30 PLANT LIFE ON THE FARM. 



le of much greater importance than it appears to be at 

 present. In any case, the fact that ammonia- solutions and 

 ammonia-vapours are absorbed by leaves with increased 

 manifestations of vital activity renders this mode of feeding 

 a matter of some consequence to the agriculturist ; and the 

 escape of ammoniacal vapour from the muck -heap may not 

 after all be the wasteful operation it is usually supposed to 

 be that is, if the circumstances are such that plants can 

 avail themselves of the exhaled vapour. 



It is a very remarkable fact that fluids which do not 

 contain nitrogen do not give rise to the movements of the 

 leaves, the changes in the protoplasm, the formation of a 

 digestive fluid, and other consequences, which Darwin has 

 discussed in his work on Insectivorous Plants. Mere 

 mechanical irritation of the leaves is not sufficient to 

 ensure the formation of the ferment requisite for digestion. 

 The different effects of salts of soda and of potash, in the 

 ease of the leaves of Drosera, are also suggestive, for 

 while soda-salts give rise to the physiological activity in 

 the leaves, potash salts do not do so, and some of them are 

 even poisonous. Neither the one nor the other is poisonous 

 to the roots, unless applied in very large quantities. Phos- 

 phate of ammonia and phosphate of soda act with remark- 

 able vigour on the leaves, while phosphate of potash is 

 quite inert, the activity in the former cases being probably 

 due to the phosphorus. 



It would thus appear that while almost all plants absorb 

 the inorganic elements, including their nitrogen, from the 

 soil, and derive their carbon from the atmosphere, there 

 are others, such as Drosera, which digest and absorb nitro- 

 genous matters by means of their leaves. Such plants 

 can even extract nitrogenous matter from pollen, seeds, 



