MECHANICAL PROBLEMS 97 
develop, the cell walls are still thin, the 
strand is itself still growing, and has not as 
yet developed those properties which will 
ultimately render it so valuable from the 
mechanical point of view. 
But just beneath the outside skin or 
epidermis we may see that the cell layers 
which make up the periphery of the outer 
rind or cortex are characterised by cell walls 
of a remarkable form. They are much 
thickened, especially at the corners where 
the cells abut on each other, and this thicken- 
ing often extends to the tangential walls, 
while the radial ones usually remain thin. 
The general impression they give is that of 
a number of concentrically arranged bands 
of thick substance (= the tangential walls) 
bound together by thin plates (= the radial 
walls). These thickened walls possess remark- 
able mechanical qualities which are very 
different from those which distinguish the 
sclerenchyma. They are much weaker, but 
this is partly compensated by their more 
advantageous position at the periphery of 
the stem. The essential feature in which 
they differ from sclerenchyma lies in the 
ease with which they can be stretched beyond 
the elastic limits, for a weight of about two 
kilograms suffices to produce a permanent 
elongation in a strand of one millimeter in 
cross section. They differ still further from 
sclerenchyma in that they do not break at 
this limit, but will stand a much stronger pull, 
G 
