MOVEMENTS OF IRRITATION. 519 



are the libriform fibres of the wood, whose strongly thickened 

 walls are frequently provided with slit-like pits. The secondary 

 wood of Tilia, for example, is very rich in libriform. fibres. 

 Finally, in many young structures, still in a state of growth, the 

 collenchyma is to be regarded as mechanical tissue. It is easily 

 recognised by the characteristic thickening of its membranes, 

 which is for the most part confined to the angles of the cells (see 

 Fig. 178). 



The mere presence of stereides, however, is by no means suffi- 

 cient to ensure the rigidity of organs with reference to flexion, 

 tension, and pressure ; the stereides must at the same time have 

 a very definite arrangement in the rest of the tissue. 



To establish constructions resistant all round to flexion (especi- 



FIG. 177. Sclerench3'ma in FIG. 178. Collenchyma in cross section, 



cross section. 



ally stems), it is sufficient if the mechanical elements are arranged 

 in a circle at the periphery of the cross section. The packing 

 between the bands is composed of parenchyma and other tissue. 

 In many organs, especially in leaves, which do not need to be 

 made resistant on all sides to flexion, the mechanical tissue is only 

 distributed on the upper and lower sides. Constructions resistant 

 to tension are secured by having the mechanical elements arranged 

 in the organs not peripherally but, on the contrary, centrally, 

 and in a single compact mass (roots, rhizomes). For construc- 

 tions resistant to pressure it is not a matter of indifference 

 whether they are exposed to longitudinal or radial pressure. 

 Roots and rhizomes developing in the soil are exposed to a radial 

 pressure, in addition to a considerable tension ; they are hence 



