34 PLANT LIFE ON THE FARM. 
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 adigestive 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 
