PHYSIOLOGY. 301 
from the sap or cambium. ‘This sap itis, there- 
fore, which causes the formation of the vessels 
and their fascicles. It is most plentiful where the 
youngest layers of vessels in the stem lie, that is 
in the inner bark. The wood which was formed 
from the outer bark becoming hard, has the sap 
not in so great a quantity. ‘The vessels of the wood 
are in general less active, they carry therefore less 
fluids, and those but slowly. The inner bark, on 
the contrary, which, possesses still young and active. 
vessels, is the only part in the plant possessed 
of life, it can therefore make with its air vessels 
the most use of the sap. Ii then the inner bark is 
injured or wounded in a ligneous plant, so that the 
air has free access to it, the plant dies. ‘The extre- 
mities ‘of the vessels in the mner bark shrink to- 
gether, and the sap alone has no power, it dries up 
entirely. In hard winters those trees have often 
been seen to die, which had their inner bark frozen, 
where those, whose pith and wood only were affected 
by the frost, not the inner bark, grew as formerly. 
From this observation we are entitled to conclude, 
that the life and duration of a tree or shrub, depends 
entirely on the health and activity of the inner bark. 
Every tree or shrub with us sends forth annually 
alarge and a small shoot. ‘The first and principal 
shoot appears in spring, the last on the contrary, 
about St John’s day, near the longest day in June. 
Hence the first has been styled the spring shoot, the 
other the. St John’s shoot. Under the equator and 
the tropics, each shoot is, in most plants of equal. 
size, and the growth of plants for this reason in the 
| : torrid 
