VEGETATIVE ORGANS OF PLANTS. 311 



ing with a cork, through which weighed quantities of 

 water are added from time to time, as required- The 

 amount of exhalation during any given interval of time 

 is learned with a close approach to accuracy by simply 

 noting the loss of weight which the plant and pot 

 together sufEer. Hales, who first experimented in this 

 manner, found that a vigorous sunflower, three and a 

 half feet high, whose foliage had an aggregate surface of 

 39 square feet, gave ofE 30 ounces av. of water in a space 

 of 13 hours, during a very warm, dry day. The average 

 "rate of perspiration" for 15 days, in July and August, 

 was 20 ounces av. At night, with "any sensible, though 

 small dew, the perspiration was nothing." Knop 

 observed a maize-plant to exhale, between May 32d and 

 September 4th, no less than 36 times its weight of water. 

 Hellriegel (at Dahme, Prussia) found that summer 

 wheat and rye, oats, beans, peas, buckwheat, red clover, 

 yellow lupine and summer colza, on the average exhaled 

 300 grams of water for 1 gram of dry matter produced 

 above ground, during the entire season of growth, when 

 stationed in a sandy soil. {Die Methode der Sandkultur, 

 p. 662.) 



Exhalation is not a regular or uniform process, but 

 varies with a number of circumstances and conditions. 

 It depends largely upon the dryness and temperature of 

 the air. When the air is in the state most favorable to 

 evaporation, the loss from the plant is rapid and large. 

 When the air is loaded with moisture, as during dewy 

 ■ nights or rainy weather, then exhalation is nearly or 

 totally, checked. 



The temperature of the soil, and even its chemical 

 composition, the condition of the leaf as to its texture, 

 age, and number of stomata, likewise affect the rate of 

 exhalation. 



Exhalation is rather incidental than necessary to the 

 life of many plants, since it may be suppressed or reduced 



