ABSORPTION 157 



tions as to methods of study, precautions, etc., by Miss ECKERSON in the 

 Botanical Gazette, 45, 1908, 50. In this paper it is shown that the common 

 greenhouse plants differ widely in their root-pressures, and still more widely 

 in their exudation quantities, while some, for one reason or another, are 

 impracticable for this study. For exudation quantity the best plants, in 

 order of excellence, are Begonia coccinea (though, it may be added, many 

 species of Begonia exude large quantities of water, much larger than any 

 other herbaceous plants known to me), Senecio Petasitis, Fuchsia, and Mar- 

 guerite. For root-pressure the best plants, taking account of high pressures, 

 ease of manipulation, and commonness of occurrence, are Fuchsia, Mar- 

 guerite, Horseshoe Geranium, though Salvia involucrata, Tomato, Heliotrope, 

 String Bean, and Senecio Petasitis also give high pressures. In general the 

 highest pressures are attained in spring, and on the morning of the second 

 day of the experiment. 



DEMONSTRATION METHODS. For exudation an effective arrangement 

 consists of a glass tube, the diameter of the stump, attached upright by rubber 

 tubing thereto; the rise of the sap is then very plain. For the demonstra- 

 tion of pressure the small gauges are not impressive, and a better arrange- 

 ment consists of a large open gauge of the same general form (including 

 the bulb), made of small-bore (2-3 mm. diameter) barometer tubing, long 

 enough to show an atmosphere of pressure; this gives a demonstration 

 leaving nothing to be desired, provided that a plant of ample exudation 

 quantity be used. 



An arrangement for making root-pressures self-registering, using an open 

 mercury gauge with a float which connects with a recording drum, is 

 described by M. B. THOMAS in the Botanical Gazette, 17, 1892, 212. It 

 does not, however, separate pressure from quantity nor allow for increasing 

 weight of the mercury column. Compare also BARANETZKY'S apparatus 

 mentioned by DETMER, 205. An instrument, called a pinometer, for observ- 

 ing simultaneously root-pressures and stem suction is described by DAR- 

 BISHIRE in Botanical Gazette, 39, 1905, 356, and an arrangement for replac- 

 ing root-pressure for severed roots is described by BLODGETT in Journal 

 of Applied Microscopy, 5, 1902, p. 1988. 



ROOT EXUDATION AND PRESSURE QUANTITIES. In the paper by Miss 

 ECKERSON, above cited, it is shown that, for herbaceous greenhouse plants, 

 the exudation varies from 1.5 cc. through a mean of 26.83 cc - t 263 cc., 

 whence may be derived a conventional expression of 1-25-300 cc, For the 

 same plants the root-pressure varies from .323 through a mean of .92 to i 605 

 atmospheres, which may be expressed conventionally as .3-.p-i.6 atmospheres. 

 The mean pressure of .9 atmospheres equals 13 pounds to the square inch. 



PRESSURE-GAUGES. Those available for physiological purposes are of 

 three types. First are the metal gauges of the steam (Bourdon) type, in 

 which the straightening of a bent flat tube under internal pressure is made 

 to move a pointer around a circular dial. These have been extensively 

 used in measuring sap-pressures (e.g., VINES, Annals of Botany, 10, 1896, 

 291, and Bulletin of the Experiment Station of Vermont, No. 103, 1903), 

 but they are only useful where large quantities of liquids are concerned. 



