34 PLANT LIFE ON THE FAKM. 



quence to the agriculturist ; and the escape of ammoni- 

 acal vapor 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 them- 

 selves of the exhaled vapor. 



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 case of the leaves of drosera, are also sug- 

 gestive, 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. Phosphate of ammonia and phosphate 

 of soda act with remarkable vigor 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 ab- 

 sorb the inorganic elements, including their nitrogen, 

 from the soil, and derive their carbon from the atmos- 

 phere, there are others, such as drosera, which digest and 

 absorb nitrogenous matters by means of their leaves. 

 Such plants can even extract nitrogenous matter from 

 pollen, seeds, and bits of leaves (Darwin). Other plants 

 absorb ammonia by means of the hairs covering their 

 leaves, and this class is probably more numerous than 

 the foregoing. Others, again, have no faculty of digest- 

 ing by their leaves, though they absorb solutions of 

 decaying animal matter by their means. Some, such as 

 the bird's nest orchis, feed on the decay of vegetable 

 matter, and are themselves nearly or quite destitute of 



