STRUCTURE OF THE STEM 73 
bark to pith, or in the reverse direction. On account, 
perhaps, of their importance to the plants, the cells of 
the medullary rays are among the longest lived of all 
vegetable cells, retaining their vitality in the beech tree 
sometimes, it is said, for more than a hundred years. 
After the inter- 
spaces between the 
first fibro- vascular 
bundles have be- 
come filled up with 
wood, the subse- 
quent growth must 
take place in the 
manner shown in 
Fig. 44. The cam- 
bium of the original 
wedges of wood, fe, 
and the cambium, ze, 
formed between 
these wedges, con- 
tinues to grow from 
its inner and from Fic. 45. Cross-Section of a Three-Year-Old 
its outer surface, Linden Twig. (Much magnified.) 
and thus causes a E, epidermis and corky layer of the bark; B, 
permanent increase ee cambium layer; #, annual rings 
in the diameter of 
the stem and a thickening of the bark, which, however, 
usually soon begins to peel off from the outside and thus 
soon attains a pretty constant thickness. 
89. The Dicotyledonous Stem, thickened by Secondary Growth. — 
Cut off, as smoothly as possible, a small branch of hickory and one 
of white oak above and below each of the rings of scars already 
