STRENGTHENING TISSUES 



129 



Strengthening Tissues. — In order to endure the strains to 

 which they are exposed, both stems and roots must have strength- 

 ening tissues so as to be tough and rigid. Strengthening cells, 

 although of different types, have much thickened walls and in 

 most cases are much elongated. 



In one kind of strengthening tissue, known as collenchyma, 

 which often occurs in the younger regions of stems, the cell 

 walls are thickened chiefly at the angles, thus leaving thin 

 portions in the side walls through which the protoplasm receives 

 enough materials to maintain hfe in spite of the modifications. 

 (Fig. 114.) 



Fig. 115. — Bast fibers of Flax. A, a portion, of a cross section of a Flax 

 stem, showing the bast fibers, e, epidermis; 6, bast fibers; w, woody part of 

 the stem; p, pith. B, longitudinal view of a number of bast fibers. Much 

 enlarged. 



A kind of strengthening tissue, in which the cell walls are quite 

 evenly thickened with cellulose, occurs in the older regions of 

 stems between the epidermis and woody cylinder, and consists 

 of bast fibers, the fibers upon which the value of Flax, Hemp, etc. 

 as fiber plants depends. Fig. 115.) Bast fibers are much elon- 

 gated cells and so spliced that they form thread-like fibers which 

 are easily combined into larger fibers for making hnen cloth, 

 twine, ropes, and other textiles. Bast fibers may occur also in 

 leaves and roots where they are usually not so prominent, how- 

 ever, as in stems. 



In the woody portions of plants, especially in all trees except 

 the evergreens, there occur along with the conductive tissues 

 wood fibers, in which the walls of the much elongated cells are not 



