106 
HISTOLOGY OF VEGETABLES. 
FIG. 93 . 
|-JV> 
^ & o r 
■ ^ ^ ^s C 
7 <z~ c"? - 
Porous vessel in 
Anthracite coal. 
a vertical section of the root of the Alder , the ducts 
as represented in Fig. 92, c, are of large size, and 
the bordered pores found on their walls, are also 
remarkable for the extent to which they are developed. 
In a section of anthracite coal, Fig. 93, 
the ducts are of great length, and all 
the pores are of an oval form. In 
the common Clematis , the ducts are 
of small size, but their termination 
by a septum or septa, situated nearly 
at an angle of 45° to the long axis 
of the duct is well shown in Fig. 92, 
a , b. Now, it often happens that these 
ducts, originally elongated cells, become 
continuous canals or vessels, by the absorption of the 
septa, but, in some cases, in which the process of 
absorption has either been incomplete, or fibres in the 
form of parallel bands, have been developed in the septa, 
these remaining at the edge of the septum, give this part 
of the duct a very peculiar appearance, somewhat like 
that of the bars of a gridiron. 
In an oblique section of a foreign wood, given me by 
Dr. Robert Brown, porous ducts abound, and the fibres 
of the septa are well shown ; they occur in parallel 
lines, like the bars of a gridiron, and, as represented in 
Fig. 94, the fibres join the walls of the pores at the 
margins of the ducts. In a section of a fossil Palm, 
from St. Vincent’s, the extremities of the ducts are 
more or less conical, and the fibre is at these points 
