CARNIVOROUS PLANTS. 
355 
the particle is inorganic the effect is only slight, and the tentacles 
soon resume their normal position. With organic substances, 
and especially minute living animals, the case is very different ; 
the embracing motion of the tentacle is compared by Darwin to 
that of the tentacles of a polyp when seizing its prey ; and they 
do not again unfold until the substance is partially or entirely 
absorbed. The behaviour of the leaves with fluids is still more 
remarkable. Distilled water produces no effect whatever. This 
might be expected ; but it is more noteworthy that the same is 
the case apparently with all organic but non-nitrogenous fluids. 
In sixty-one experiments recorded in Mr. Darwin’s work, with 
gum-arabic, sugar, starch, dilute alcohol, olive oil, and a de- 
coction of tea, the tentacles were not inflected in a single case. 
The case is very different with all nitrogenous fluids. Milk, 
albumen, infusion of meat, mucus, saliva, and isinglass, produced 
inflection in every instance, after a longer or shorter time. In 
fact the leaves of Drosera furnish a test for nitrogen in solu- 
tions, rivalling in delicacy any of the performances of the 
spectroscope. Experiments, which were repeated over and over 
again to ensure accuracy, and with the utmost care to eliminate 
every possible source of error, show that the 
1 
14,400 
of a grain 
(•00445 milligram) of carbonate of ammonia absorbed by a 
gland, is sufficient to induce inflection in the basal part of the 
same tentacle ; and, if immersed in a solution of this substance 
for a few hours, the same effect is produced by the 
1 
268,800 
of 
a grain (*00024 mg.). Immersion in nitrate of ammonia of 
such strength that each gland can absorb only the - — of 
s s J 691,200 
a grain (*0000937 mg.) excites movements in each tentacle. 
With phosphate of ammonia the result is still more extraordi- 
nary. A minute drop containing of a grain (*000423 
mg.), if held for a few seconds in contact with a gland, causes 
the tentacle bearing this gland to be inflected. If a leaf is im- 
mersed for a few hours, and sometimes for a shorter time, in a 
solution so weak that each gland can absorb only the — — 
s J 19,760,000 
of a grain (*00000328 mg.), this is enough to excite the tentacle 
into movement, so that it becomes closely inflected. 
The facts connected with the list of non-nitrogenous sub- 
stances which cause and which do not cause inflection of the 
tentacles are very curious. Nine salts of soda with which Mr. 
Darwin experimented all caused inflection, while none of seven 
corresponding salts of potassa did so ; the salts of metals as a 
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