84 



A PRIMER OF BIOLOGY 



the construction of girders, rails, masts, columns, 

 &c. The same table shows us that wood and bone 

 are able to resist tearing and crushing forces about 

 equally, but that wood is stronger than bone in its 

 power of resisting a tearing force, while bone is 

 stronger than wood in its pow y er of resisting a crushing 

 force. How important this point is we shall see 

 later, when we come to consider the strains to which 

 these skeletal substances are subjected in the plant 

 and animal respectively. 



(Note. The relative values are obtained by 

 dividing the figures given in the first two columns 

 by the specific gravity of the materials, viz., steel, 

 7'2 ; bone, T9 ; oak wood, '9. The values given 

 are approximate only. The table is adapted from 

 Macalister, " Encyc. Brit." Art. Anatomy.) 



Bearing these fundamental data in mind, the next 



point of importance is to determine how we may 



best arrange a given amount of material, bone or 



wood, as the case may be, so as to combine economy 



of material and effectiveness to resist crushing or 



Arrange- bending. Obviously the plant has to withstand 



sk^ietsfi more tearing than crushing. Thus, pressure of 



material, wind tends to bend the stem of a plant, that is, to 



