154 T. G. YUNCKER 



complete seal at the edges. The space in the paper collar around 

 each plant was filled with a fine grade of sand for the purpose of 

 reducing evaporation. In a seal of this kind no paraffin comes in 

 contact with the plant and it was found by checks that it was 

 impossible to detect any loss of water from such a seal. 



Watering and weighing. The plants were watered by means 

 of a graduate and a funnel. The correct amount of water to be 

 added, determined by weighing, was measured into the graduate 

 and added to the potometers through the tube for that purpose. 

 The weighing was done with what is known as a ''micrometer" 

 scale. This scale weighs accurately to within five grams up to 

 eighteen kilos, and proved very efficient for rapid weighing. 



TRANSPIRATION EXPERIMENTS 



These were made to determine the correlation existing between 

 the amount of water transpired per unit area of leaf surface and 

 the moisture content of the soil. Also to determine the water 

 equivalent of the plants, or the ratio of dry matter produced to 

 the amount of water used by the plants under the various mois- 

 ture contents of the soil. 



The potometers in the series run for the day and night transpi- 

 ration were watered at each night weighing. The others were 

 weighed and watered every other day during the greater portion 

 of the period of the experiment. The last two weeks, however, 

 when the plants were transpiring greater amounts, due to in- 

 creased area of transpiratory surface, they were watered each 

 day. To compensate for the increase in weight of the plants by 

 growth it was figured by previous experiments that approxi- 

 mately one gram of material per plant was added each day. 

 Therefore, at intervals of ten days sufficient water was added to 

 make up for the increased weight of the crop. This was merely 

 for the purpose of reducing error. One Series of thirty potome- 

 ters was weighed both morning and night to determine the 

 relation of nocturnal to diurnal transpiration. The weighings 

 were taken as nearly as possible at the same time of day each 

 time for the morning and night weighings so that the amount of 

 error would be as small as possible. The potometers of the three 



