MECHANICAL PROBLEMS 95 



kilograms, and still spring back when the 

 weight is removed. They elongate more 

 than a steel wire would do under the same 

 stress, and they differ in another respect from 

 steel, in that they break if loaded only a 

 little beyond their elastic limits. 



To understand how effective a system of 

 such strands really is in enabling the stem 

 to withstand bending stresses, or to recover 

 its original position when the force (e. g. that 

 of the wind) is withdrawn, we must consider 

 the way in which they are arranged, and what 

 actually happens when a stem is made to 

 bend. In the first place the sclerenchymatous 

 strands form a tissue system, and in the second 

 place the strands cannot shift from their 

 relative positions, being prevented from doing 

 so by the surrounding cells of the stem which 

 occupy the space between them. If we there- 

 fore consider the condition of two of these 

 strands situated on, let us say, the east and 

 west sides of the stem they : may together be 

 regarded as forming a girder, the relatively 

 weak tissue of the stem lying in the east and 

 west plane forming the " webbing " or lattice- 

 work of the girder. Now when a girder of 

 this construction is bent, the concave side 

 is shortened or squeezed, while the convex 

 side is lengthened or pulled. The intervening 

 webbing merely serves to hold the two bars or 

 flanges in their relative positions and is itself 

 subject to less and less stress the nearer the 

 middle line between the two flanges is reached. 



