This page has been validated.



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. (Müller.)

The first change is that the cambium in the vascular bundles becomes continuous across through the medul-