66 



GROWTH AND WORK OF PLANTS 



Fig. 61. 



Section of oak tree 

 showing annual rings and 

 medullary rays. 



chyma cells which are alive in the sap wood 

 and usually dead in the heart wood.* The 

 cells of the pith rays are very much elon- 

 gated radially; they are flattened by the 

 lateral pressure of the bundles and they 

 present the smooth shining surfaces in 

 radially split wood: second, the porosity of 

 the wood, which appears to the eye (unless 

 it is a very hard compact heavy wood) to 

 have numerous minute pores: third, the 

 presence of numerous concentric rings, 

 called annual rings. 



106. Growth in thickness and the 

 formation of annual rings. In woody 

 stems the fibre-vascular bundles lie very 

 closely side by side so that the woody part 

 of one bundle practically touches that of 

 the two adjacent ones. They dp not quite 

 touch, however, for the pith ray is between. 

 But the parenchyma of the pith rays is 

 squeezed into thin plates by the crowding 

 of the bundles. This brings the bundles of 

 the first year's growth of the stem so near 

 together that they form a ring visible to the 

 eye. Since the bundle is an open one the 

 cambium grows each year, adding on new 

 wood on the inside and new bast on the 

 outside. This causes the growth in thick- 

 ness of woody stems since each successive 

 year the cambium places on a new layer of 

 wood on the outsidef of the old and a new 



* In some trees the pith ray cells remain alive for 

 many years. 



j-For this reason dicotyledons are called by some exogenous (growing 

 on the outside), while the monocotyledons are endogenous (growing on the 

 inside). 



