ORGANS OF SENSATION. 107 



this cause, succeeded in retarding the restoration of a 

 leaf which he had caused to droop, by blacking the 

 springs of the foot-stalk, so as to interrupt the influence 

 of light. 



M. Dutrochet found by experiment on the leaves of 

 the kidney bean, with the air pump, that they folded 

 and nodded in proportion to the quantity of air in 

 their air cells, independent of light and heat, whose 

 influence it would thus appear is only exerted through 

 the medium of the air. 



It is certain that the leaves and young shoots of 

 plants turn to the light ' ; that a leaf, when forcibly 

 placed with its underside uppermost, twists round to 

 regain its position ; and that twining plants will only 

 twine in one direction, from right to left, or left to 

 right, according to the species. 



M. Marcet, of Geneva, in experimenting upon the 

 poisoning of plants, found that metallic poisons, such as 

 arsenic, corrode and destroy them as they do animals, 

 while vegetable poisons destroy irritability, also as in 

 animals. M. Macaire of Geneva confirmed and ex- 

 tended these researches, and found that what is termed 

 the sleep of plants, is thereby destroyed. Drs. Chris- 

 tison and Turner found similar effects produced by 

 poisonous gases, some acting as irritants and some as 

 narcotics. The irritant gases destroy first the parts 

 least plentifully supplied with moisture ; the narcotic 

 gases destroy vegetable life by attacking it throughout 

 the whole plant at once. 



(1) See fuller illustrations in the ALPHABET OF SCIENTIFIC 

 GARDENING, pp. 17 & 28. 



