196 DISEASE IN I'LANTS. 



tliose next below grow out under the released 

 pressure, and these give rise to cells which become 

 cork. As the growth and cell-division continue 

 in the cells below this thin elastic cork-layer, they 

 form a soft herbaceous cushion or callus looking 

 like a thickened lip to each margin of the cut. 

 Each lip soon meets its opposite neighbour, and 

 the wound is closed over, a slight projection with 

 a median axial depression alone appearing on the 

 surface. The depression contains the trapped- 

 in callus-cork squeezed more and more in the 

 plane of the cut as the two lips of callus press 

 one against the other, and sections across the 

 stem and perpendicular to the axis of the cut 

 show that this thin cork, like a bit of brown 

 paper, alone intervenes between the cambium, 

 phloem and cortex respectively of each lip, as each 

 layer attempts to bridge over the interval. If 

 the healing proceeds normally, these layers, each 

 pressing against the trapped cork-film, and grow- 

 ing more and more in thickness, shear the cork- 

 layer and tear its cells asunder, and very soon 

 we find odd cells of the cambium of one lip 

 meeting cambium cells of the other, phloem 

 meeting phloem, and cortex cortex, and the 

 normal thickening of the now fused layers 

 previously separated by the knife goes on as if 

 nothing had happened, the only external sign 

 of the wound being a slight ridge-like eleva- 

 tion, and, internally, traces of the dead cells and 

 cork trapped here and there beneath the ridge. 

 When the conjoined cambium resumes the develop- 



