88 



A PRIMER OF BIOLOGY 



the mode of deposition of the skeleton or mechanical 

 tissue in such plants are extremely numerous as 

 may be seen from the examples illustrated in Fig. 40. 



Even in forest trees it 

 not infrequently hap- 

 pens that the central 

 wood decays, and an 

 old tree may be quite 

 hollow in the centre 

 and yet be quite able 

 to support the super- 

 incumbent weight of 

 branches and leaves. 



Roots, on the other 

 hand, have to with- 

 stand a rectilinear 

 pull, and are not 

 subjected to bending 

 at all, and engineers 

 tell us that the tissue 

 required to resist such 

 a strain should be 

 centrally placed. This 

 is precisely the ar- 

 rangement adopted 

 in the root (Fig. 41). 

 Even when there is 

 a central pith it may 

 become hardened, or 

 sclerotic, while the softer tissues are ' peripherally 

 placed. 



We may now turn to the consideration of another 

 Principle engineering example^the arch or rafter. An ex- 

 arch 6 cellent illustration is obtained from the human 

 ankle (Fig. 42). In every roof (Fig. 43) where the 



June as 



Clad, i urn 



FIG. 40. Distribution of skeletal 

 tissue in plant stems. (After Van 

 Tieghem.) 



