WATER-RELATION BETWEEN PLANT AND SOIL. 13 



upon a balance and make weighings, as frequently as might be requisite, 

 to determine variations in the weight of the entire system — ^plant, soil, 

 irrigator, and pot. The water loss from the irrigator reservoir was 

 read by filling to a file-mark on the bottle neck, from a burette. For 

 any relatively short period of time the loss by transpiration may be 

 taken as equal to the loss in weight for that period plus the corre- 

 sponding loss from the bottle. Slight alterations in weight caused by 

 respiration and photosynthesis are neglected, as are also the errors 

 introduced by thermometer effects, in the cup tube and bottle, due to 

 temperature influence on the contained water. As will appear later, 

 the variation in weight was not always manifested as a loss, in which 

 case the transpirational water loss is of course the bottle loss minus 

 the corresponding gain in weight. 



These experiments were carried out in one of the experiment houses 

 of the Laboratory of Plant Physiology of the Johns Hopkins University. 

 The cultures were sealed January 24, 1913. At this time there were 

 nine cylinders in the series. Three cylinders contained each three 

 healthy plants of Vicia faha about 15 cm. high, grown from seed 

 planted in the cylinders January 2. Three cylinders contained each 

 three healthy plants of Coleus hlumei (a pale green variety) about 10 

 cm. high. These were from rooted cuttings, planted in the cylinders 

 January 10. The remaining three cylinders each contained three 

 plants of Pelargonium hyhridum, about 10 cm. high. The bottom of 

 the irrigator cup was, in all cases, 50 cm. above the water surface in 

 the reservoir; the cup itself was about 12 cm. high. Thus, all water 

 entering the soil was removed from the cup against a pressure of from 

 50 to 62 cm. of water. An ordinary standardized porous cup atmom- 

 eter, mounted to draw from a burette, was operated alongside the 

 cultures to give integrated measures of the evaporating power of the 

 air for the various periods. 



The plants were allowed to grow and weighings and irrigator and 

 atmometer readings were frequently made, until February 23. Begin- 

 ning with the ninth hour on this day and continuing to include the 

 tenth hour on the following day, hourly observations were obtained. 

 All the plants were in a flourishing condition at this time, and no wilting 

 occurred during this day. 



After the completion of this 25-hour series of measurements the 

 plants were allowed to grow for a month, generally with daily readings 

 and weighings. On February 26, however, the heights of the supplying 

 water columns were altered, being changed from 50 to 100 cm., for the 

 Coleus and Pelargonium plants, and from 50 to 5 cm. for those of Vicia. 



Beginning with the ninth hour, March 23, and continuing through 

 the eighteenth hour of the following day, a second series of hourly 

 measurements was carried out. The plants of Vicia showed pro- 

 nounced wilting during the hottest hours of this day. After the com- 



