126 
UPWARD GROWTH OF THE HEAD, ETC. [Part II. 
resin in the fir : and the woods of different trees 
do not differ more in their proper constituents 
than the barks of different trees; and, possibly , 
as the first herbaceous envelope is burst and 
destroyed, the next ring of pith assumes its 
functions. 
Let the practical man guard these external 
piths from external injury. Besides the gnaw¬ 
ing of horses, cattle like to find soft-barked trees, 
such as Scotch firs, &c., of a size that they can 
take between their horns to rub their foreheads 
against, and do infinite mischief in this way. 
Trees that are too large for this are compara¬ 
tively safe, as the side rubbing of cattle does not 
injure them so much; besides, that the dead 
epidermis of old trees is a great defence to them. 
It is the mechanical injury which is to be 
guarded against: the idea of chemical poisoning 
from animal oil is a fancy. 
