NATURE OF PLANTS 73 



of the cell contents. The furrows and seams that occur in the 

 bark of most trees are caused by the continued activity of the 

 cork cambium and the cambium of the vascular bundles which 

 adds each season new cells to the stem and so pushes out the 

 cork cells. In this way it comes about that the cork cells are 

 pushed further and further from the center of the stem and 

 since they are not capable of dividing they are pulled apart, 

 forming the characteristic furrows in the bark. It will often 

 be noticed that the bark of some trees, see the cone-bearing 

 trees, sycamore, etc., cleaves off in shell-like scales. This is 

 due to the formation of a new cambium that joins on to the old 

 cambium in the form of crescents (Fig. 48). Consequently a? 

 this bark is pushed out it breaks along these successive crescent 

 shaped cambiums and finally cleaves off in scales. In some 

 trees the cork cells break off about as fast as they are formed, so 

 that a comparatively thin layer of cork cells remains attached to 

 the trunk. Such trees have a smooth bark, as in the beech. 

 In other cases the cork cells are formed in great abundance, 

 and owing to the adhesion of the cells, thick layers of cork are 

 formed, as in the oaks, and giant trees of the Pacific. 



It is evident that this mantle of impervious cork cells must 

 prevent the access of the atmosphere to the stem. We have seen 



FIG. 49. Cross-section of the outer part of a stem showing the early 

 development of a lenticel. Note the irregular character and loose arrange- 

 ment of the cells below the stoma and the cork cambium, c, extending out 

 on either side of the lenticel. 



that all living cells respire. In some plants the air spaces ex- 

 tending from the leaves to all regions of the stem are sufficient 

 to bring about an adequate interchange of gases but in the 



