardboard) not to heat and expand the air in the tube. The ascent 

 of the water is to be observed, and when its extreme height is reached, 

 the pressure exerted by it upon the closed air is to be calculated by 

 Boyle's law. The final reading should be made as nearly as possible 

 at the same temperature at which the tube was sealed.* 

 A source of error in this experiment is the vapor-tension in the tube, 



* \Vith such gauges as these, my students have often obtained with young Ricinus 

 plants, in November, a root-pressure equal to an atmosphere. 



