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of the wind) is withdrawn, we must consider 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. 
