POROUS OR DOTTED TISSUE. 
105 
POROUS OR DOTTED DUCTS. 
There are, in addition to the spiral vessels of which 
I have now spoken, other tubes or canals of large 
size, termed ducts, which give to woods their diffe- 
rent degrees of porosity, and these are visible even 
to the naked eye. Some of them are marked by an 
internal deposit of fibre in addition to the pores, but 
unlike the spiral vessels, they are incapable of unroll- 
ing. The usual markings 
exhibited are either simple or 
bordered pores. When these 
vessels are examined with a 
low power, the pores appear 
as dots, and hence the name 
“ dotted ducts ” has been 
given to them, the term 
“ duct,” being restricted to 
such vessels as are incapable 
of being unrolled. 
In the Cissampelos Pa- 
reira , or Pareira Brava , the 
ducts form bundles, and are 
remarkable for the minute 
sels, or ducts, from the stem of a 
Clematis ; c , portion of a porous size of their pores ; the ex- 
duct from the Alder ; d , pores of 
the same magnified 500 diauie- tremity of each duct being 
ters ; e , three bordered pores from 
a fossil wood. more or less flattened. In 
fig. 92 . 
a b, portions of two porous ves- 
