2048 Journal of Applied Microscopy 



3. Rate of Transpiration Measured by the Potometer. Construct a potometer as 

 shown in Fig. 5; the outer end of the long glass tube should be several centi- 

 meters higher than where it bends to pass through the cork. The whole appar- 

 atus should stand on a tray which can be carried from one place to another. 

 The rate of transpiration can be calculated by measuring the recession of the 

 water in the long arm of the glass tube. Inject a leaf of the horse-chestnut 

 {Aescuhis HippocastenHt7t) or a plant stem, as in the preceding experiment, tie it 

 firmly into the short piece of rubber tubing on the T-tube. When the experi- 

 ment is working well make the following tests : 



id) the rate of transpiration in a cool atmosphere, 15°-20°C. 

 (b) the rate of transpiration in a warm atmosphere, 23°-2S°C. 

 (r) the effect of wind on transpiration ; in this test the temperature of the 

 wind must be taken into account. 



4. Rate of Transpiration Measured by Weighing. A potted plant is to have 

 the pot enclosed in rubber sheeting or oiled paper, the sheeting being tied 

 around the base of the plant stem, thus preventing evaporation of moisture from 

 crock and earth. The plant is then to be balanced on a weighing scale and a 

 comparative study made of the rate of transpiration in cool air, in warm air, and 

 in a current of air as in the preceding experiment. Finally determine the effect 

 on transpiration of putting a bell-jar or a wrapping of paper over the plant. 



,5. Restoration of the Transpiration Current. Allow two horse-chestnut leaves 

 or cut stems to become somewhat wilted, place one with its cut end in a bottle of 

 water, fill a U-tube with water and firmly fasten the other leaf in one end of the 

 tube. Inject the plant by pouring mercury into the open end of the U-tube 

 until there is a mercury pressure of 5-7 cm. The injected plant will revive in 

 less than an hour, the time should be noted and compared with that required for 

 the uninjected plant. 



6. The Path of the Transpiration Stream. Place a translucent stem or a stem 

 bearing white flowers in a jar of dilute aqueous solution of eosin or methylene 

 blue. If no stems of Impatiens or Begonia can be obtained use seedlings of 

 Indian corn which have attained a length of 15-25 cm. After half an hour the 

 colored liquid can be detected in the fibro-vascular bundles of the plant. 



7. Influence of the Epidermis on Transpiration. Select two apples and two 

 potatoes, paring one of each ; weigh each object carefully, then set all away in 

 the same place for twenty-four hours or more. At the end of this period weigh 

 out again and compute the loss of weight. 



8. The Function of Stomata on Transpiration. Determine by microscopical 

 examination some plant whose leaves have stomata on one surface only. Soak a 

 few square centimeters of filter paper in an aqueous solution of cobaltouschlorid 

 and dry it in an oven or over a radiator ; when the paper is dry note its color, then 

 notice the change in color produced by moist vapor, e. g., steam or the breath. 



From the paper cut two circles 1-2 cm. in diameter and fasten them by 

 means of cover-glasses and weak wire clamps to the opposite sides of a leaf of 

 the plant selected. Within a few minutes the paper covering the stomata will 

 be bleached by the aqueous vapor escaping therefrom, while the paper on the 

 opposite side will retain its color. Howard S. Reed. 



Botanical Laboratory, University of Michigan. 



