138 DROSERA ROTUNDIFOLIA. (Cuar. VII. 
which bits of bone and phosphate of lime affect the leaves. 
The inflection excited by the other salts of ammonia is’ pro- 
bably due solely to their nitrogen,—on the same principle that: 
nitrogenous organic fluids act powerfully, whilst non-nitro- 
genous organic fluids are powerless. As such minute doses 
of the salts of ammonia affect the leaves, we may feel almost 
sure that Drosera absorbs and profits by the amount, though 
small, which is present in rain-water, in the same manner as. 
other plants absorb these same salts by their roots. 
The smallness of the doses of the nitrate, and more 
especially of the phosphate of ammonia, which cause the ten- 
tacles of immersed leaves to be inflected, is perhaps the most 
remarkable fact recorded in this volume. When we see that. 
much less than the millionth* of a grain of the phosphate, 
absorbed by a gland of one of the exterior tentacles, causes. 
it to bend, it may be thought that the effects of the solution 
on the glands of the disc have been overlooked ; namely, the 
transmission of a motor impulse from them to the exterior 
tentacles. No doubt the movements of the latter are thus. 
aided ; but the aid thus rendered must be insignificant; for 
we know that a drop containing as much as the 5,4, of a 
grain placed on the disc is only just able to cause the outer 
tentacles of a highly sensitive leaf to bend. It is certainly 
a most surprising fact that the pyagoooo Of a grain, or in 
round numbers the one-twenty-millionth of a grain (+ 00000383. 
mg.), of the phosphate should affect any plant or indeed any 
animal; and as this salt contains 35°33 per cent. of water of 
crystallisation, the efficient elements are reduced to 353¢/513¢ 
of a grain, or in round numbers to one-thirty-millionth of a. 
grain (-00000216 mg.). The solution, moreover, in these 
experiments was diluted in the proportion of one part of the: 
salt to 2,187,500 of water, or one grain to 5000 oz. The 
reader will perhaps best realise this degree of dilution by 
remembering that 5000 oz. would more than fill a 31-gallon 
cask; and that to this large body of water one grain of the 
salt was added ; only half a drachm, or thirty minims, of the 
solution being poured over the leaf. Yet this amount. 
* It is scarcely possible to realise 4 in. in length, and stretch it along 
what a million means. The best the wall ofa large hall; then mark 
illustration which I have met with is off at one end the tenth of an inch.. 
that given by Mr. Croll, who says, This tenth will represent a hundred, 
—take a narrow strip of paper 83 ft. and the entire strip a million. 
