264 TIMBBES AND THEIR USES 



and an outer epidermis. Between the bundles 

 there is tissue connecting the pith with the 

 cortex, and known as the primary medullary or 

 pith rays. 



Taking a still older part of the same plant, 

 and making a cross section, it is obvious that 

 certain changes have occurred. We can stiU 

 distinguish the pith, the ring of bundles, the 

 primary medullary rays, the cortex and the 

 epidermis. In addition careful observation wiU 

 show that each bundle is cormected to its neigh- 

 bour by a layer of minute cells which have 

 passed right across the primary medullary rays. 

 As a result the bundles appear to be suspended 

 on a complete ring of tissue ; this tissue is the 

 cambium, the meristematic tissue which forms 

 wood inwardly and bast outwardly. The cam- 

 bium within ^the bundles is still known as fas- 

 cicular cambium, while that which connects 

 neighbouring bundles is called interfascicular 

 cambium. When a young stem is peeled, it is 

 along the ring of cambium that the peehng 

 takes place, and the gummy substance with 

 which the exposed surfaces are covered is the 

 protoplasm contained in the cambium cells. 

 Sometimes, though wrongly, the whole of the 

 tissues lying outside the cambium ring are called 

 the bark ; we shall show later why this is in- 

 correct. 



