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, 

 Q 



