IX.] HEALING OF WOUNDS BY OCCLUSION. 215 



Now this callus (Fig. 29, Cal) is in all cases some- 

 thing more than mere cambium or rather, as the 

 cambium extends by cell-divisions from the cut edge 

 of the wound, its outer parts develop into cortex, and 



Cal, 



FIG. 29. The same piece of stem four years later. The cushion-like development, 

 Ca/, resulting from the overgrowth of the cambium and cortical tissues of the cut 

 branch, has extended some distance fro:n the edges, and is covering in the exposed 

 wood. B is the dead outer corky tissue, incapable of growth, and partially 

 cracked under the pressures exerted by the thickening of the stem. The latter is 

 somewhat swollen transversely, owing to the release of pressure in this region, 

 enabling the cambium to develop a little more actively here ; the quicker growth 

 of the occluding cushion in the horizontal direction is due to the same cause. 



its inner parts into wood, as in the normal case. The 

 consequence is that we have in the callus, slowly 

 creeping out from the margins of the wound, new 



