56 STRUCTURAL BOTANY 



all over, but there are a few small narrow pits where 

 the wall is left thin. These cells, unlike the vessels, 

 keep their living contents, and do not fuse with one 

 another. The chief function of the fibrous cells is a 

 mechanical one ; they serve to give rigidity to the stem. 

 At the same time their protoplasm retains its activity, 

 and consequently these cells are able to form and store 

 up starch for future use. Tissue composed of cells of 

 this shape, with long pointed ends overlapping one 

 another, is called prosenchyma, and is commonly found 

 where mechanical strength is required (see Fig. 36, 

 p. 94). 



3. The Xylem-Parenchyma 



Besides the vessels and fibrous cells, the xylem also 

 contains cells with square ends. They are of consider- 

 able length, though very much shorter than the fibrous 

 cells, and have rather thick walls with small pits. They 

 always contain protoplasm and a nucleus, and often 

 starch granules as well, and form the chief tissue in 

 the wood in which starch is stored up. They do not 

 undergo fusion. These square-ended cells constitute 

 the woody or xylem parenchyma, the word parenchyma 

 being used for any cellular tissue with cells of a rounded, 

 square, or oblong form. Cells of this kind are always 

 found in contact with the vessels (see Fig. 36). 



The woody character of the cell-walls of the xylem 

 is due to the presence .of a substance called lignine, 

 a chemical modification of cellulose, which greatly 

 alters its properties. Lignified cell- walls are much 

 harder and stiffer than those of pure cellulose. Such 

 walls are characteristic of the xylem, which is a sup- 

 porting as well as a conducting tissue, and thus forms 

 part of the skeleton of the plant. But we often find 

 cells with lignified walls outside the vascular bundle 



