Chap. VII. SUMMAKY, SALTS OF AMMONIA. 169 



efficient ; its power no doubt depending quite as much 

 on the phosphorus as on th^ 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 3V4-0 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- stretch it along the wall of a large 



ise what a million means. The hall; then mark ofl* at one end 



best illustration which I have met the tenth of an inch. This tenth 



with is that given by Mr. Croll, will represent a hundred, and the 



who says, — Take a narrow strip of entire strip a million, 

 paper 83 ft. 4 in. in length, and 



