168 MECHANICAL SYSTEM 



a definite pressure per unit cross-section. In nature this ideal condi- 

 tion can scarcely ever be realised : for almost inevitably some accessory 

 circumstance, sucb as the effect of a lateral pressure, or a slight 

 asymmetry of construction, will produce a small deflection, which is 

 thereupon at once accentuated by the action of the load. Thus any 

 columnar structures which occur in the plant-body must be constructed 

 so as to withstand bending, and the mechanical principles discussed in 

 the section dealing with inflexibility must apply equally in their case. 



2. Resistance to radial 'pressure. 



[f a cylindrical body is to withstand radial or crushing pressures, 

 it must evidently have its resistant (dements arranged in the form of 

 a strong peripheral shell. It will be seen later on that all subterranean 

 and submerged organs have to be protected in this way against the 

 radial pressure of the surrounding soil or water. The pressure-resisting 

 hollow cylinder or tube must not of course be confused with the similar 

 structure which affords protection against bending strains. As already 

 explained, one may imagine the latter to be produced from a purely 

 mechanical point of view by the lateral fusion of a number of isolated 

 girders; this explanation does not, however, account for the origin of 

 the pressure-proof hollow cylinder, which cannot in any circumstance- 

 be replaced by a ring of isolated girders. 



IV. ARRANGEMENT OF MECHANICAL TISSUES. 



A detailed examination of the mechanical system in the various 

 organs of the plant-body, and particularly in the stem, leaf and root, 

 makes it quite evident that the disposition of the mechanical tissues 

 conforms exactly to the structural principles which have been dis- 

 cussed in the preceding sections. There is indeed a remarkably close 

 agreement in this respect between theoretical expectation and observed 

 fact. In no department of physiological anatomy are more striking 

 and obvious adaptations to be met with, mainly because there is no 

 section of physiology in which the physical laws that underlie the 

 physiological adaptations have been determined with greater complete- 

 ness and precision. 



Before the mechanical system can be considered in detail, a few 

 wolds must be devoted to the general structure and arrangement of 

 mechanical tissues. 



The specialised mechanical elements, namely bast-fibres and col- 

 leuchymatous cells, are generally united to form more or less massive 

 strands or bundles, which are as a rule elongated in the same direction 

 as their component cells. The disposition of mechanical strands within 



