CELL-CONTENTS AND FORMS OF CELLS. 295 



tissues. The cork cells in different trees are variously developed 

 and accordingly two types of bork formation may be distinguished. 

 In sycamore, cherry and plum trees the cork cells are only slightly 

 thickened (Fig. 159) and the periderm in the form of layers 



FIG. 160. White oak bark with the fissured corky layers (bork). 



separates from the tree annually. In the oaks, chestnuts and 

 tulip poplar the cork cells (Fig. 159) are thick walled and com- 

 pactly arranged so' that, under the stress of growth and thickness 

 of the bark, the layers of periderm are split longitudinally, giving 



