i 7 8 



BOTANY OF THE LIVING PLANT 



Each develops from a single embryonic, or cambial cell. As it elon- 

 gates, its pointed ends slide between those of other fibrous cells, taking 

 a sinuous course. The result is that the cells of the strand interlock, 

 and when a longitudinal stress 1 is applied, the resistant rods press 

 laterally upon one another, so that the greater the stress the more 

 closely are they united. Mechanically such strands act like solid 

 metal wires. 



Fig. 112 



A , Transverse section of sclerenchvma of stem of Sunflower. The larger sections 

 show cells cut through the middle of the fibre, the smaller near to the pointed end. 

 B, the same in longitudinal section, showing the pointed ends of the cells. Small 

 pits are present, in surface view, and in section. F. O. B. ( x 300.) 



Characteristic features of the resistance of plant-fibres, as contrasted 

 with those of certain metal wires of similar transverse section, show 

 how different is the behaviour of the two under stress. 



1 Stress and strain. When a force is applied to a piece of material, the 

 material is said to be under stress. In mechanical construction those portions 

 intended to be under pull are called ties, and those under push, struts. Ties are 



