Cuap. VIL SUMMARY, SALTS OF AMMONIA. 169 
efficient ; its power no doubt depending quite as much 
on the phosphorus as on the nitrogen which it contains. 
We may infer that this is the case, from the energetic 
manner in which bits of bone and phosphate of lime 
affect the leaves. The inflection excited by the other 
salts of ammonia is probably due solely to their nitro- 
gen,—on the same principle that nitrogenous organic , 
fluids act powerfully, whilst non-nitrogenous 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 tentacles 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 solu- 
tion 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 move- 
ments 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 ,,';, of a grain placed 
on the disc is only just able to cause the outer ten- 
tacles of a highly sensitive leaf to bend. It is cer- 
* It is scarcely possible to real- 
ise what a million means. The 
best illustration which I have met 
' with is that given by Mr. Croll, 
who says,—Take a narrow strip of 
paper 83 ft. 4 in. in length, and 
stretch it along the wall of a large 
hall; then mark off at one end 
the tenth of an inch. This tenth 
will represent a hundred, and the 
entire strip a million. 
