308 
PHYSIOLOGY. 
BOOK II. 
roots of plants contain a large proportion of azotised matter, 
which is so abundant in the spongioles, as immediately to give 
off ammoniacal vapours when decomposed by aid of heat. 
Aerial roots, especially those of many species of Pothos, con- 
tain more than such as are subterranean. This azotised 
matter is almost or entirely insoluble in water, and adheres 
inseparably to the cellular tissue : it is most abundant at the 
points of the spongioles, and gradually disappears in the in- 
terior of the root. It appears essential to the life of plants, 
and its large proportion at the extremities of the roots may 
help to explain why azotised manures are so peculiarly 
efficient. It also shows how the well known destructive effects 
of tannin upon roots take place, by precipitating the azotised 
matter, which is essential to the existence of roots. 
