POROUS OR DOTTED DUCTS. 
107 
often developed into broad flattened bands, which are 
well represented in Fig. 95. One of the most strik- 
ing examples of the gridiron tissue is to be met with 
FIG. 94. fig. 95. 
Termination of two porous ducts, the fibres Termination of two ducts of 
of the septa being transverse and parallel. a fossil Palm. 
in vertical sections of the White Birch; in this tree 
almost all the vessels have septa composed of parallel 
fibres. 
Peculiar forms of porous ducts are met with in the 
Stigmariee ; they are frequently of great length, and in 
some specimens there is an appearance of the fibre 
between the pores of one duct passing across to join 
those of a neighbouring duct. 
All the pores of the ducts I have hitherto mentioned 
have been either of the simple or bordered kind ; but 
certain peculiarities occur in some plants which re- 
quire notice. In the Clematis , represented by a h , in 
Fig. 92, an oblique line or marking passes through the 
centre of each pore ; but in a specimen of fossil wood 
