316 ASSIMILATION. 



protoplasm which contains coloring-mater, as for instance the 

 chlorophyll granules, the colored granules in algae, etc. 



834. Artificial light and assimilation. De CandoUe ^ exposed 

 the submerged leaves of several species of plants to the light 

 emitted by six Argand lamps, and failed to obtain thereby any 

 evolution of gas. He estimated that the lamps had about five 

 sixths of the intensity of sunlight. In this experiment the 

 light, though insufficient to cause the evolution of gas, restored 

 etiolated plants to their original green color. 



835. When, however, a submerged water plant is exposed to 

 the j-ays from a calcium light '^ (as that of an ordinary projecting 

 lantern), there is a copious evolution of gas from its leaves. The 

 light from burning magnesium wire is also sufficient to cause the 

 decomposition of carbonic acid and the evolution of oxygen.^ 



836. The influence of the electi-ic light upon assimilation has 

 been investigated by numerous observers. Deherain, who ex- 

 perimented in the Palais de ITndustrie, in Paris, found that the 

 total assimilation produced in tlie leaves of Anacharis Canaden- 

 sis, during an exposure for five da^'s, was not equal to that which 

 followed exposure to sunhght for a single hour.* Siemens has 

 shown that (1) many plants do not require any period of rest 

 during the day, but thrive under continued illumination by elec- 

 tric light and sun-light ; (2) electric light, properl3- regulated, 

 accelerates growth, and produces upon plants eflects comparable 

 to those produced hy sun-light.° 



837. Temperature and assimilation proper. In certain cases 

 the minimum temperature at which assimilation can take place is 

 only slightly above the freezing-point of water. Boussingault ^ 

 found that the leaves of the larch decompose carbonic acid at 

 a temperature of from 0°.5-2°.5 C. ; while Kraus' gives the 



1 Mem. pres. par divers Savans, h, I'Institut des Sciences, tome i., 1806, 

 p. 333, and Physiologie vegetale, 1832, p. 131. 



Biot, in 1840 (Froriep's Notizen, xiii. 10), when measuring, in Spain, the 

 length of a degree of latitude, found that the light from the powerful signal- 

 ling apparatus used was not sufficient to cause any evolution of gas from sub- 

 merged plants of Agave Americana. 



^ Prillieux ; Comptes Eendns, Ixix., 1869, p. 408. 



* Heinrich : Versuchs-Stationen, xiii., 1871, p. 153. 



* Annales Agronomiques, tome vii., 1881, p. 385. 



5 Proceedings of the Royal Society, xxx., pp. 210, 295, and Report of tlie 

 British Association fur tlie Advancement of Science, 1881, p. 474. 



6 Ann. des Sc. nat., ser. 5, tome x., 1868, p. 336. 



' Kraus (Pringsh. Jalirb., ^■ii., p. 522) placed seedlings of Lepidium sativum 

 in the dim light of the back of a room, wliere after six days the cotyledons 



