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 life in spite of the modifications. 
(Fig. 114.) 
Fic. 115. — Bast fibers of Flax. 4, a portion of a cross section of a Flax 
stem, showing the bast fibers. ¢, 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 linen 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 
