246 



PLANT PHYSIOLOGY 



plant surfaces. That such losses do take place can be easily 

 demonstrated : 



1. If a plant is well watered and placed under a bell jar, care 

 being taken to cover the pot and soil with oiled paper so that no 

 water loss can take place from these sources, in a short time drops 



r\ 



[ 



7 



a 



of water will be seen to collect on the sides of the 

 jar, enlarge, and run down. These drops of 

 water have passed off from the plant in the 

 form of water vapor and condensed on the glass 

 surface of the jar. 



2. Another easy and convenient way to dem- 

 onstrate this water loss is to place a well-watered 

 plant on the pan of a balance, taking the same 

 precautions as before to check all evaporation 

 except from the plant itself. After the pans 

 have been balanced, it will be noticed that in a 

 very short time the one holding the plant be- 

 gins to rise as water is lost. 



3. One may also note the rate of transpiration 



indirectly by means of the potometer. A branch 



is fastened in a vessel filled with water to which 



a horizontal glass tube is attached. As water 



evaporates more is drawn in from the vessel and 



consequently the meniscus at the end of the 



column of water in the tube shifts its position 



glazed shell and nearer the bottle. This is not an accurate 

 tube are filled with ,, , r . . ,. i ., 



water and inverted metnod of measuring transpiration because it 



in the glass jar measures water used by the plant rather than 



which is also filled wa ter lost. 



^epurpose of the 4 ' An ^her method much in use is the co- 

 small glass tube, bait chloride one. In this case paper impreg- 

 and why is the up- nated with cobalt chloride is placed in close 

 per e en own. con ^ ac £ w ^ n ^ ne j ea f sur f ace anc } the time noted 



for the blue paper to change to pink as water vapor from the 

 leaf is absorbed. 



Atmometers. — In transpiration studies it is often convenient 

 to compare the plant with a dead evaporating surface, and, for 

 this purpose, porous clay cylinders called atmometers (Fig. 14) 

 are filled with water and connected with a water reservoir. These 

 instruments have been much employed by Livingston and others 



Fig. 14 

 mometer. 



—An at- 

 The un- 



