THE TREE ITS SHOOT-SYSTEM. 



101 



is very thin from side to side, and therefore stands out 

 like a knife, with an upper and a lower edge (Fig. 27). 

 Now imagine the primary vascular bundles replaced. 



FIG. 27. Tangential longitudinal section of oak wood, magnified fifty 

 diameters, and showing the transverse sections of the medullary rays, 

 cut as they project towards the observer. (Mtlller.) 



The first change is that the cambium in the vascular 

 bundles becomes continuous across through the medul- 



