even to myself, and I anxiously sought for every source of error. The
salt was in some cases weighed for me by a chemist in an excellent
balance; and fresh water was measured many times with care. The
observations were repeated during several years. Two of my sons, who
were as incredulous as myself, compared several lots of leaves
simultaneously immersed in the weaker solutions and in water, and
declared that there could be no doubt about the difference in their
appearance. I hope that some one may hereafter be induced to repeat my
experiments; in this case he should select young and vigorous leaves,
with the glands surrounded by abundant secretion. The leaves should be
carefully cut off and laid gently in watch-glasses, and a measured
quantity of the solution and of water poured over each. The water used
must be as absolutely pure as it can be made. It is to be especially
observed that the experiments with the weaker solutions ought to be
tried after several days of very warm weather. Those with the weakest
solutions should be made on plants which have been kept for a
considerable time in a warm greenhouse, or cool hothouse; but this is
by no means necessary for trials with solutions of moderate strength.
I beg the reader to observe that the sensitiveness or irritability of
the tentacles was ascertained by three different methods indirectly by
drops placed on the disc, directly by drops applied to the glands of
the outer tentacles, and by the immersion of whole leaves; and it was
found by these three methods that the nitrate was more powerful than
the carbonate, and the phosphate much more powerful than the nitrate;
this result being intelligible from the difference in the amount of
nitrogen in the first two salts, and from the presence of phosphorus in
the third. It may aid the [page 172] reader s faith to turn to the
experiments with a solution of one grain of the phosphate to 1000 oz.
of water, and he will there find decisive evidence that the
one-four-millionth of a grain is sufficient to cause the inflection of
a single tentacle. There is, therefore, nothing very improbable in the
fifth of this weight, or the one-twenty-millionth of a grain, acting on
the tentacle of a highly sensitive leaf. Again, two of the leaves in
the solution of one grain to 3000 oz., and three of the leaves in the
solution of one grain to 5000 oz., were affected, not only far more
than the leaves tried at the same time in water, but incomparably more
than any five leaves which can be picked out of the 173 observed by me
153