INSECTIVOROUS AND CLIMBING PLANTS. 317 



gooo'oooo or even the TmnrWor of a g rain of P nos ' 

 phate or nitrate of ammonia ! 



By varied experiments it was found that the nitrate 

 of ammonia was more powerful than the carbonate, 

 and the phosphate 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. There is 

 nothing surprising in the absorption of such extremely 

 dilute solutions by a gland. As our author remarks : 

 " All physiologists admit that the roots of plants ab- 

 sorb the salts of ammonia brought to them by the rain ; 

 and fourteen gallons of rain-water [i. e., early rain- 

 water] contain a grain of ammonia ; therefore, only a 

 little more than twice as much as in the weakest solu- 

 tion employed by me. The fact which appears truly 

 wonderful is that the ^-g Woooo of a grain of the 

 phosphate of ammonia, including less than 3 o o 0*0 o o o 

 of efficient matter [if the water of crystallization 

 is deducted], when absorbed by a gland, should in- 

 duce some change in it which leads to a motor im- 

 pulse being transmitted down the whole length of 

 the tentacle, causing its basal part to bend, often 

 through an angle of 180°." But odoriferous particles 

 which act upon the nerves of animals must be infinite- 

 ly smaller, and by these a dog a quarter of a mile to 

 the leeward of a deer perceives his presence by some 

 change in the olfactory nerves transmitted through 

 them to the brain. 



When Mr. Darwin obtained these results, fourteen 

 years ago, he could claim for Drosera a power and 

 delicacy in the detection of minute quantities of a sub- 



