198 BOTANY. 



increase or decrease of temperature, and electrical discharges, 

 may cause movements. Those movements which are brought 

 about by changes in the amount of light constitute what are 

 known as the " sleep" and " waking" of plants. Thus the 

 leaves of the Sensitive Plant close up in darkness exactly as 

 from a concussion, but they remain closed until the reap- 

 pearance of the light. 



260. — The power of movement, whether spontaneous or 

 paratonic, may be temporarily suspended by certain external 

 conditions. Thus, according to Sachs, transitory rigidity 

 or immobility takes place under the following conditions : 



1. Low Temperature. In Mimosa pudica rigidity com- 

 mences at about 15° Cent. (59° Fahr.), in Desmodium gyrans 

 at about 23° Cent. (72° Fahr.). 



2. High Temperature. Mimosa slowly becomes rigid at 

 40° Cent. (104° Fahr.), and very quickly at 50° Cent. (122° 

 F..hr.). 



3. Darkness. Long exposure to darkness (twenty-four 

 hours or more) produces a rigidity which is only removed by 

 a long exposure to light. 



4. Insufficient Moisture. "When the supply of water to 

 the roots of the Sensitive Plant is too little, a partial, and 

 sometimes almost complete, immobility is produced, which is 

 soon removed, however, by copious watering. 



5. insufficient Supply of Oxygen. In a vacuum, or in an 

 atmosphere of nitrogen, hydrogen, ammoniacal gas, etc., 

 jnotile organs become immobile. On the other hand, in 

 pure oxygen rigidity takes place also. 



6. Anmsthetics. In the vapor of ether or chloroform the 

 leaves of the Sensitive Plant become immobile, but in the 

 air they soon regain their motility. 



Mr. Darwin's experiments* upon the leaves of Drosera and 

 Dioneea are confirmatory of the foregoing statements. The 

 sensitive tentacles of the former and leaf-blades of the lat- 

 ter were rendered insensible to the peculiar stimulus of con- 

 tact with soluble nitrogenous bodies when subjected to most 

 of the above-mentioned conditions. 



' Insectivorous Plants." London, 1875. Chap. IV., IX., and XIII. 



