82 



A PRIMER OF BIOLOGY 



giving rigidity in an un jointed and, in itself, immobile 



framework. 



We may now turn our attention to the general 



nature of the material of which the skeleton is com- 

 posed in the two types 

 of organism. The mate- 

 rial in the one case is 

 mainly bone, in the other 

 mainly woodland we may 

 consider these two sub- 

 stances from two points 

 of view, (a) chemical 

 composition, and (6) 

 structure. First, as to 

 chemical composition. 

 If a piece of bone and 

 a piece of wood be 

 placed in a furnace and 

 burned so far as they 

 will burn, we find that, 

 at the end of the opera- 

 tion, we still have a 

 bone, though it has lost 

 considerably in weight, 

 whilst the outline of 

 the piece of wood is 

 entirely lost. Further, 

 the ash left over after 

 burning weighs only a 



FIQ. 35. Wood of the Plane small fraction of the 

 tree in tangential longitudinal origina l b l ock . Chemical 

 section. ( x 75.) i / i. 



analysis, in fact, shows 



us that, whilst two-thirds or more of the dry bone is 

 composed of mineral matter, not more than a twentieth 

 of the dry weight of the wood is inorganic in charac- 



