PLASTICITY OF SHOOT AND ROOT 203 



experiment described in all books for demonstrating photo- 

 synthesis. Select a living potted plant with large, clear, green 

 leaves ; keep it two nights and a day in darkness (to empty 

 leaves of starch), then bring into bright sunlight, covering one 

 leaf above and below with tinfoil, in the upper fold of which 

 a figure or letter has been cut ; expose this all day to bright 

 light ; at evening drop this leaf into nearly boiling water for 

 five minutes (to kill it and swell starch), then place it in 

 strong alcohol warmed over a water bath, which will take 

 out the green in a few minutes and leave it white, or it may 

 simply be left in alcohol until next day. Then place it in a 

 solution of iodine (made by dissolving a little potassic iodide 

 in water and adding solid iodine until it is of a dark wine 

 color), which turns starch dark blue. The letter or other 

 mark exposed to light will stand out dark blue on a white 

 ground. This may be varied in many ways, as described in 

 different books. 



Experiment No. 5 is rather difficult to demonstrate well, 

 and the only practicable method is that of collecting in an 

 inverted test-tube over water the bubbles from cut shoots of 

 Anacharis, Cabomba, or some other water plant, which rise 

 and displace the water, as described in all works on physi- 

 ology. The gas must then be tested, which may be done by 

 transferring the test-tube to a very small vessel (slipped 

 under it) and inserting into it caustic potash, when the 

 rise of the liquid will show how much of the gas is carbon 

 dioxide (a very small quantity). If, now, enough pyrogallic 

 acid is added to make with the potash a concentrated solu- 

 tion, the further rise will show how much oxygen (really 

 nearly all of the remainder) is present. This test is difficult 

 to apply, but it is more certain than the usual lighted splinter, 

 or phosphorus, test. 



