a 

THE STEM 135 
ARRANGEMENT OF MECHANICAL TISSUES 
Mechanical tissues. The plant body obtains strength from 
three general types of cells: 
1. Parenchyma cells, whose rigidity is due almost entirely to 
osmotic pressure. Parenchyma tissue is weak and so must be 
present in considerable bulk in order to give any great amount 
of strength. 
2. Collenchyma cells, which are living cells and have their 
walls thickened at the angles where three or four cells meet 
(Fig. 25). These cells get their rigidity from the thickened 
walls, as well as from turgor, and are therefore much better 
strengthening material than are parenchyma cells. As they are 
living cells whose walls can be stretched, they are especially 
fitted for strengthening the growing portions of a plant. 
3. Thick-walled dead cells (Figs. 117, 119), including the 
sclerenchyma of the cortex and pericycle and the wood fibers. 
The wood fibers are elongated dead cells with very thick walls. 
Sclerenchyma cells are stronger than parenchyma or collenchyma 
cells and are the principal strengthening material of old stems. 
As they are dead cells with very thick walls, they are not 
suited to give strength to growing parts. Thick-walled tracheids 
may be very similar to sclerenchyma in their mechanical property. 
Girders. The arrangement of the strengthening material is 
different in leaves, in stems, and in roots, and is suited to the 
special stresses which these various organs have to withstand. 
In order to understand this arrangement it will be convenient 
to consider the stresses occurring in a girder, or beam. If a beam 
of wood or other material is supported at both ends and weighted 
in the middle, the upper surface will be subjected to compression 
and the lower surface to tension, or stretching. Going from the 
upper surface to the lower, we find that the compression decreases 
as the center is approached and at the center it becomes zero. 
The stress is then changed to tension, which gradually increases 
toward the lower surface. It will thus be seen that the greatest 
stresses in a beam are at the upper and lower surfaces and the 
