ADJUSTMENT TO WATER 65 



total loss for the period of experiment. The plant will usually 

 require watering only once or twice during the experiment, but 

 the soil should be aerated every day by blowing air through the 

 thistle tube. When water is added, the amount must be care- 

 fully measured and recorded. For the sake of comparing differ- 

 ent plants, and since the leaf surface varies on account of 

 growth and the withering of old leaves, it is very desirable to 

 express the amount of transpiration upon the basis of a square 

 centimeter or decimeter of surface. The total leaf surface is 

 found as indicated in Experiment 15; it is usually unnecessary 

 to allow for the stem surface. The transpiration for a square 

 decimeter of surface is then found by dividing the water loss 

 for the time concerned by the number of square decimeters of 

 total area. 



82. Measuring transpiration in the field. The measurement 

 of transpiration in the field, i.e., of a plant in its own habitat, 

 may be made in the same way. The inconvenience is greater 

 only because of the need of taking the scales into the field, or 

 of bringing the plant into the laboratory, and of seeing that the 

 plant runs no risk of being disturbed or destroyed. The plants 

 to be studied are carefully dug in the spring, when they are small, 

 and transferred to pots of a size that will necessitate little or no 

 repotting. The pots are sunken in the holes from which the plants 

 are taken. Readings of transpiration may be made as soon 

 as the plant is well established. The surface of the pot alone 

 is covered with rubber, since the sides are protected by the soil. 

 The plant is weighed and watered from time to time in the way 

 already described. Care must be taken to free the surface of 

 the pot from earth when it is taken from the soil for each weigh- 

 ing. The transpiration is likewise expressed in grams per square 

 decimeter of leaf surface. It is further very desirable that the 

 plants for experiment be located in the stations where physical 

 factor readings are taken. This will make it possible to di.s- 

 cover the causes of the variations in the amount of transpirnlion 

 shown by plants of different habitats. 



Experiment 19. Influence of factors upon the rate of transpiration. 

 Select five well-grown sunflower plants as nearly alike as possible. 

 Cover the pots with rubber as indicated above and w(>igh e:icli one. 

 Place one plant as a check in a sunny moist place in the plant -house. 

 Alongside of it, place two dishes of measured surface, one containing a 



