How Substances are Transported and Removed 203 



plants; for the necessity that it be regulated and minimized in 

 places where water is habitually scanty, as occurs conspicuously 

 in deserts, has resulted in the development of protective adapta- 

 tions which, as the weird aspect of desert plants abundantly 

 attests, affect the forms, sizes, and other structural features of 

 plants more profoundly than does any other influence whatsoever 



FIG. 69. A transpirograph iii action. The loss of a gram of water from the plant permits 

 that end of the balance to rise and close an electric circuit ; this acts, through an electro- 

 magnet, to force a pen against a revolving time-drum (seen on the left of the stand), 

 and at the same time to drop a spherical gram weight from a cylindrical reservoir 

 into the box under the scale pan, which is thus depressed, again breaking the circuit. 

 Thus a record is made on the time-drum at each moment when the plant has lost a 

 gram of water. 



excepting only Photosynthesis. But this subject belongs really 

 with a later chapter (on Protection), where it will be treated in 

 detail. 



The results of all experiments on transpiration show remarkable 

 variations in its amount; but it soon becomes evident that such 

 variations are correlated closely with changes in external condi- 

 tions. This can be tested by weighing the plants while kept 

 under somewhat extreme conditions of heat or cold, humidity or 

 dryness, light or darkness; and the results are all the clearer if one 

 makes use of some form of self-recording instrument, one of 



