336 EFFECT OF CHEMICAL AGENTS [Cn. XI 



compounds are placed upon the leaves of the sundew, Drosera, 

 the tentacles remain uniuflected; but when a drop of a nitroge- 

 nous fluid, such as milk, urine, albumen, infusion of raw meat, 

 saliva, or isinglass, is placed on the leaf, the tentacles quickly 

 bend inwards over the drop. DARWIN now set to work sys- 

 tematically to determine which salts and acids cause and 

 which do not cause inflection. Of nine salts of ammonia tried, 

 all caused inflection of the tentacles, and of these the phosphate 

 of ammonia was the most powerful. Sodium salts in general 

 cause inflection while potassium salts do not. The earthy 

 salts are in general inoperative, as are likewise those of lead, 

 manganese, and cobalt. The more or less poisonous salts of 

 silver, mercury, gold, copper, nickel, platinum, and chromic 

 and arsenious acids produce great inflection with extreme quick- 

 ness. Other substances which caused inflection were nitric, 

 hydrochloric, iodic, sulphuric, phosphoric, boracic, and many 

 organic acids ; gallic, tannic, tartaric, citric, and uric acids 

 alone being inoperative. In all these cases, where a bending 

 of the tentacles over the drop occurs, the turning must be 

 regarded as a response to the stimulus of the chemical sub- 

 stance. An excitation proceeds from the irritated region to 

 the protoplasm upon whose imbibitory activity the turning of 

 the tentacles depends. 



2. Chemotropism of Roots. -- - Attention was directed to the 

 fact that roots turn towards or from chemical substances by 

 MOLISCH ('84), who experimented with gases. When grains 

 of maize or peas are sprouted in water, their roots will turn 



FIG. 91. Seedling of Zea, whose radicle originally was just touching the water 

 obliquely with its apex and thereafter nutated in characteristic fashion, keeping 

 close to the air. (From MOLISCH, '84.) 



