AMOUNT OF TRANSPIRATION. 



271 



and will not open when the plant, still kept in darkness, is 

 subjected to a higher temperature, say from 27 to 30 C. 



731. From the foregoing, it appears (1) that stomata are 

 delicately balanced valves, which are exceedingly sensitive to 

 external influences; (2) that in wilted leaves they are partially 

 closed ; (3) that in most cases, on the application of liquid 

 water, stomata which are open close ; (4) that strong light causes 

 stomata to open widely ; (5) that a sudden shock causes them to 

 close. 



732. Amount of water given off in transpiration. 1 This is 

 determined chiefly by the balance. 



In the oft-cited experiment of Hales, 2 in 1724, the amount 



1 The earliest experiments upon this subject appear to have been those by 

 Woodward in 1699 (Philosophical Transactions). They were made from July 

 to October, and gave the following results (here reduced for convenience to 

 grams) : 



Woodward's most interesting observations relate to the ratio of growth to 

 evaporation when plants are cultivated in different kinds of water. Thus 

 when mint was grown in water mixed with garden earth, the ratio of growth 

 to evaporation was 1 : 52 ; but when it was grown in distilled water, 1 : 214. 



2 "July 3, 1724, in order to find out the quantity imbibed and perspired 

 by the Sun-Flower, I took a garden-pot with a large Sun-Flower, 3 feet -+- 

 high, which was purposely planted in it when young ; it was of the large 

 annual kind. 



" I covered the pot with a plate of thin milled lead, and cemented all the 

 joints fast, so as no vapour could pass, but only air, thro' a small glass tube 

 nine inches long, which was fixed purposely near the stem of the plant, to 

 make a free communication with the outward air, and that under the leaden 

 plate. 



"I cemented also another short glass tube into the plate, two inches long 

 and one inch in diameter. Thro' this tube I watered the plant, and then 

 stopped it up with a cork ; I stopped up also the holes at the bottom of the 

 pot with corks. 



" I weighed this pot and plant morning and evening, for fifteen several days, 

 from July 3, to Aug. 8, after which I cut off the plant close to the leaden 

 plate, and then covered the stump well with cement ; and upon weighing 

 found there perspired thro' the unglazed porous pot two ounces every twelve 



