38 THE TISSUES OF PLANTS 
joining cells is very thin and permeable to water except 
a button-like thickening, in the center. When seen in 
surface view, a bordered pit shows a double circle, the 
smaller inner one being the opening into the pit and the 
outer circle, the outer edge of the diaphragm. 
49. Special mention must be made of the tracheids of 
Conifers (Spruces, Pines, etc.). 
These are shaped and thickened 
like wood fibers but differ in 
possessing on their radial faces 
one or more longitudinal rows 
of bordered pits. They com- 
bine the functions of tracheids 
and fibrous tissue, serving both 
for conduction of water and for 
Pio. | Mcp Trapheery tissue mechanical support. 
50. Sieve Tissue. In almost 
all of the higher plants and in many of the more massive 
lower plants, there are found rows of elongated rather 
wide cells whose transverse separating walls are pierced 
by numerous larger or smaller perforations. Where two 
such cells lie side by side parts of the lat- 
eral separating wall will often show simi- 
lar perforated areas. These are the so- 
called sieve plates which give the name to 
this tissue. The walls of the sieve tubes, _ |] 
as the elongated cells are called, are usu- "*® 
ally rather thin. The sieve plates, on the 4. 16. "Sieve 
contrary, arerather thick. In surface view 
they look like a sort of network. In some cases, the 
meshes of the net are perforations, in others, they are 
thin walled areas perforated by several to many fine holes. 
The mature sieve tubes have the walls lined with a thick 
layer of cytoplasm in which the nucleus is imbedded. 
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