"ROOT-PRESSURE" — WHITE 493 



Since roots of this sort developed pressures in excess of 30 pounds 

 to the square inch, it was evident that something more than the 

 simple hose seal between glass and root would have to be devised to 

 stand the pressures. After many trials, the assembly shown in 

 plate 1, figure 3, was developed. The base of the manometer had a 

 hole about 1 mm in diameter and 15 mm deep to receive the base of 

 the root. Above this the opening tapered abruptly to a bore of about 

 0.25 mm. A centimeter back of the tip on the outside was a collar 

 of glass. The specially made rubber connecting tube had a corre- 

 sponding collar inside, with the hole about 1.5 mm in diameter at 

 the end which was to fit over the glass and only about 0.1 mm in 

 diameter where it was to enclose the root. In setting up an experi- 

 ment, this tube was moistened with glycerine and the end of the 

 manometer inserted completely through it so that the tip protruded. 

 The base of the root was then inserted into the manometer and the 

 rubber pushed back until it enclosed the root for a distance of about 

 a centimeter and the glass for an equal distance. A strong linen 

 thread was bound tightly around the rubber both over the root and 

 over the glass. The rubber distributed the pressure so that this 

 binding did not crush the root, while the glass collar prevented the 

 rubber from slipping off. The whole was then enclosed in a metal 

 clamp, which had collars to press into the rubber at both ends. This 

 effectively protected the rubber from being ruptured by the pressures 

 applied. The manometer was then inserted into a 500 ml Erlen- 

 meyer flask and the upper end attached to a metal manifold. Pres- 

 sure was applied to the manifold from a compressed air tank and 

 observation begun. The manifold used took 4 manometers at a 

 time. 



In the first series of experiments with this apparatus, a gage reading 

 to 100 pounds per square inch was used, on the supposition that this 

 would suffice to record any pressure obtained. Figure 3 represents 

 the results of one experiment with such a setup. The rise of the 

 water column was observed for 24 hours and the secretion curve, 

 which showed a definite diurnal variation in slope, 2 plotted. Pres- 

 sure of one atmosphere was applied at 4 p. m. and, since the column 

 continued to rise, a second atmosphere was applied at 5 p. m. and the 

 apparatus left over night. Under a pressure of 2 atm. the column 

 rose as rapidly that night as it had in the corresponding period of the 



1 This diurnal rhythm was observed in all experiments where readings were made at frequent enough 

 intervals to permit its detection. It seems to be a regular characteristic of the secretion process. The 

 roots used were not protected against the diurnal variations in temperature (24°-28° 0., June 1936) and 

 illumination characteristic of a laboratory room with NE. exposure. Nevertheless, since the mean daily 

 temperatures often varied more than did the hourly temperatures within single 24-hour periods, without 

 producing corresponding variations in secretion rate, it seems improbable that this rhythm is the result of 

 temperature variations. It is difficult, though not impossible, to imagine how an organ without chloro- 

 phyll and whose growth rate has been shown to be independent of illumination of the intensity obtained in 

 the laboratory (White, 1937) could have this one process so markedly affected by illumination. This 

 diurnal rhythm remains an interesting but as yet unexplained feature of the secretion process. 



