MECHANICAL TISSUES 31 



Fibers occur both in and around phloem and in xylem. Those that 

 occur in the phloem or bark are called bast cells while those that occur 

 in the xylem are called wood fibers. The commercial fibers such as 

 flax, hemp, and jute, out of w^hich thread, twine and ropes are made, 

 are bast fibers. The individual fibers are only 1 or 2 mm. in length 

 but they have pointed ends and are dovetailed together in such a way 

 as to give great strength. When mature their walls are very thick; 

 so thick that the lumen of the cell is extremely narrow. 



Sclereids are formed by the uniform thickening of the walls of 

 ordinary parenchyma cells and are therefore more or less isodia- 

 metric. They are found in the hard shell of nuts, in the seed of the 

 date, and in the bark of trees. Sometimes isolated sclereids occur in 

 places where they do not seem to be of any use, as, for example, the 

 stone cells that occur in the fruit of the pear. In the petiole of the 

 water lily leaf peculiar star-shaped or irregularly branched sclereids 

 are found, their branches projecting into the large air spaces (Fig. 9). 



Collenchyma cells occur in many herbaceous stems, in petioles, 

 and in young woody stems, usually just inside the epidermis. They 

 are living cells, often contain chloroplasts, and may perform various 

 functions besides those of mechanical tissue. 



As a rule mechanical tissues are arranged in the plant in what 

 seems to be the most efficient way. A vertical stem needs to have 

 mechanical tissue placed where it will produce stiffness and prevent 

 the stem from being too easily bent over by the wind. Now, when a 

 vertical column is bent ov^er, if it does not break, there must be a 

 stretching on one side and a compression on the other. Both the 

 compression and the stretching decrease toward the center until they 

 become zero at the center of the column. ^Mechanical tissue right 

 at the center, therefore, would not produce stiffness and it is a well 

 known fact that mechanical tissue is very seldom found at the center 

 of a stem. The center is filled with pith and the mechanical tissue is 

 near the outside. Furthermore, it is well known to mechanical 

 engineers that the most economical way to construct a column, that 

 is, the way to get maximum strength in proportion to the material 

 used, is to make it hollow but with a wall whose thickness is equal to 

 about one-seventh the total diameter of the column. We find that 

 many herbaceous stems closely conform to this mechanical principle 

 by having the layer of supporting tissue equal in thickness to about 

 one-seventh the diameter of the stem. Stems that are square, like 

 those of many mints, often have most of the mechanical tissue in the 

 angles and this is, of course, the very best place for it (Fig. 10). 



