154 Transactions of the [Sess. 



upon tliem, the marginal glands curved inwards. It would appear, 

 from what Darwin has been able to make out, that an object must 

 come in contact not only with the secretion surrounding the gland, 

 . but with the gland itself ; and this is brought about by the object 

 absorbing the secretion, and thus sinking through it to the surface of 

 the gland. Particles which merely rest on the secretion, and do not 

 come into actual contact with the gdand, never produce any effect ; 

 and the same may be said of one or more contacts with any hard 

 substance. Excessively small closes of certain organic fluids and 

 saline solutions cause strongly marked inflection. DarAvin found that 

 the phosphate of ammonia was by far the most powerful in causing 

 this. When a leaf was immersed in thirty minims of a solution of 

 one part by weight of the salt to 21,875,000 parts of water, the 

 absorption of the twenty-millionth of a grain by a gland was suffi- 

 cient to cause the tentacle bearing it to curve to the centre of the 

 leaf. The amount of heat which the leaves will stand without being 

 injured is also remarkable. Darwin found that when they were 

 immersed in water at a temperature of betAveen 115° and 125° Fahr. 

 they were quickly inflected, and the protoplasm became aggregated ; 

 but Avhen afterwards placed in cold water, they slowly expanded. 

 When exposed to a temperature of 130°, inflection did not imme- 

 diately take place ; but when afterwards placed in cold water, they 

 often became inflected, and then re-expanded. When placed in cold 

 water after exposure to a temperature of 145°, they sometimes be- 

 came slightly though slowly inflected ; but when placed in water at a 

 temperature of 150° for a short time, they Avere killed. 



The secretion surrounding the glands is extremely viscid, so that 

 an insect alighting on the leaf is immediately entangled amongst the 

 glands, which, on becoming excited, transmit a motor impulse to all 

 the surrounding tentacles, which immediately bend over and soon 

 kill it. The time during which the tentacles remain inflected depends 

 on the age and vigour of the leaf, and Darwin mentions that they so 

 remain for a much longer time over soluble nitrogenous substances 

 than over those which yield no such matter. The time varies from 

 one to seven clays, and he states that he has seen the glands of the 

 same leaf inflected three successive times over insects placed on the 

 disc. The leaves are more quickly inflected over animal substances, 

 and they remain so for a longer time during very Avarm Aveather than 

 during cold weather. A living insect is more efficient in causing 

 inflection than a dead one, as it struggles and presses against the 

 glands of many tentacles ; and an insect such as a fly, with thin 

 integuments, is more efficient in causing prolonged inflection than a 

 beetle with a thick coat. 



When an organic or inorganic object is placed on certain glands of 

 a leaf, the secretion from the other glands is increased in quantity, 

 and becomes acid, and this takes place before they come in contact 



