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 



FlG \ P iti7St e a a c r h y eids5 8Sue mechanical support. 



60. 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 

 contrary, are rather 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. 



FIG. 16. Sieve 

 tissue. 



