36 THE WORK OF THE LEAF [chap. 



plant possessing only a single stem, a geranium for 

 example, and tie up the pot in waterproof paper or 

 rubber sheet so drawn round the stem of the plant that 

 no water can evaporate from the surface of the pot or 

 the soil in it. Counterpoise the pot on a large balance, 

 and note the loss of weight at intervals for the next few 

 days, comparing the loss per hour by day or by night, 

 and again during periods of varying temperature, etc. 

 Instead of weighing the water lost, a form of apparatus 

 may be devised to render visible the rate at which the 

 plant is giving off or " transpiring " water, so that com- 

 parative measurements may be made much more quickly. 

 An eight-ounce bottle is provided with a rubber cork 

 pierced with three holes, through the largest of which a 

 wooden rod passes, while a second carries a horizontal 

 piece of capillary tubing. A young branch of a plant 

 or the stem of a large leaf is now passed through the 

 third opening, the bottle is filled with water, and the cork 

 is forced in so that no bubbles of air are trapped beneath 

 the cork. When the apparatus has been left to itself for 

 a time, until it has become adjusted to the temperature 

 of the room, the water in the horizontal tube will be 

 seen to be continually receding because of the water 

 taken up by the stem and transpired from the leaves. 

 By means of a scale on the horizontal tube the rate at 

 which the leaves are transpiring may be measured, and 

 by putting the apparatus in a cooler room, by shielding 

 it from the light, etc., the effect of various factors upon 

 the transpiration process may be gauged. By means of 

 the wooden rod, the water in the bottle may be forced 

 back into the capillary tube each time a fresh measure- 

 ment is required. Measurements should not be made 

 until some little time has elapsed after each change, in 

 order that the plant may have adjusted itself to the new 

 condition. In this way it can be shown that a current of 



